Investment casting, also called lost-wax casting, is one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys. Lost foam casting is a modern form of investment casting that eliminates certain steps in the process.
Inlet-outlet cover of a valve for a nuclear power station produced using investment castingThe process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process require little surface finishing and only minor machining.
Contents[hide]1 Applications 2 History 3 The process 4 External links
[edit] ApplicationsInvestment casting is used in the aerospace and power generation industries to produce single-crystal turbine blades, which have more creep resistance than equiaxed castings. It is also widely used by firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost. Other industries that use standard investment-cast parts include military, medical, commercial and automotive.
Investment casting offers high production rates, particularly for small or highly complex components, and extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness) with very little machining. The drawbacks include the specialized equipment, costly refractories and binders, many operations to make a mold, and occasional minute defects.
[edit] HistoryInvestment casting dates back thousands of years. Its earliest use was for idols, ornaments and jewellery, using natural beeswax for patterns, clay for the moulds and manually operated bellows for stoking furnaces. Examples have been found in India's Harappan Civilisation (2000 BC - 2500 BC) idols, Egypt's tombs of Tutankhamun (1333 – 1324 BC), in Mesopotamia, Mexico, and the Benin civilization in Africa where the process produced detailed artwork of copper, bronze and gold.
The earliest known text that describes the investment casting process (Schedula Diversarum Artium) was written around 1100 A.D. by Theophilus Presbyter, a monk who described various manufacturing processes, including the recipe for parchment. This book was used by sculptor and goldsmith Benvenuto Cellini (1500 - 1571), who detailed in his autobiography the investment casting process he used for the Perseus and the Head of Medusa sculpture that stands in the Loggia dei Lanzi in Florence, Italy.
Investment casting came into use as a modern industrial process in the late 19th century, when dentists began using it to make crowns and inlays, as described by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. Its use was accelerated by Dr. William H. Taggart of Chicago, whose 1907 paper described his development of a technique. He also formulated a wax pattern compound of excellent properties, developed an investment material, and invented an air-pressure casting machine.
In the 1940s, World War II increased the demand for precision net shape manufacturing and specialized alloys that could not be shaped by traditional methods, or that required too much machining. Industry turned to investment casting. After the war, its use spread to many commercial and industrial applications that used complex metal parts. For example, Sturm, Ruger, founded in 1949, based much of its manufacturing on the then newly-adopted technology, rising to dominance in the firearms manufacturing world through the elimination of labor-intensive machining of firearms as had been common practice in the firearms industry.
Modern investment casting techniques stem from the development in the United Kingdom of a shell process using wax patterns known as the Investment X Process. This method resolved the problem of wax removal by enveloping a completed and dried shell in a vapor degreaser. The vapor permeated the shell to dissolve and melt the wax. This process has been evolved over years into the current process of melting out the virgin wax in an autoclave or furnace.
[edit] The process Fig 1. Shell for cast turbocharger rotorA pattern of the component to be cast is produced by injection-molding special waxes into a metal die. Pre-formed ceramic cores can be included in the wax pattern as it is molded, which can create intricate hollows within the finished casting. As many as several hundred patterns may be assembled into a tree around a wax runner system (riser & sprue). Once a tree has been assembled, a pour cup is attached.
Fig 2. View of the ceramic impression in a turbocharger shellThe completed tree is dipped, or invested, by hand or via robotic control into a ceramic slurry of ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying) or a hybrid of these controlled for pH and viscosity. A fine sand is applied to the invested tree in a fluidised bed, rain tower sander, or by hand. During the primary coat(s), the sand will typically be a zircon-based, as zirconium is less likely to react with the molten metal when poured into the shell. The stuccoed tree is then allowed to dry before re-dipping in slurry and applying secondary coats of mullite, Molochite, chamotte or fused silica refractory material. This process is repeated until the shell is thick enough to withstand the mechanical shock of receiving the molten metal. Dry times generally range from 24 to 48 hours, and total production from two days to one week.
Completed turbocharger rotorAfter the shell (Fig 1.) has been constructed, the wax is removed in an autoclave or furnace (hence, the lost-wax process). Most shell failures occur at this point, as the fragile stuccoed shell is subjected to extremes of temperature and, in an autoclave, pressure. The shell is then fired at temperatures of around 1,100 degrees Celsius to induce chemical and physical changes in the set refractory materials forming a ceramic shell. This leaves a ceramic impression (Fig 2.) of the part to be cast. Most foundries remove the shells from the furnace while still hot and pour the molten metal into the ceramic shell. Various methods of pouring the molten metal include vacuum casting, anti-gravity casting, tilt casting, gravity pouring, pressure assisted pouring, centrifugal casting. After the molten metal cools, the shell is removed. This is generally done with waterjets, vibration, grit blasting or chemical dissolution. The cooled parts are removed from the tree by sawing them free or by dipping them in liquid nitrogen and breaking them off with a hammer and chisel. The parts are then finished. Many cast parts require grinding of the gate and runner bar attachments. Because molten metal cools slowly, it does not finish as hard as some forging and machining processes. Cast parts often are subsequently hardened by heat treatment, surface hardening, or HIP (Hot Isostatic Pressing) hardening (Known as HIPping). The parts are inspected by eye or in special cases by X-ray at the foundry or by specialty firms.
2008-07-21
Injection molding
Injection molding (British: moulding) is a manufacturing technique for making parts from both thermoplastic and thermosetting plastic materials in production. Molten plastic is injected at high pressure into a mold, which is the inverse of the product's shape. After a product is designed, usually by an industrial designer or an engineer, molds are made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. Injection molding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture. Injection molding typically is capable of tolerances equivalent to an IT Grade of about 9–14.
Standard two plates tooling – core and cavity are inserts in a mold base – "Family mold" of 5 different partsThe most commonly used thermoplastic materials are polystyrene (low cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a ter-polymer or mixture of compounds used for everything from Lego parts to electronics housings), polyamide (chemically resistant, heat resistant, tough and flexible – used for combs), polypropylene (tough and flexible – used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser).
Injection molding can also be used to manufacture parts from aluminium or brass (die casting). The melting points of these metals are much higher than those of plastics; this makes for substantially shorter mold lifetimes despite the use of specialized steels. Nonetheless, the costs compare quite favorably to sand casting, particularly for smaller parts.
Contents [hide]1 Equipment 1.1 Mold 1.2 Design 1.3 Machining 1.4 Cost 2 Injection process 2.1 Injection molding cycle 2.2 Molding trial 2.3 Molding defects 3 History 4 See also 5 Notes 6 References 7 External links 7.1 Associations
[edit] Equipment Paper clip mold opened in molding machine; the nozzle is visible at rightMain article: Injection molding machineInjection molding machines, also known as presses, hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can generate. This pressure keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations.
[edit] MoldMold (Tool and/or Mold) is the common term used to describe the production tooling used to produce plastic parts in molding.
Traditionally, molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copper alloy. The choice of material to build a mold is primarily one of economics. Steel molds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium molds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for molding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated. The molds can be manufactured by either CNC machining or by using Electrical Discharge Machining processes
[edit] DesignMolds separate into two sides at a parting line, the A side, and the B side, to permit the part to be extracted. Plastic resin enters the mold through a sprue in the A plate, branches out between the two sides through channels called runners, and enters each part cavity through one or more specialized gates. Inside each cavity, the resin flows around protrusions (called cores) and conforms to the cavity geometry to form the desired part. The amount of resin required to fill the sprue, runner and cavities of a mold is a shot. When a core shuts off against an opposing mold cavity or core, a hole results in the part. Air in the cavities when the mold closes escapes through very slight gaps between the plates and pins, into shallow plenums called vents. To permit removal of the part, its features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such overhangs when the mold opens. Sides of the part that appear parallel with the direction of draw (the direction in which the core and cavity separate from each other) are typically angled slightly with (draft) to ease release of the part from the mold, and examination of most plastic household objects will reveal this. Parts with bucket-like features tend to shrink onto the cores that form them while cooling, and cling to those cores when the cavity is pulled away. The mold is usually designed so that the molded part reliably remains on the ejector (B) side of the mold when it opens, and draws the runner and the sprue out of the (A) side along with the parts. The part then falls freely when ejected from the (B) side. Tunnel gates tunnel sharply below the parting surface of the B side at the tip of each runner so that the gate is sheared off of the part when both are ejected. Ejector pins are the most popular method for removing the part from the B side core(s), but air ejection, and stripper plates can also be used depending on the application. Most ejector plates are found on the moving half of the tool, but they can be placed on the fixed half if spring loaded. For thermoplastics, coolant, usually water with corrosion inhibitors, circulates through passageways bored through the main plates on both sides of the mold to enable temperature control and rapid part solidification.
To ease maintenance and venting, cavities and cores are divided into pieces, called inserts, and subassemblies, also called inserts, blocks, or chase blocks. By substituting interchangeable inserts, one mold may make several variations of the same part.
More complex parts are formed using more complex molds. These may have sections called slides, that move into a cavity perpendicular to the draw direction, to form overhanging part features. Slides are then withdrawn to allow the part to be released when the mold opens. Slides are typically guided and retained between rails called gibs, and are moved when the mold opens and closes by angled rods called horn pins and locked in place by locking blocks, both of which move cross the mold from the opposite side.
Some molds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one-piece tires and wheels.
2-shot or multi shot molds are designed to "overmold" within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This can be achieved by having pairs of identical cores and pairs of different cavities within the mold. After injection of the first material, the component is rotated on the core from the one cavity to another. The second cavity differs from the first in that the detail for the second material is included. The second material is then injected into the additional cavity detail before the completed part is ejected from the mold. Common applications include "soft-grip" toothbrushes and freelander grab handles.
The core and cavity, along with injection and cooling hoses form the mold tool. While large tools are very heavy weighing hundreds and sometimes thousands of pounds, they usually require the use of a forklift or overhead crane, they can be hoisted into molding machines for production and removed when molding is complete or the tool needs repairing.
A mold can produce several copies of the same parts in a single "shot". The number of "impressions" in the mold of that part is often incorrectly referred to as cavitation. A tool with one impression will often be called a single cavity (impression) tool. A mold with 2 or more cavities of the same parts will likely be referred to as multiple cavity tooling. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities.
In some cases multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts are not the same but often part of a family of parts (to be used in the same product for example).
[edit] MachiningMolds are built through two main methods: standard machining and EDM machining. Standard Machining, in its conventional form, has historically been the method of building injection molds. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.
The electrical discharge machining (EDM) or spark erosion process has become widely used in mold making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes erosion of the mold surface in the inverse shape of the electrode.
[edit] CostThe cost of manufacturing molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others.
[edit] Injection process Small injection molder showing hopper, nozzle and die area[edit] Injection molding cycleFor the injection molding cycle to begin, four criteria must be met: mold open, ejector pins retracted, shot built, and carriage forward. When these criteria are met, the cycle begins with the mold closing. This is typically done as fast as possible with a slow down near the end of travel. Mold safety is low speed and low pressure mold closing. It usually begins just before the leader pins of the mold and must be set properly to prevent accidental mold damage. When the mold halves touch clamp tonnage is built. Next, molten plastic material is injected into the mold. The material travels into the mold via the sprue bushing, then the runner system delivers the material to the gate. The gate directs the material into the mold cavity to form the desired part. This injection usually occurs under velocity control. When the part is nearly full, injection control is switched from velocity control to pressure control. This is referred to as the pack/hold phase of the cycle. Pressure must be maintained on the material until the gate solidifies to prevent material from flowing back out of the cavity. Cooling time is dependent primarily on the wall thickness of the part. During the cooling portion of the cycle after the gate has solidified, plastication takes place. Plastication is the process of melting material and preparing the next shot. The material begins in the hopper and enters the barrel through the feed throat. The feed throat must be cooled to prevent plastic pellets from fusing together from the barrel heat. The barrel contains a screw that primarily uses shear to melt the pellets and consists of three sections. The first section is the feed section which conveys the pellets forward and allows barrel heat to soften the pellets. The flight depth is uniform and deepest in this section. The next section is the transition section and is responsible for melting the material through shear. The flight depth continuously decreases in this section, compressing the material. The final section is the metering section which features a shallow flight depth, improves the melt quality and color dispersion. At the front of the screw is the non-return valve which allows the screw to act as both an extruder and a plunger. When the screw is moving backwards to build a shot, the non-return assembly allows material to flow in front of the screw creating a melt pool or shot. During injection, the non-return assembly prevents the shot from flowing back into the screw sections. Once the shot has been built and the cooling time has timed out, the mold opens. Mold opening must occur slow-fast-slow. The mold must be opened slowly to release the vacuum that is caused by the injection molding process and prevent the part from staying on the stationary mold half. This is undesirable because the ejection system is on the moving mold half. Then the mold is opened as far as needed, if robots are not being used, the mold only has to open far enough for the part to be removed. A slowdown near the end of travel must be utilized to compensate for the momentum of the mold. Without slowing down the machine cannot maintain accurate positions and may slam to a stop damaging the machine. Once the mold is open, the ejector pins are moved forward, ejecting the part. When the ejector pins retract, all criteria for a molding cycle have been met and the next cycle can begin.
The basic injection cycle is as follows: Mold close – injection carriage forward – inject plastic – metering – carriage retract – mold open – eject part(s) Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels that assist in cooling the mold and the heated plastic solidifies into the part. Improper cooling can result in distorted molding. The cycle is completed when the mold opens and the part is ejected with the assistance of ejector pins within the mold.
The resin, or raw material for injection molding, is most commonly supplied in pellet or granule form. Resin pellets are poured into the feed hopper, a large open bottomed container, which is attached to the back end of a cylindrical, horizontal barrel. A screw within this barrel is rotated by a motor, feeding pellets up the screw's grooves. The depth of the screw flights decreases toward the end of the screw nearest the mold, compressing the heated plastic. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Electric heater bands attached to the outside of the barrel assist in the heating and temperature control during the melting process.
The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut or twisted off and recycled, sometimes being granulated next to the mold machine. Some molds are designed so that the part is automatically stripped through action of the mold.
[edit] Molding trialWhen filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, a technician/tool setter usually starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off has occurred, then holding pressure is increased until the parts are free of sinks and part weight has been achieved. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future.
Process optimization is done using the following methods. Injection speeds are usually determined by performing viscosity curves. Process windows are performed varying the melt temperatures and holding pressures. Pressure drop studies are done to check if the machine has enough pressure to move the screw at the set rate. Gate seal or gate freeze studies are done to optimize the holding time. A cooling time study is done to optimize the cooling time.
[edit] Molding defectsInjection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)
Molding Defects Alternative name Descriptions Causes Blister Blistering Raised or layered zone on surface of the part Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater Burn Marks Air Burn/ Gas Burn Black or brown burnt areas on the part located at furthest points from gate Tool lacks venting, injection speed is too high Colour Streaks Localized change of colour Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only Delamination Thin mica like layers formed in part wall Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond Flash Burrs Excess material in thin layer exceeding normal part geometry Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces. Embedded contaminates Embedded Particulates Foreign particle (burnt material or other) embedded in the part Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection Flow marks Directionally "off tone" wavy lines or patterns Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times) Jetting Deformed part by turbulent flow of material Poor tool design, gate position or runner. Injection speed set too high. Polymer degradation polymer breakdown from hydrolysis, oxidation etc Excess water in the granules, excessive temperatures in barrel Silver streaks Circular pattern around gate caused by hot gas Moisture in the material, usually when hygroscopic resins are dried improperly Sink Marks Localized depression (In thicker zones) Holding time/pressure too low, cooling time too low, with sprueless hot runners this can also be caused by the gate temperature being set too high Short shot Non-Fill / Short mold Partial part Lack of material, injection speed or pressure too low Splay Marks Splash mark / Silver Streaks Circular pattern around gate caused by hot gas Caused by the material (plastic) being damped prior to injection Stringiness Stringing String like remain from previous shot transfer in new shot Nozzle temperature too high. Gate hasn't frozen off Voids Empty space within part (Air pocket) Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness). Weld line Knit Line / Meld Line Discolored line where two flow fronts meet Mold/material temperatures set too low (the material is cold when they meet, so they don't bond) Warping Twisting Distorted part Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)
[edit] HistoryIn 1868 John Wesley Hyatt became the first to inject hot celluloid into a mold, producing billiard balls. He and his brother Isaiah patented an injection molding machine that used a plunger in 1872,[1] and the process remained more or less the same until 1946, when James Hendry built the first screw injection molding machine, revolutionizing the plastics industry.[2] Roughly 95% of all molding machines now use screws to efficiently heat, mix, and inject plastic into molds.
Standard two plates tooling – core and cavity are inserts in a mold base – "Family mold" of 5 different partsThe most commonly used thermoplastic materials are polystyrene (low cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a ter-polymer or mixture of compounds used for everything from Lego parts to electronics housings), polyamide (chemically resistant, heat resistant, tough and flexible – used for combs), polypropylene (tough and flexible – used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser).
Injection molding can also be used to manufacture parts from aluminium or brass (die casting). The melting points of these metals are much higher than those of plastics; this makes for substantially shorter mold lifetimes despite the use of specialized steels. Nonetheless, the costs compare quite favorably to sand casting, particularly for smaller parts.
Contents [hide]1 Equipment 1.1 Mold 1.2 Design 1.3 Machining 1.4 Cost 2 Injection process 2.1 Injection molding cycle 2.2 Molding trial 2.3 Molding defects 3 History 4 See also 5 Notes 6 References 7 External links 7.1 Associations
[edit] Equipment Paper clip mold opened in molding machine; the nozzle is visible at rightMain article: Injection molding machineInjection molding machines, also known as presses, hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can generate. This pressure keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations.
[edit] MoldMold (Tool and/or Mold) is the common term used to describe the production tooling used to produce plastic parts in molding.
Traditionally, molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copper alloy. The choice of material to build a mold is primarily one of economics. Steel molds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium molds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for molding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated. The molds can be manufactured by either CNC machining or by using Electrical Discharge Machining processes
[edit] DesignMolds separate into two sides at a parting line, the A side, and the B side, to permit the part to be extracted. Plastic resin enters the mold through a sprue in the A plate, branches out between the two sides through channels called runners, and enters each part cavity through one or more specialized gates. Inside each cavity, the resin flows around protrusions (called cores) and conforms to the cavity geometry to form the desired part. The amount of resin required to fill the sprue, runner and cavities of a mold is a shot. When a core shuts off against an opposing mold cavity or core, a hole results in the part. Air in the cavities when the mold closes escapes through very slight gaps between the plates and pins, into shallow plenums called vents. To permit removal of the part, its features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such overhangs when the mold opens. Sides of the part that appear parallel with the direction of draw (the direction in which the core and cavity separate from each other) are typically angled slightly with (draft) to ease release of the part from the mold, and examination of most plastic household objects will reveal this. Parts with bucket-like features tend to shrink onto the cores that form them while cooling, and cling to those cores when the cavity is pulled away. The mold is usually designed so that the molded part reliably remains on the ejector (B) side of the mold when it opens, and draws the runner and the sprue out of the (A) side along with the parts. The part then falls freely when ejected from the (B) side. Tunnel gates tunnel sharply below the parting surface of the B side at the tip of each runner so that the gate is sheared off of the part when both are ejected. Ejector pins are the most popular method for removing the part from the B side core(s), but air ejection, and stripper plates can also be used depending on the application. Most ejector plates are found on the moving half of the tool, but they can be placed on the fixed half if spring loaded. For thermoplastics, coolant, usually water with corrosion inhibitors, circulates through passageways bored through the main plates on both sides of the mold to enable temperature control and rapid part solidification.
To ease maintenance and venting, cavities and cores are divided into pieces, called inserts, and subassemblies, also called inserts, blocks, or chase blocks. By substituting interchangeable inserts, one mold may make several variations of the same part.
More complex parts are formed using more complex molds. These may have sections called slides, that move into a cavity perpendicular to the draw direction, to form overhanging part features. Slides are then withdrawn to allow the part to be released when the mold opens. Slides are typically guided and retained between rails called gibs, and are moved when the mold opens and closes by angled rods called horn pins and locked in place by locking blocks, both of which move cross the mold from the opposite side.
Some molds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one-piece tires and wheels.
2-shot or multi shot molds are designed to "overmold" within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This can be achieved by having pairs of identical cores and pairs of different cavities within the mold. After injection of the first material, the component is rotated on the core from the one cavity to another. The second cavity differs from the first in that the detail for the second material is included. The second material is then injected into the additional cavity detail before the completed part is ejected from the mold. Common applications include "soft-grip" toothbrushes and freelander grab handles.
The core and cavity, along with injection and cooling hoses form the mold tool. While large tools are very heavy weighing hundreds and sometimes thousands of pounds, they usually require the use of a forklift or overhead crane, they can be hoisted into molding machines for production and removed when molding is complete or the tool needs repairing.
A mold can produce several copies of the same parts in a single "shot". The number of "impressions" in the mold of that part is often incorrectly referred to as cavitation. A tool with one impression will often be called a single cavity (impression) tool. A mold with 2 or more cavities of the same parts will likely be referred to as multiple cavity tooling. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities.
In some cases multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts are not the same but often part of a family of parts (to be used in the same product for example).
[edit] MachiningMolds are built through two main methods: standard machining and EDM machining. Standard Machining, in its conventional form, has historically been the method of building injection molds. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.
The electrical discharge machining (EDM) or spark erosion process has become widely used in mold making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes erosion of the mold surface in the inverse shape of the electrode.
[edit] CostThe cost of manufacturing molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others.
[edit] Injection process Small injection molder showing hopper, nozzle and die area[edit] Injection molding cycleFor the injection molding cycle to begin, four criteria must be met: mold open, ejector pins retracted, shot built, and carriage forward. When these criteria are met, the cycle begins with the mold closing. This is typically done as fast as possible with a slow down near the end of travel. Mold safety is low speed and low pressure mold closing. It usually begins just before the leader pins of the mold and must be set properly to prevent accidental mold damage. When the mold halves touch clamp tonnage is built. Next, molten plastic material is injected into the mold. The material travels into the mold via the sprue bushing, then the runner system delivers the material to the gate. The gate directs the material into the mold cavity to form the desired part. This injection usually occurs under velocity control. When the part is nearly full, injection control is switched from velocity control to pressure control. This is referred to as the pack/hold phase of the cycle. Pressure must be maintained on the material until the gate solidifies to prevent material from flowing back out of the cavity. Cooling time is dependent primarily on the wall thickness of the part. During the cooling portion of the cycle after the gate has solidified, plastication takes place. Plastication is the process of melting material and preparing the next shot. The material begins in the hopper and enters the barrel through the feed throat. The feed throat must be cooled to prevent plastic pellets from fusing together from the barrel heat. The barrel contains a screw that primarily uses shear to melt the pellets and consists of three sections. The first section is the feed section which conveys the pellets forward and allows barrel heat to soften the pellets. The flight depth is uniform and deepest in this section. The next section is the transition section and is responsible for melting the material through shear. The flight depth continuously decreases in this section, compressing the material. The final section is the metering section which features a shallow flight depth, improves the melt quality and color dispersion. At the front of the screw is the non-return valve which allows the screw to act as both an extruder and a plunger. When the screw is moving backwards to build a shot, the non-return assembly allows material to flow in front of the screw creating a melt pool or shot. During injection, the non-return assembly prevents the shot from flowing back into the screw sections. Once the shot has been built and the cooling time has timed out, the mold opens. Mold opening must occur slow-fast-slow. The mold must be opened slowly to release the vacuum that is caused by the injection molding process and prevent the part from staying on the stationary mold half. This is undesirable because the ejection system is on the moving mold half. Then the mold is opened as far as needed, if robots are not being used, the mold only has to open far enough for the part to be removed. A slowdown near the end of travel must be utilized to compensate for the momentum of the mold. Without slowing down the machine cannot maintain accurate positions and may slam to a stop damaging the machine. Once the mold is open, the ejector pins are moved forward, ejecting the part. When the ejector pins retract, all criteria for a molding cycle have been met and the next cycle can begin.
The basic injection cycle is as follows: Mold close – injection carriage forward – inject plastic – metering – carriage retract – mold open – eject part(s) Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels that assist in cooling the mold and the heated plastic solidifies into the part. Improper cooling can result in distorted molding. The cycle is completed when the mold opens and the part is ejected with the assistance of ejector pins within the mold.
The resin, or raw material for injection molding, is most commonly supplied in pellet or granule form. Resin pellets are poured into the feed hopper, a large open bottomed container, which is attached to the back end of a cylindrical, horizontal barrel. A screw within this barrel is rotated by a motor, feeding pellets up the screw's grooves. The depth of the screw flights decreases toward the end of the screw nearest the mold, compressing the heated plastic. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Electric heater bands attached to the outside of the barrel assist in the heating and temperature control during the melting process.
The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut or twisted off and recycled, sometimes being granulated next to the mold machine. Some molds are designed so that the part is automatically stripped through action of the mold.
[edit] Molding trialWhen filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, a technician/tool setter usually starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off has occurred, then holding pressure is increased until the parts are free of sinks and part weight has been achieved. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future.
Process optimization is done using the following methods. Injection speeds are usually determined by performing viscosity curves. Process windows are performed varying the melt temperatures and holding pressures. Pressure drop studies are done to check if the machine has enough pressure to move the screw at the set rate. Gate seal or gate freeze studies are done to optimize the holding time. A cooling time study is done to optimize the cooling time.
[edit] Molding defectsInjection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)
Molding Defects Alternative name Descriptions Causes Blister Blistering Raised or layered zone on surface of the part Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater Burn Marks Air Burn/ Gas Burn Black or brown burnt areas on the part located at furthest points from gate Tool lacks venting, injection speed is too high Colour Streaks Localized change of colour Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only Delamination Thin mica like layers formed in part wall Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond Flash Burrs Excess material in thin layer exceeding normal part geometry Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces. Embedded contaminates Embedded Particulates Foreign particle (burnt material or other) embedded in the part Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection Flow marks Directionally "off tone" wavy lines or patterns Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times) Jetting Deformed part by turbulent flow of material Poor tool design, gate position or runner. Injection speed set too high. Polymer degradation polymer breakdown from hydrolysis, oxidation etc Excess water in the granules, excessive temperatures in barrel Silver streaks Circular pattern around gate caused by hot gas Moisture in the material, usually when hygroscopic resins are dried improperly Sink Marks Localized depression (In thicker zones) Holding time/pressure too low, cooling time too low, with sprueless hot runners this can also be caused by the gate temperature being set too high Short shot Non-Fill / Short mold Partial part Lack of material, injection speed or pressure too low Splay Marks Splash mark / Silver Streaks Circular pattern around gate caused by hot gas Caused by the material (plastic) being damped prior to injection Stringiness Stringing String like remain from previous shot transfer in new shot Nozzle temperature too high. Gate hasn't frozen off Voids Empty space within part (Air pocket) Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness). Weld line Knit Line / Meld Line Discolored line where two flow fronts meet Mold/material temperatures set too low (the material is cold when they meet, so they don't bond) Warping Twisting Distorted part Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)
[edit] HistoryIn 1868 John Wesley Hyatt became the first to inject hot celluloid into a mold, producing billiard balls. He and his brother Isaiah patented an injection molding machine that used a plunger in 1872,[1] and the process remained more or less the same until 1946, when James Hendry built the first screw injection molding machine, revolutionizing the plastics industry.[2] Roughly 95% of all molding machines now use screws to efficiently heat, mix, and inject plastic into molds.
Extrusion
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes is its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms finished parts with an excellent surface finish.[1]
Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.
Commonly extruded materials include metals, polymers, ceramics, and foodstuffs.
Contents [hide]1 History 2 Process 2.1 Hot extrusion 2.2 Cold extrusion 2.3 Warm extrusion 2.4 Equipment 2.5 Extrusion defects 3 Materials 3.1 Metal 3.2 Plastic 3.3 Ceramic 3.4 Food 4 Design 5 See also 6 References 7 External links
[edit] HistoryIn 1797, Joseph Bramah patented the first extrusion process for making lead pipe. It involved preheating the metal and then forcing it through a die via a hand driven plunger. The process wasn't developed until 1820 when Thomas Burr constructed the first hydraulic powered press. At this time the process was called squirting. In 1894, Alexander Dick expanded the extrusion process to copper and brass alloys.[2]
[edit] Process Extrusion of a round blank through a die.The process begins by heating the stock material. It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterwards the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.[2]
[edit] Hot extrusionHot extrusion is done at an elevated temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 250 to 12,000 tons. Pressures range from 5,000 to 100,000 psi, therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep.[1]
Hot extrusion temperature for various metals[1] Material Temperature [F° (C°)] Magnesium 650-850 Aluminium 650-900 Copper 1200-2000 Steel 2200-2400 Titanium 1300-2100 Nickel 1900-2200 Refractory alloys up to 4000
The extrusion process is generally economical when producing between several pounds and many tons, depending on the material being extruded. There is a crossover point where rolling becomes more economical. For instance, some steels becomes more economical to roll if producing more than 50,000 lb.[2]
[edit] Cold extrusionCold extrusion is done at room temperature or near room temperature. The advantages of this over hot extrusion are the lack of oxidation, higher strength due to cold working, closer tolerances, good surface finish, and fast extrusion speeds if the material is subject to hot shortness.[1]
Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel.
Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber cylinders, automotive pistons, and gear blanks.
[edit] Warm extrusionWarm extrusion is done above room temperature, but below the recrystallization temperature of the material. It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties.[3]
[edit] EquipmentThere are many different variations of extrusion equipment. They vary by four major characteristics:[1]
Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram its called "indirect extrusion". The position of the press, either vertical or horizontal. The type of drive, either hydraulic or mechanical. The type of load applied, either conventional (variable) or hydrostatic. A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (for steel alloys and titanium alloys for example), oil pressure (for aluminum), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.
There are several methods for forming internal cavities in extrusions. One way is to have the mandrel integrated into the ram. If a solid billet is used as the feed material then it must first be pierced by the mandrel before extruding through the die. A special press is used in order to control the mandrel independently from the ram.[1] Another method is using whats known as a "spider die, porthole die and bridge die". During extrusion, the metal divides and flows around the supports for the internal mandrel. (This is much like water in a river flowing around a large rock and rejoining downstream.)
Typical extrusion presses cost more than $100,000, whereas dies can cost up to $2000.
[edit] Extrusion defectsSurface cracking - When the surface of an extrusion splits. This often caused by the extrusion temperature, friction, or speed being too high. It can also happen at lower temperatures if the extruded product temporarily sticks to the die. Pipe - A flow pattern that draws the surface oxides and impurities to the center of the product. Such a pattern is often cause by high friction or cooling of the outer regions of the billet. Internal cracking - When the center of the extrusion develops cracks or voids. These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die. (A similar situation to the necked region in a tensile stress specimen.)
[edit] Materials
[edit] MetalMetal extrusion is used by industry for various purposes such as:
Copper pipe for plumbing Aluminium extrusion profiles for tracks, frames, rails, mullions, and extrusion-type heat sinking devices Steel rods or track Titanium aircraft components including seat tracks, engine rings, and other structural parts Magnesium and aluminium alloys usually have a 30 μin. RMS or better surface finish. Titanium and steel can achieve a 125 μin. RMS.[1]
In 1935,[citation needed] Ugine Sejournet, of France, invented a process which uses glass as a lubricant for extruding steel. The Ugine-Sejournet, or Sejournet, process is now used for other materials that have melting temperatures higher than steel or that require a narrow range of temperatures to extrude. The process starts by heating the materials to the extruding temperature and then rolling it in glass powder. The glass melts and forms a thin film, 20 to 30 mils (0.5 to 0.75 mm), in order to separate it from chamber walls and allow it to act as a lubricant. A thick solid glass ring that is 0.25 to 0.75 in (6 to 18 mm) thick is placed in the chamber on the die to lubricate the extrusion as it is forced through the die. A second advantage of this glass ring is its ability to insulate the heat of the billet from the die. The extrusion will have a 1 mil thick layer of glass, which can be easily removed once it cools.[3]
Another breakthrough in lubrication is the use of phosphate coatings. With this process, in conjunction with glass lubrication, steel can be cold extruded. The phosphate coat absorbs the liquid glass to offer even better lubricating properties.[3]
[edit] Plastic Sectional view of a plastic extruder showing the componentsMain article: Plastics extrusionPlastic extrusion commonly uses plastic chips or pellets, which are usually dried in a hopper before going to the feed screw. The polymer resin is heated to molten state by a combination of heating elements and shear heating from the extrusion screw. The screw forces the resin through a die, forming the resin into the desired shape. The extrudate is cooled and solidified as it is pulled through the die or water tank. In some cases (such as fibre-reinforced tubes) the extrudate is pulled through a very long die, in a process called pultrusion.
A multitude of polymers are used in the production of plastic tubing, pipes, rods, rails, seals, and sheets or films.
[edit] Ceramic Green Play-Doh with can and accessory extruder toyCeramic can also be formed into shapes via extrusion. Terracotta extrusion is used to produce pipes. Many modern bricks are also manufactured using a brick extrusion process.[4] Some Play-Doh toy products also make use of extrusion.
[edit] Food Please help improve this article or section by expanding it.Further information might be found on the talk page or at requests for expansion. (January 2008)
Extrusion has found great application in food processing. Products such as pastas, breakfast cereals, Fig Newtons, prefab cookie dough, Sevai, Idiappam, jalebi and ready-to-eat snacks are now manufactured by extrusion. Krispy Kreme doughnuts are also manufactured by extrusion to keep the doughnuts uniform in shape and size. Softer foods such as meringue have long been "piped" using pastry bags. Extrusion is also used with grains such as wheat, corn, and rice. In feed industry it is used for process with floating and slow sinking feed.
[edit] DesignThe following guidelines should be followed to produce a quality extrusion. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section (called the circumscribing circle). This diameter, in turn, controls the size of the die required, which ultimately determines if the part will fit in a given press. For example, a larger press can handle 25 inch diameter circumscribing circles for aluminium and 22 in. diameter circles for steel and titanium.[1]
Thicker sections generally need an increased section size. In order for the material to flow properly legs should not be more than ten times longer than their thickness. If the cross-section is asymmetrical, adjacent sections should be as close to the same size as possible. Sharp corners should be avoided; for aluminium and magnesium the minimum radius should be 1/64 in. and for steel corners should be 0.030 in. and fillets should be 0.125 in. The following table lists the minimum cross-section and thickness for various materials.[1]
Material Minimum cross-section [sq. in.] Minimum thickness [in.] Carbon steels 0.40 0.120 Stainless steel 0.45-0.70 0.120-0.187 Titanium 0.50 0.150 Aluminium <0.40 0.040 Magnesium <0.40 0.040
Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.
Commonly extruded materials include metals, polymers, ceramics, and foodstuffs.
Contents [hide]1 History 2 Process 2.1 Hot extrusion 2.2 Cold extrusion 2.3 Warm extrusion 2.4 Equipment 2.5 Extrusion defects 3 Materials 3.1 Metal 3.2 Plastic 3.3 Ceramic 3.4 Food 4 Design 5 See also 6 References 7 External links
[edit] HistoryIn 1797, Joseph Bramah patented the first extrusion process for making lead pipe. It involved preheating the metal and then forcing it through a die via a hand driven plunger. The process wasn't developed until 1820 when Thomas Burr constructed the first hydraulic powered press. At this time the process was called squirting. In 1894, Alexander Dick expanded the extrusion process to copper and brass alloys.[2]
[edit] Process Extrusion of a round blank through a die.The process begins by heating the stock material. It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterwards the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.[2]
[edit] Hot extrusionHot extrusion is done at an elevated temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 250 to 12,000 tons. Pressures range from 5,000 to 100,000 psi, therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep.[1]
Hot extrusion temperature for various metals[1] Material Temperature [F° (C°)] Magnesium 650-850 Aluminium 650-900 Copper 1200-2000 Steel 2200-2400 Titanium 1300-2100 Nickel 1900-2200 Refractory alloys up to 4000
The extrusion process is generally economical when producing between several pounds and many tons, depending on the material being extruded. There is a crossover point where rolling becomes more economical. For instance, some steels becomes more economical to roll if producing more than 50,000 lb.[2]
[edit] Cold extrusionCold extrusion is done at room temperature or near room temperature. The advantages of this over hot extrusion are the lack of oxidation, higher strength due to cold working, closer tolerances, good surface finish, and fast extrusion speeds if the material is subject to hot shortness.[1]
Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel.
Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber cylinders, automotive pistons, and gear blanks.
[edit] Warm extrusionWarm extrusion is done above room temperature, but below the recrystallization temperature of the material. It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties.[3]
[edit] EquipmentThere are many different variations of extrusion equipment. They vary by four major characteristics:[1]
Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram its called "indirect extrusion". The position of the press, either vertical or horizontal. The type of drive, either hydraulic or mechanical. The type of load applied, either conventional (variable) or hydrostatic. A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (for steel alloys and titanium alloys for example), oil pressure (for aluminum), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.
There are several methods for forming internal cavities in extrusions. One way is to have the mandrel integrated into the ram. If a solid billet is used as the feed material then it must first be pierced by the mandrel before extruding through the die. A special press is used in order to control the mandrel independently from the ram.[1] Another method is using whats known as a "spider die, porthole die and bridge die". During extrusion, the metal divides and flows around the supports for the internal mandrel. (This is much like water in a river flowing around a large rock and rejoining downstream.)
Typical extrusion presses cost more than $100,000, whereas dies can cost up to $2000.
[edit] Extrusion defectsSurface cracking - When the surface of an extrusion splits. This often caused by the extrusion temperature, friction, or speed being too high. It can also happen at lower temperatures if the extruded product temporarily sticks to the die. Pipe - A flow pattern that draws the surface oxides and impurities to the center of the product. Such a pattern is often cause by high friction or cooling of the outer regions of the billet. Internal cracking - When the center of the extrusion develops cracks or voids. These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die. (A similar situation to the necked region in a tensile stress specimen.)
[edit] Materials
[edit] MetalMetal extrusion is used by industry for various purposes such as:
Copper pipe for plumbing Aluminium extrusion profiles for tracks, frames, rails, mullions, and extrusion-type heat sinking devices Steel rods or track Titanium aircraft components including seat tracks, engine rings, and other structural parts Magnesium and aluminium alloys usually have a 30 μin. RMS or better surface finish. Titanium and steel can achieve a 125 μin. RMS.[1]
In 1935,[citation needed] Ugine Sejournet, of France, invented a process which uses glass as a lubricant for extruding steel. The Ugine-Sejournet, or Sejournet, process is now used for other materials that have melting temperatures higher than steel or that require a narrow range of temperatures to extrude. The process starts by heating the materials to the extruding temperature and then rolling it in glass powder. The glass melts and forms a thin film, 20 to 30 mils (0.5 to 0.75 mm), in order to separate it from chamber walls and allow it to act as a lubricant. A thick solid glass ring that is 0.25 to 0.75 in (6 to 18 mm) thick is placed in the chamber on the die to lubricate the extrusion as it is forced through the die. A second advantage of this glass ring is its ability to insulate the heat of the billet from the die. The extrusion will have a 1 mil thick layer of glass, which can be easily removed once it cools.[3]
Another breakthrough in lubrication is the use of phosphate coatings. With this process, in conjunction with glass lubrication, steel can be cold extruded. The phosphate coat absorbs the liquid glass to offer even better lubricating properties.[3]
[edit] Plastic Sectional view of a plastic extruder showing the componentsMain article: Plastics extrusionPlastic extrusion commonly uses plastic chips or pellets, which are usually dried in a hopper before going to the feed screw. The polymer resin is heated to molten state by a combination of heating elements and shear heating from the extrusion screw. The screw forces the resin through a die, forming the resin into the desired shape. The extrudate is cooled and solidified as it is pulled through the die or water tank. In some cases (such as fibre-reinforced tubes) the extrudate is pulled through a very long die, in a process called pultrusion.
A multitude of polymers are used in the production of plastic tubing, pipes, rods, rails, seals, and sheets or films.
[edit] Ceramic Green Play-Doh with can and accessory extruder toyCeramic can also be formed into shapes via extrusion. Terracotta extrusion is used to produce pipes. Many modern bricks are also manufactured using a brick extrusion process.[4] Some Play-Doh toy products also make use of extrusion.
[edit] Food Please help improve this article or section by expanding it.Further information might be found on the talk page or at requests for expansion. (January 2008)
Extrusion has found great application in food processing. Products such as pastas, breakfast cereals, Fig Newtons, prefab cookie dough, Sevai, Idiappam, jalebi and ready-to-eat snacks are now manufactured by extrusion. Krispy Kreme doughnuts are also manufactured by extrusion to keep the doughnuts uniform in shape and size. Softer foods such as meringue have long been "piped" using pastry bags. Extrusion is also used with grains such as wheat, corn, and rice. In feed industry it is used for process with floating and slow sinking feed.
[edit] DesignThe following guidelines should be followed to produce a quality extrusion. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section (called the circumscribing circle). This diameter, in turn, controls the size of the die required, which ultimately determines if the part will fit in a given press. For example, a larger press can handle 25 inch diameter circumscribing circles for aluminium and 22 in. diameter circles for steel and titanium.[1]
Thicker sections generally need an increased section size. In order for the material to flow properly legs should not be more than ten times longer than their thickness. If the cross-section is asymmetrical, adjacent sections should be as close to the same size as possible. Sharp corners should be avoided; for aluminium and magnesium the minimum radius should be 1/64 in. and for steel corners should be 0.030 in. and fillets should be 0.125 in. The following table lists the minimum cross-section and thickness for various materials.[1]
Material Minimum cross-section [sq. in.] Minimum thickness [in.] Carbon steels 0.40 0.120 Stainless steel 0.45-0.70 0.120-0.187 Titanium 0.50 0.150 Aluminium <0.40 0.040 Magnesium <0.40 0.040
Forging
Forging is the term for shaping metal by using localized compressive forces. Cold forging is done at room temperature or near room temperature. Hot forging is done at a high temperature, which makes metal easier to shape and less likely to fracture. Warm forging is done at intermediate temperature between room temperature and hot forging temperatures. Forged parts can range in weight from less than a kilogram to 170 metric tons. Forged parts usually require further processing to achieve a finished part.
Contents [hide]1 History 2 Advantages and disadvantages 2.1 Hot forging 2.2 Cold forging 3 Processes 3.1 Open-die drop-hammer forging 3.2 Impression-die drop-hammer forging 3.2.1 Design of impression-die forgings and tooling 3.3 Press forging 3.4 Upset forging 3.5 Automatic hot forging 3.6 Roll forging 3.7 Net-shape and near-net-shape forging 4 Equipment 5 References
[edit] HistoryForging is one of the oldest known metalworking processes.
Forging was done historically by a smith using hammer and anvil, and though the use of water power in the production and working of iron dates to the 12th century CE, the hammer and anvil are not obsolete. The smithy has evolved over centuries to the forge shop with engineered processes, production equipment, tooling, raw materials and products to meet the demands of modern industry.
In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam. These hammers are large, having reciprocating weights in the thousands of pounds. Smaller power hammers, 500 pounds or less reciprocating weight, and hydraulic presses are common in art smithies as well. Steam hammers are becoming obsolete.
[edit] Advantages and disadvantagesForging results in metal that is stronger than cast or machined metal parts. This stems from the grain flow caused through forging. As the metal is pounded the grains deform to follow the shape of the part, thus the grains are unbroken throughout the part. Some modern parts take advantage of this for a high strength-to-weight ratio.
Many metals are forged cold, but iron and its alloys are almost always forged hot. This is for two reasons: first, if work hardening were allowed to progress, hard materials such as iron and steel would become extremely difficult to work with; secondly, steel can be strengthened by other means than cold-working, thus it is more economical to hot forge and then heat treat. Alloys that are amenable to precipitation hardening, such as most alloys of aluminium and titanium, can also be hot forged then hardened. Other materials must be strengthened by the forging process itself.
[edit] Hot forgingHot forging is defined as working a metal above its recrystallization temperature. The main advantage of hot forging is that as the metal is deformed the strain-hardening effects are negated by the recrystallization process. Other advantages include:
Decrease in yield strength, therefore it is easier to work and takes less energy (force) Increase in ductility Elevated temperatures increase diffusion which can remove or reduce chemical inhomogeneities Pores may reduce in size or close completely during deformation In steel, the weak, ductile, FCC (face-centered-cubic) austenite is deformed instead of the strong BCC (body-centered-cubic) ferrite at lower temperatures The disadvantages of hot forging are:
Undesirable reactions between the metal and the surrounding atmosphere Less precise tolerances due to thermal contraction and warping from uneven cooling Grain structure may vary throughout the metal due to many various reasons
[edit] Cold forgingMain article: Cold formingCold forging is defined as working a metal below its recrystallization temperature, but usually around room temperature.
Advantages:
No heating required Better surface finish Superior dimensional control Better reproducibility and interchangeability Directional properties can be imparted into the metal Contamination problems are minimized Disadvantages:
Higher forces are required Heavier and more powerful equipment and stronger tooling are required Metal is less ductile Metal surfaces must be clean and scale-free Intermediate anneals may be required to compensate for loss of ductility that accompanies strain hardening The imparted directional properties may be detrimental Undesirable residual stress may be produced
[edit] Processes Scan of sectioned, forged connecting rod that has been etched to show grain flow.There are many different kinds of forging processes available, however they can be grouped into three main classes:
Drawn out: length increases, cross-section decreases Upset: Length decreases, cross-section increases Squeezed in closed compression dies: produces multidirectional flow Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.
[edit] Open-die drop-hammer forgingOpen-die forging is also known as smith forging. In open-die forging a hammer comes down and deforms the workpieces, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the working surfaces of the forge that contract the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. Therefore the operator needs to orient and position the workpiece to get the desired shape. The dies are usually flat in shape but may have a specially shaped surface for specialized operations; for instance the die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool.
Open-die forging lends itself to short runs and is appropriate for art smithing and custom work. Other times open-die forging is used to rough shape ingots to prepare it for further operations. This can also orient the grains to increase strength in the required direction.
[edit] Impression-die drop-hammer forgingImpression-die forging is also called closed-die forging. In impression-die work metal is placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. Depending on the size and complexity of the part the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities; this is called flash. The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging the flash is trimmed off.
In commercial impression-die forging the workpiece is usually moved though a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called a edging, fullering, or bending impression. The following cavities are called blocking cavities in which the workpiece is working into a shape that more and more resembles the final product. These stages usually impart the workpiece with generous bends and large fillets. The final shape is forged in a final or finisher impression cavity. If there is only a short run of parts to be done it may be more economical for the die to lack a final impression cavity and rather machine the final features.
Impression-die forging has been further improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging.
One variation of impression-die forging is called flashless forging, or true closed-die forging. In this type of forging the die cavities are completely closed, which keeps the workpiece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process included: additional cost due to a more complex die design, the need for better lubrication, and better workpiece placement.
There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging preform from liquid metal. The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed and then quenched to room temperature to harden the part. Another variation follows the same process as outlined above, except the preform is produced by the spraying deposition of metal droplet into shaped collectors (similar to the Osprey Process).
Closed-die forging has a high initial cost due to the creation of dies and required design work to make working die cavities. However, it has low recurring costs for each part, thus forgings become more economical with more volume. This is one of the major reasons forgings are often used in the automotive and tool industry. Another reason forgings are common in these industrial sectors is because forgings generally have about a 20% higher strength to weight ratio compared to cast or machined parts of the same material.
[edit] Design of impression-die forgings and toolingForging dies are usually made of high-alloy or tool steel. Dies must be impact resistant, wear resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling. In order to produce a better, more economical die the following rules should be followed:
The dies should part along a single, flat plane if at all possible, If not the parting plan should follow the contour of the part. The parting surface should be a plane through the center of the forging and not near an upper or lower edge. Adequate draft should be provided; a good guideline is at least 3° for aluminum and 5° to 7° for steel Generous fillets and radii should be used Ribs should be low and wide The various sections should be balanced to avoid extreme difference in metal flow Full advantage should be taken of fiver flow lines Dimensional tolerances should not be closer than necessary The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. It should be noted that the dimensions across the paring plane are affected by the closure of the dies, and are therefore dependent die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.
Mass [kg (lbs)] Minus tolerance [mm (in.)] Plus tolerance [mm (in.)] 0.45 (1) 0.15 (0.006) 0.48 (0.018) 0.91 (2) 0.20 (0.008) 0.61 (0.024) 2.27 (5) 0.25 (0.010) 0.76 (0.030) 4.54 (10) 0.28 (0.011) 0.84 (0.033) 9.07 (20) 0.33 (0.013) 0.99 (0.039) 22.68 (50) 0.48 (0.019) 1.45 (0.057) 45.36 (100) 0.74 (0.029) 2.21 (0.087)
A lubricant is always used when forging to reduce friction and wear. It is also used to as a thermal barrier to restrict heat transfer from the workpiece to the die. Finally the lubricant acts as a parting compound to prevent the part from sticking in one of the dies.
[edit] Press forgingPress forging is an operation characterized by the process of deformation which consists of a lot of heating and cooling. During the process, the material is slowly condensed into a shape by increasing pressure. There are two dies; one stationary and one pushed towards the other, which compresses the part.
Press forging is variation of drop-hammer forging. Unlike drop-hammer forging, press forges work slowly by applying continuous pressure or force. The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot. Cold press forging is done on cold annealed steel and hot press forging is done on a large armored plate.
The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete workpiece. Drop-hammer forging usually only deforms the surfaces of the workpiece in contact with the hammer and anvil; the interior of the workpiece will stay relatively undeformed. Another advantage to the process includes the knowledge of the new parts strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled. There are a few disadvantages to this process, most stemming from the workpiece being in contact with the dies for such an extended period of time. The operation is a time consuming process due to the amount of steps and how long each of them take. The workpiece will cool faster because the dies are in contact with workpiece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the workpiece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The workpiece may also need to be reheated. When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in press forging, the greater percentage of work is used in the work piece. Another advantage is that the operation can be used to create any size part because there is no limit to the size of the press forging machine. New press forging techniques have been able to create a higher degree of mechanical and orientation integrity. By the constraint of oxidation to the outer most layers of the part material, reduced levels of microcracking take place in the finished part.
Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation.
[edit] Upset forgingUpset forging increases the diameter of the workpiece by compressing its length.
Based on number of pieces produced this is the most widely used forging process.
Upset forging is usually done in special high speed machines. The machines are usually set up to work in the horizontal plane, to facilitate the quick exchange of workpieces from one station to the next. The initial workpiece is usually wire or rod, but some machines can accept bars up to 25 cm (10 in.) in diameter. The standard upsetting machine employs split dies that contain multiple cavities. The dies open enough to allow the workpiece to move from one cavity to the next; the dies then close and the heading tool, or ram, then moves longitudinally against the bar, upsetting it into the cavity. If all of the cavities are utilized on every cycle then a finished part will be produced with every cycle, which is why this process is ideal for mass production.
A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.
The following three rules must be followed when designing parts to be upset forged:
The length of unsupported metal that can be upset in one blow without injurious buckling should be limited to three times the diameter of the bar. Lengths of stock greater than three times the diameter may be upset successfully provided that the diameter of the upset is not more than 1.5 times the diameter of the stock. In an upset requiring stock length greater than three times the diameter of the stock, and where the diameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupported metal beyond the face of the die must not exceed the diameter of the bar.
[edit] Automatic hot forgingThe automatic hot forging process involves feeding mill-length steel bars (typically 7 m or 24 ft long) into one end of the machine at room temperature and hot forged products emerge from the other end. This all occurs very quickly; small parts can be made at a rate of 180 parts per minute (ppm) and larger can be made at a rate of 90 ppm. The parts can be solid or hollow, round or symmetrical, up to 6 kg (12 lbs), and up to 18 cm (7 in.) in diameter. The main advantages to this process are its high output rate and ability to accept low cost materials. Little labor is required to operate the machinery. There is no flash produced so material savings are between 20 - 30% over conventional forging. The final product is a consistent 1050 °C (1900 °F) so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging). Tolerances are usually ±0.3 mm (±0.012 in.), surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly double that of conventional forging because contact times are on the order of 6/100 of a second. The downside to the process is it only feasible on smaller symmetric parts and cost; the initial investment can be over $10 million, so large quantities are required to justify this process.
The process starts by heating up the bar to 1200 to 1300 °C (2200 to 2350 °F) in less than 60 seconds using high power induction coils. It is then descaled with rollers, sheared into blanks, and transferred several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary). This process can also be couple with high speed cold forming operations. Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.
Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for LP gas cylinders.[1] Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.[2]
[edit] Roll forgingRoll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves. A heated bar is inserted into the rolls and when it hits a stop the rolls rotate and the bar is progressively shaped as it is rolled out of the machine. The work piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves. This continues until the desired shape and size is achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the workpiece.
Examples of products produced using this method include axles, tapered levers and leaf springs.
[edit] Net-shape and near-net-shape forgingThis process is also known as precision forging. This process was developed to minimize cost and waste associated with post forging operations. Therefore the final product from a precision forging needs little to no final machining. Cost savings are gained from the use of less material, and thus less scrap, the overall decrease in energy used, and the reduction or elimination of machining. Precision forging also requires less or a draft, 1° to 0°. The downsize of this process is its cost, therefore it is only implemented if significant cost reduction can be achieved.
[edit] Equipment Hydraulic drop-hammer (a) Material flow of a conventionally forged disc; (b) Material flow of a impactor forged disc.The most common thought of forging equipment is the hammer and anvil. The principles behind the hammer and anvil are still used today in drop-hammer equipment. The principle behind the machine is very simple, raise the hammer and then drop it or propel it into the workpiece, which rests on the anvil. The main variations between drop-hammers is in the way that the hammer is powered; the most common being air and steam hammers. Drop-hammers usually operate in the vertical position. The main reason for this is because excess energy (energy that isn't used to deform the workpiece) that isn't released as heat or sound needs to be transmitted to the foundation. Moreover, a large machine base is needed to absorb the impacts.
To overcome some of the shortcomings of the drop-hammer the counterblow machine or impactor is used. In a counterblow machine both the hammer and anvil move and the workpiece is held between them. Here excess energy becomes recoil. This allows for the machine to work horizontally and consist of a smaller base. Other advantages include less noise, heat and vibrations. It also produces a distinctly different flow pattern. Both of these machines can be used for open die or closed die forging.
A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system difference forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 tons). Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press is its flexibility and greater capacity. The disadvantages are that it is slower, larger, and more costly to operate.
The roll forging, upsetting, and automatic hot forging processes all use specialized machinery.
Contents [hide]1 History 2 Advantages and disadvantages 2.1 Hot forging 2.2 Cold forging 3 Processes 3.1 Open-die drop-hammer forging 3.2 Impression-die drop-hammer forging 3.2.1 Design of impression-die forgings and tooling 3.3 Press forging 3.4 Upset forging 3.5 Automatic hot forging 3.6 Roll forging 3.7 Net-shape and near-net-shape forging 4 Equipment 5 References
[edit] HistoryForging is one of the oldest known metalworking processes.
Forging was done historically by a smith using hammer and anvil, and though the use of water power in the production and working of iron dates to the 12th century CE, the hammer and anvil are not obsolete. The smithy has evolved over centuries to the forge shop with engineered processes, production equipment, tooling, raw materials and products to meet the demands of modern industry.
In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam. These hammers are large, having reciprocating weights in the thousands of pounds. Smaller power hammers, 500 pounds or less reciprocating weight, and hydraulic presses are common in art smithies as well. Steam hammers are becoming obsolete.
[edit] Advantages and disadvantagesForging results in metal that is stronger than cast or machined metal parts. This stems from the grain flow caused through forging. As the metal is pounded the grains deform to follow the shape of the part, thus the grains are unbroken throughout the part. Some modern parts take advantage of this for a high strength-to-weight ratio.
Many metals are forged cold, but iron and its alloys are almost always forged hot. This is for two reasons: first, if work hardening were allowed to progress, hard materials such as iron and steel would become extremely difficult to work with; secondly, steel can be strengthened by other means than cold-working, thus it is more economical to hot forge and then heat treat. Alloys that are amenable to precipitation hardening, such as most alloys of aluminium and titanium, can also be hot forged then hardened. Other materials must be strengthened by the forging process itself.
[edit] Hot forgingHot forging is defined as working a metal above its recrystallization temperature. The main advantage of hot forging is that as the metal is deformed the strain-hardening effects are negated by the recrystallization process. Other advantages include:
Decrease in yield strength, therefore it is easier to work and takes less energy (force) Increase in ductility Elevated temperatures increase diffusion which can remove or reduce chemical inhomogeneities Pores may reduce in size or close completely during deformation In steel, the weak, ductile, FCC (face-centered-cubic) austenite is deformed instead of the strong BCC (body-centered-cubic) ferrite at lower temperatures The disadvantages of hot forging are:
Undesirable reactions between the metal and the surrounding atmosphere Less precise tolerances due to thermal contraction and warping from uneven cooling Grain structure may vary throughout the metal due to many various reasons
[edit] Cold forgingMain article: Cold formingCold forging is defined as working a metal below its recrystallization temperature, but usually around room temperature.
Advantages:
No heating required Better surface finish Superior dimensional control Better reproducibility and interchangeability Directional properties can be imparted into the metal Contamination problems are minimized Disadvantages:
Higher forces are required Heavier and more powerful equipment and stronger tooling are required Metal is less ductile Metal surfaces must be clean and scale-free Intermediate anneals may be required to compensate for loss of ductility that accompanies strain hardening The imparted directional properties may be detrimental Undesirable residual stress may be produced
[edit] Processes Scan of sectioned, forged connecting rod that has been etched to show grain flow.There are many different kinds of forging processes available, however they can be grouped into three main classes:
Drawn out: length increases, cross-section decreases Upset: Length decreases, cross-section increases Squeezed in closed compression dies: produces multidirectional flow Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.
[edit] Open-die drop-hammer forgingOpen-die forging is also known as smith forging. In open-die forging a hammer comes down and deforms the workpieces, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the working surfaces of the forge that contract the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. Therefore the operator needs to orient and position the workpiece to get the desired shape. The dies are usually flat in shape but may have a specially shaped surface for specialized operations; for instance the die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool.
Open-die forging lends itself to short runs and is appropriate for art smithing and custom work. Other times open-die forging is used to rough shape ingots to prepare it for further operations. This can also orient the grains to increase strength in the required direction.
[edit] Impression-die drop-hammer forgingImpression-die forging is also called closed-die forging. In impression-die work metal is placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. Depending on the size and complexity of the part the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities; this is called flash. The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging the flash is trimmed off.
In commercial impression-die forging the workpiece is usually moved though a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called a edging, fullering, or bending impression. The following cavities are called blocking cavities in which the workpiece is working into a shape that more and more resembles the final product. These stages usually impart the workpiece with generous bends and large fillets. The final shape is forged in a final or finisher impression cavity. If there is only a short run of parts to be done it may be more economical for the die to lack a final impression cavity and rather machine the final features.
Impression-die forging has been further improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging.
One variation of impression-die forging is called flashless forging, or true closed-die forging. In this type of forging the die cavities are completely closed, which keeps the workpiece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process included: additional cost due to a more complex die design, the need for better lubrication, and better workpiece placement.
There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging preform from liquid metal. The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed and then quenched to room temperature to harden the part. Another variation follows the same process as outlined above, except the preform is produced by the spraying deposition of metal droplet into shaped collectors (similar to the Osprey Process).
Closed-die forging has a high initial cost due to the creation of dies and required design work to make working die cavities. However, it has low recurring costs for each part, thus forgings become more economical with more volume. This is one of the major reasons forgings are often used in the automotive and tool industry. Another reason forgings are common in these industrial sectors is because forgings generally have about a 20% higher strength to weight ratio compared to cast or machined parts of the same material.
[edit] Design of impression-die forgings and toolingForging dies are usually made of high-alloy or tool steel. Dies must be impact resistant, wear resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling. In order to produce a better, more economical die the following rules should be followed:
The dies should part along a single, flat plane if at all possible, If not the parting plan should follow the contour of the part. The parting surface should be a plane through the center of the forging and not near an upper or lower edge. Adequate draft should be provided; a good guideline is at least 3° for aluminum and 5° to 7° for steel Generous fillets and radii should be used Ribs should be low and wide The various sections should be balanced to avoid extreme difference in metal flow Full advantage should be taken of fiver flow lines Dimensional tolerances should not be closer than necessary The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. It should be noted that the dimensions across the paring plane are affected by the closure of the dies, and are therefore dependent die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.
Mass [kg (lbs)] Minus tolerance [mm (in.)] Plus tolerance [mm (in.)] 0.45 (1) 0.15 (0.006) 0.48 (0.018) 0.91 (2) 0.20 (0.008) 0.61 (0.024) 2.27 (5) 0.25 (0.010) 0.76 (0.030) 4.54 (10) 0.28 (0.011) 0.84 (0.033) 9.07 (20) 0.33 (0.013) 0.99 (0.039) 22.68 (50) 0.48 (0.019) 1.45 (0.057) 45.36 (100) 0.74 (0.029) 2.21 (0.087)
A lubricant is always used when forging to reduce friction and wear. It is also used to as a thermal barrier to restrict heat transfer from the workpiece to the die. Finally the lubricant acts as a parting compound to prevent the part from sticking in one of the dies.
[edit] Press forgingPress forging is an operation characterized by the process of deformation which consists of a lot of heating and cooling. During the process, the material is slowly condensed into a shape by increasing pressure. There are two dies; one stationary and one pushed towards the other, which compresses the part.
Press forging is variation of drop-hammer forging. Unlike drop-hammer forging, press forges work slowly by applying continuous pressure or force. The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot. Cold press forging is done on cold annealed steel and hot press forging is done on a large armored plate.
The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete workpiece. Drop-hammer forging usually only deforms the surfaces of the workpiece in contact with the hammer and anvil; the interior of the workpiece will stay relatively undeformed. Another advantage to the process includes the knowledge of the new parts strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled. There are a few disadvantages to this process, most stemming from the workpiece being in contact with the dies for such an extended period of time. The operation is a time consuming process due to the amount of steps and how long each of them take. The workpiece will cool faster because the dies are in contact with workpiece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the workpiece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The workpiece may also need to be reheated. When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in press forging, the greater percentage of work is used in the work piece. Another advantage is that the operation can be used to create any size part because there is no limit to the size of the press forging machine. New press forging techniques have been able to create a higher degree of mechanical and orientation integrity. By the constraint of oxidation to the outer most layers of the part material, reduced levels of microcracking take place in the finished part.
Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation.
[edit] Upset forgingUpset forging increases the diameter of the workpiece by compressing its length.
Based on number of pieces produced this is the most widely used forging process.
Upset forging is usually done in special high speed machines. The machines are usually set up to work in the horizontal plane, to facilitate the quick exchange of workpieces from one station to the next. The initial workpiece is usually wire or rod, but some machines can accept bars up to 25 cm (10 in.) in diameter. The standard upsetting machine employs split dies that contain multiple cavities. The dies open enough to allow the workpiece to move from one cavity to the next; the dies then close and the heading tool, or ram, then moves longitudinally against the bar, upsetting it into the cavity. If all of the cavities are utilized on every cycle then a finished part will be produced with every cycle, which is why this process is ideal for mass production.
A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.
The following three rules must be followed when designing parts to be upset forged:
The length of unsupported metal that can be upset in one blow without injurious buckling should be limited to three times the diameter of the bar. Lengths of stock greater than three times the diameter may be upset successfully provided that the diameter of the upset is not more than 1.5 times the diameter of the stock. In an upset requiring stock length greater than three times the diameter of the stock, and where the diameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupported metal beyond the face of the die must not exceed the diameter of the bar.
[edit] Automatic hot forgingThe automatic hot forging process involves feeding mill-length steel bars (typically 7 m or 24 ft long) into one end of the machine at room temperature and hot forged products emerge from the other end. This all occurs very quickly; small parts can be made at a rate of 180 parts per minute (ppm) and larger can be made at a rate of 90 ppm. The parts can be solid or hollow, round or symmetrical, up to 6 kg (12 lbs), and up to 18 cm (7 in.) in diameter. The main advantages to this process are its high output rate and ability to accept low cost materials. Little labor is required to operate the machinery. There is no flash produced so material savings are between 20 - 30% over conventional forging. The final product is a consistent 1050 °C (1900 °F) so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging). Tolerances are usually ±0.3 mm (±0.012 in.), surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly double that of conventional forging because contact times are on the order of 6/100 of a second. The downside to the process is it only feasible on smaller symmetric parts and cost; the initial investment can be over $10 million, so large quantities are required to justify this process.
The process starts by heating up the bar to 1200 to 1300 °C (2200 to 2350 °F) in less than 60 seconds using high power induction coils. It is then descaled with rollers, sheared into blanks, and transferred several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary). This process can also be couple with high speed cold forming operations. Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.
Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for LP gas cylinders.[1] Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.[2]
[edit] Roll forgingRoll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves. A heated bar is inserted into the rolls and when it hits a stop the rolls rotate and the bar is progressively shaped as it is rolled out of the machine. The work piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves. This continues until the desired shape and size is achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the workpiece.
Examples of products produced using this method include axles, tapered levers and leaf springs.
[edit] Net-shape and near-net-shape forgingThis process is also known as precision forging. This process was developed to minimize cost and waste associated with post forging operations. Therefore the final product from a precision forging needs little to no final machining. Cost savings are gained from the use of less material, and thus less scrap, the overall decrease in energy used, and the reduction or elimination of machining. Precision forging also requires less or a draft, 1° to 0°. The downsize of this process is its cost, therefore it is only implemented if significant cost reduction can be achieved.
[edit] Equipment Hydraulic drop-hammer (a) Material flow of a conventionally forged disc; (b) Material flow of a impactor forged disc.The most common thought of forging equipment is the hammer and anvil. The principles behind the hammer and anvil are still used today in drop-hammer equipment. The principle behind the machine is very simple, raise the hammer and then drop it or propel it into the workpiece, which rests on the anvil. The main variations between drop-hammers is in the way that the hammer is powered; the most common being air and steam hammers. Drop-hammers usually operate in the vertical position. The main reason for this is because excess energy (energy that isn't used to deform the workpiece) that isn't released as heat or sound needs to be transmitted to the foundation. Moreover, a large machine base is needed to absorb the impacts.
To overcome some of the shortcomings of the drop-hammer the counterblow machine or impactor is used. In a counterblow machine both the hammer and anvil move and the workpiece is held between them. Here excess energy becomes recoil. This allows for the machine to work horizontally and consist of a smaller base. Other advantages include less noise, heat and vibrations. It also produces a distinctly different flow pattern. Both of these machines can be used for open die or closed die forging.
A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system difference forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 tons). Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press is its flexibility and greater capacity. The disadvantages are that it is slower, larger, and more costly to operate.
The roll forging, upsetting, and automatic hot forging processes all use specialized machinery.
Powder metallurgy
Powder metallurgy is a forming and fabrication technique consisting of three major processing stages. First, the primary material is physically powdered, divided into many small individual particles. Next, the powder is injected into a mold or passed through a die to produce a weakly cohesive structure (via cold welding) very near the dimensions of the object ultimately to be manufactured. Finally, the end part is formed by applying pressure, high temperature, long setting times (during which self-welding occurs), or any combination thereof.
Two main techniques used to form and consolidate the powder are Sintering and Metal Injection Molding.
Contents [hide]1 History and capabilities 1.1 Powder metallurgy in space-based manufacturing 2 Powder production techniques 2.1 Atomization 2.2 Centrifugal disintegration 2.3 Other techniques 2.4 Powder production in space-based manufacturing 3 Powder pressing 4 Sintering 5 Continuous powder processing 6 Special products 7 References 8 External resources
[edit] History and capabilitiesThe history of powder metallurgy and the art of metals and ceramics sintering are intimately related. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. There is evidence that iron powders were fused into hard objects as early as 1200 B.C. In these early manufacturing operations, iron was extracted by hand from metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.
A much wider range of products can be obtained from powder processes than from direct alloying of fused materials. In melting operations the "phase rule" applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminium/iron powders poses major problems. Other substances that are especially reactive with atmospheric oxygen, such as tin, are sinterable in special atmospheres or with temporary coatings.
In powder metallurgy or ceramics it is possible to fabricate components which otherwise would decompose or disintegrate. All considerations of solid-liquid phase changes can be ignored, so powder processes are more flexible than casting, extrusion, or forging techniques. Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic, and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds. Competitive characteristics of manufacturing processing (e.g., tool wear, complexity, or vendor options) also may be closely regulated.
Powder Metallurgy products are today used in a wide range of industries, from automotive and aerospace applications to power tools and household appliances. Each year the international PM awards highlight the developing capabilities of the technology.[1]
[edit] Powder metallurgy in space-based manufacturingPowder metallurgy in the microgravity and vacuum conditions of orbit or on the Moon offer several potential advantages over similar applications on Earth. For example, due to the absence of atmosphere (and therefore, the elimination of undesirable reactivity with atmospheric gases), cold-welding effects will be far more pronounced and dependable due to the absence of surface coatings. Gravitational settling in polydiameter powder mixtures can largely be avoided, permitting the use of broader ranges of grain sizes in the initial compact and producing correspondingly lower porosities. Finally, it should be possible to selectively coat particles with special films which artificially inhibit contact welding until the powder mixture is properly shaped. (The film is then removed by low heat or by chemical means, forming the powder in microgravity conditions without a mold.)
[edit] Powder production techniquesAny fusible material can be atomized. Several techniques have been developed which permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. Powders may be prepared by comminution, grinding, chemical reactions, or electrolytic deposition. Several of the melting and mechanical procedures are clearly adaptable to operations in space or on the Moon.
Powders of the elements Ti, V, Th, Nb, Ta, Ca, and U have been produced by high-temperature reduction of the corresponding nitrides and carbides. Fe, Ni, U, and Be submicrometre powders are obtained by reducing metallic oxalates and formates. Exceedingly fine particles also have been prepared by directing a stream of molten metal through a high-temperature plasma jet or flame, simultaneously atomizing and comminuting the material. On Earth various chemical- and flame-associated powdering processes are adopted in part to prevent serious degradation of particle surfaces by atmospheric oxygen.
[edit] AtomizationAtomization is accomplished by forcing a molten metal stream through an orifice at moderate pressures. A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. On Earth, air and powder streams are segregated using gravity or cyclonic separation. Most atomized powders are annealed, which helps reduce the oxide and carbon content. The water atomized particles are smaller, cleaner, and nonporous and have a greater breadth of size, which allows better compacting.
Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance index used is the Reynolds number R = fvd/n, where f = fluid density, v = velocity of the exit stream, d = diameter of the opening, and n = absolute viscosity. At low R the liquid jet oscillates, but at higher velocities the stream becomes turbulent and breaks into droplets. Pumping energy is applied to droplet formation with very low efficiency (on the order of 1%) and control over the size distribution of the metal particles produced is rather poor. Other techniques such as nozzle vibration, nozzle asymmetry, multiple impinging streams, or molten-metal injection into ambient gas are all available to increase atomization efficiency, produce finer grains, and to narrow the particle size distribution. Unfortunately, it is difficult to eject metals through orifices smaller than a few millimeters in diameter, which in practice limits the minimum size of powder grains to approximately 10 μm. Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and remelting a significant fraction of the grain boundary.
[edit] Centrifugal disintegrationCentrifugal disintegration of molten particles offers one way around these problems. Extensive experience is available with iron, steel, and aluminium. Metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle. Opposite the spindle tip is an electrode from which an arc is established which heats the metal rod. As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls. A circulating gas sweeps particles from the chamber. Similar techniques could be employed in space or on the Moon. The chamber wall could be rotated to force new powders into remote collection vessels (DeCarmo, 1979), and the electrode could be replaced by a solar mirror focused at the end of the rod.
An alternative approach capable of producing a very narrow distribution of grain sizes but with low throughput consists of a rapidly spinning bowl heated to well above the melting point of the material to be powdered. Liquid metal, introduced onto the surface of the basin near the center at flow rates adjusted to permit a thin metal film to skim evenly up the walls and over the edge, breaks into droplets, each approximately the thickness of the film.
[edit] Other techniquesAnother powder-production technique involves a thin jet of liquid metal intersected by high-speed streams of atomized water which break the jet into drops and cool the powder before it reaches the bottom of the bin. In subsequent operations the powder is dried. This is called water atomisation. The advantage is that metal solidifies faster than by gas atomisation since thermal conductivity of water is some magnitudes higher. The solidification rate is inversely proportional to the particle size. As a consequence, one can obtain smaller particles by water atomisation. The smaller the particles, the more homogeneous the microstructure will be. Notice that particles will have a more irregular shape and the particle size distribution will be wider. In addition, some surface contamination can occur by oxidation skin formation. Powder can be reduced by some kind of preconsolidation treatment as annealing.
Finally, mills are now available which can impart enormous rotational torques on powders, on the order of 2.0×107 rpm. Such forces cause grains to disintegrate into yet finer particles.
[edit] Powder production in space-based manufacturingPowders prepared in the vacuum of space will largely avoid this problem, and the availability of zero-g may suggest alternative techniques for the production of spherical or unusually shaped grains.
Two powdering techniques which appear especially applicable to space manufacturing are atomization and centrifugal disintegration. Direct Solar energy can be used to melt the working materials, so the most energy-intensive portion of the operation requires a minimum of capital equipment mass per unit of output rate since low-mass solar collectors can be employed either on the Moon or in space. The two major energy input stages - powder manufacturing and sintering - require 19 MJ/kg and 17 MJ/kg, respectively. At a mean energy cost of $0.007/MJ, this corresponds to about $0.13/kg. Major savings might be possible in space using solar energy.
[edit] Powder pressingAlthough many products such as pills and tablets for medical use are cold-pressed directly from powdered materials, normally the resulting compact is only strong enough to allow subsequent heating and sintering. Release of the compact from its mold is usually accompanied by small volume increase called "spring-back."
In the typical powder pressing process a powder compaction press is employed with tools and dies. Normally, a die cavity that is closed on one end (vertical die, bottom end closed by a punch tool) is filled with powder. The powder is then compacted into a shape and then ejected from the die cavity. Various components can be formed with the powder compaction process. Some examples of these parts are bearings, bushings, gears, pistons, levers, and brackets. When pressing these shapes, very good dimensional and weight control are maintained. In a number of these applications the parts may require very little additional work for their intended use; making for very cost efficient manufacturing.
In some pressing operations (such as hot isostatic pressing) compact formation and sintering occur simultaneously. This procedure, together with explosion-driven compressive techniques, is used extensively in the production of high-temperature and high-strength parts such as turbine blades for jet engines. In most applications of powder metallurgy the compact is hot-pressed, heated to a temperature above which the materials cannot remain work-hardened. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. Also it permits better dimensional control of the product, lessened sensitivity to physical characteristics of starting materials, and allows powder to be driven to higher densities than with cold pressing, resulting in higher strength. Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres during forming and cooling stages.
[edit] SinteringMain article: sinteringSolid State Sintering is the process of taking metal in the form of a powder and placing it into a mold or die. Once compacted into the mold the material is placed under a high heat for a long period of time. Under heat bonding takes place between the porous aggregate particles takes and once cooled the powder has bonded to form a solid piece.
Sintering can be considered to proceed in three stages. During the first, neck growth proceeds rapidly but powder particles remain discrete. During the second, most densification occurs, the structure recrystallizes and particles diffuse into each other. During the third, isolated pores tend to become spheroidal and densification continues at a much lower rate. The words Solid State in Solid State Sintering simply refer to the state the material is in when it bonds, solid meaning the material was not turned molten to bond together as alloys are formed.[2]
One recently developed technique for high-speed sintering involves passing high electrical current through a powder to preferentially heat the asperities. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders.
[edit] Continuous powder processingThe phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however, the term refers to processes whose products are much longer in one physical dimension than in the other two. Compression, rolling, and extrusion are the most common examples.
In a simple compression process, powder flows from a bin onto a two-walled channel and is repeatedly compressed vertically by a horizontally stationary punch. After stripping the compress from the conveyor the compact is introduced into a sintering furnace. An even easier approach is to spray powder onto a moving belt and sinter it without compression. Good methods for stripping cold-pressed materials from moving belts are hard to find. One alternative that avoids the belt-stripping difficulty altogether is the manufacture of metal sheets using opposed hydraulic rams, although weakness lines across the sheet may arise during successive press operations.
Powders can also be rolled to produce sheets. The powdered metal is fed into a two-high rolling mill and is compacted into strip at up to 100 feet per minute. [3] The strip is then sintered and subjected to another rolling and sintering.[4] Rolling is commonly used to produce sheet metal for electrical and electronic components as well as coins. [5]Considerable work also has been done on rolling multiple layers of different materials simultaneously into sheets.
Extrusion processes are of two general types. In one type, the powder is mixed with a binder or plasticizer at room temperature; in the other, the powder is extruded at elevated temperatures without fortification. Extrusions with binders are used extensively in the preparation of tungsten-carbide composites. Tubes, complex sections, and spiral drill shapes are manufactured in extended lengths and diameters varying from 0.05-30 cm. Hard metal wires 0.01 cm diam have been drawn from powder stock. At the opposite extreme, large extrusions on a tonnage basis may be feasible.
There appears to be no limitation to the variety of metals and alloys that can be extruded, provided the temperatures and pressures involved are within the capabilities of die materials. Extrusion lengths may range from 3 to 30 m and diameters from 0.2 to 1 m. Modern presses are largely automatic and operate at high speeds (on the order of m/s).
Extrusion Temperatures Of Common Metals And Alloys Metals and alloys Temperature of extrusion, K Aluminium and alloys 673-773 Magnesium and alloys 573-673 Copper 1073-1153 Brasses 923-1123 Nickel brasses 1023-1173 Cupro-nickel 1173-1273 Nickel 1383-1433 Monel 1373-1403 Inconel 1443-1473 Steels 1323-1523
[edit] Special productsMany special products are possible with powder metallurgy technology. A nonexhaustive list includes Al2O3 whiskers coated with very thin oxide layers for improved refractories; iron compacts with Al2O3 coatings for improved high-temperature creep strength; light bulb filaments made with powder technology; linings for friction brakes; metal glasses for high-strength films and ribbons; heat shields for spacecraft reentry into Earth's atmosphere; electrical contacts for handling large current flows; magnets; microwave ferrites; filters for gases; and bearings which can be infiltrated with lubricants.
Extremely thin films and tiny spheres exhibit high strength. One application of this observation is to coat brittle materials in whisker form with a submicrometre film of much softer metal (e.g., cobalt-coated tungsten). The surface strain of the thin layer places the harder metal under compression, so that when the entire composite is sintered the rupture strength increases markedly. With this method, strengths on the order of 2.8 GPa versus 550 MPa have been observed for, respectively, coated (25% Co) and uncoated tungsten carbides. It is interesting to consider whether similarly strong materials could be manufactured from aluminium films stretched thin over glass fibers (materials relatively abundant in space).
Two main techniques used to form and consolidate the powder are Sintering and Metal Injection Molding.
Contents [hide]1 History and capabilities 1.1 Powder metallurgy in space-based manufacturing 2 Powder production techniques 2.1 Atomization 2.2 Centrifugal disintegration 2.3 Other techniques 2.4 Powder production in space-based manufacturing 3 Powder pressing 4 Sintering 5 Continuous powder processing 6 Special products 7 References 8 External resources
[edit] History and capabilitiesThe history of powder metallurgy and the art of metals and ceramics sintering are intimately related. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. There is evidence that iron powders were fused into hard objects as early as 1200 B.C. In these early manufacturing operations, iron was extracted by hand from metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.
A much wider range of products can be obtained from powder processes than from direct alloying of fused materials. In melting operations the "phase rule" applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminium/iron powders poses major problems. Other substances that are especially reactive with atmospheric oxygen, such as tin, are sinterable in special atmospheres or with temporary coatings.
In powder metallurgy or ceramics it is possible to fabricate components which otherwise would decompose or disintegrate. All considerations of solid-liquid phase changes can be ignored, so powder processes are more flexible than casting, extrusion, or forging techniques. Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic, and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds. Competitive characteristics of manufacturing processing (e.g., tool wear, complexity, or vendor options) also may be closely regulated.
Powder Metallurgy products are today used in a wide range of industries, from automotive and aerospace applications to power tools and household appliances. Each year the international PM awards highlight the developing capabilities of the technology.[1]
[edit] Powder metallurgy in space-based manufacturingPowder metallurgy in the microgravity and vacuum conditions of orbit or on the Moon offer several potential advantages over similar applications on Earth. For example, due to the absence of atmosphere (and therefore, the elimination of undesirable reactivity with atmospheric gases), cold-welding effects will be far more pronounced and dependable due to the absence of surface coatings. Gravitational settling in polydiameter powder mixtures can largely be avoided, permitting the use of broader ranges of grain sizes in the initial compact and producing correspondingly lower porosities. Finally, it should be possible to selectively coat particles with special films which artificially inhibit contact welding until the powder mixture is properly shaped. (The film is then removed by low heat or by chemical means, forming the powder in microgravity conditions without a mold.)
[edit] Powder production techniquesAny fusible material can be atomized. Several techniques have been developed which permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. Powders may be prepared by comminution, grinding, chemical reactions, or electrolytic deposition. Several of the melting and mechanical procedures are clearly adaptable to operations in space or on the Moon.
Powders of the elements Ti, V, Th, Nb, Ta, Ca, and U have been produced by high-temperature reduction of the corresponding nitrides and carbides. Fe, Ni, U, and Be submicrometre powders are obtained by reducing metallic oxalates and formates. Exceedingly fine particles also have been prepared by directing a stream of molten metal through a high-temperature plasma jet or flame, simultaneously atomizing and comminuting the material. On Earth various chemical- and flame-associated powdering processes are adopted in part to prevent serious degradation of particle surfaces by atmospheric oxygen.
[edit] AtomizationAtomization is accomplished by forcing a molten metal stream through an orifice at moderate pressures. A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. On Earth, air and powder streams are segregated using gravity or cyclonic separation. Most atomized powders are annealed, which helps reduce the oxide and carbon content. The water atomized particles are smaller, cleaner, and nonporous and have a greater breadth of size, which allows better compacting.
Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance index used is the Reynolds number R = fvd/n, where f = fluid density, v = velocity of the exit stream, d = diameter of the opening, and n = absolute viscosity. At low R the liquid jet oscillates, but at higher velocities the stream becomes turbulent and breaks into droplets. Pumping energy is applied to droplet formation with very low efficiency (on the order of 1%) and control over the size distribution of the metal particles produced is rather poor. Other techniques such as nozzle vibration, nozzle asymmetry, multiple impinging streams, or molten-metal injection into ambient gas are all available to increase atomization efficiency, produce finer grains, and to narrow the particle size distribution. Unfortunately, it is difficult to eject metals through orifices smaller than a few millimeters in diameter, which in practice limits the minimum size of powder grains to approximately 10 μm. Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and remelting a significant fraction of the grain boundary.
[edit] Centrifugal disintegrationCentrifugal disintegration of molten particles offers one way around these problems. Extensive experience is available with iron, steel, and aluminium. Metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle. Opposite the spindle tip is an electrode from which an arc is established which heats the metal rod. As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls. A circulating gas sweeps particles from the chamber. Similar techniques could be employed in space or on the Moon. The chamber wall could be rotated to force new powders into remote collection vessels (DeCarmo, 1979), and the electrode could be replaced by a solar mirror focused at the end of the rod.
An alternative approach capable of producing a very narrow distribution of grain sizes but with low throughput consists of a rapidly spinning bowl heated to well above the melting point of the material to be powdered. Liquid metal, introduced onto the surface of the basin near the center at flow rates adjusted to permit a thin metal film to skim evenly up the walls and over the edge, breaks into droplets, each approximately the thickness of the film.
[edit] Other techniquesAnother powder-production technique involves a thin jet of liquid metal intersected by high-speed streams of atomized water which break the jet into drops and cool the powder before it reaches the bottom of the bin. In subsequent operations the powder is dried. This is called water atomisation. The advantage is that metal solidifies faster than by gas atomisation since thermal conductivity of water is some magnitudes higher. The solidification rate is inversely proportional to the particle size. As a consequence, one can obtain smaller particles by water atomisation. The smaller the particles, the more homogeneous the microstructure will be. Notice that particles will have a more irregular shape and the particle size distribution will be wider. In addition, some surface contamination can occur by oxidation skin formation. Powder can be reduced by some kind of preconsolidation treatment as annealing.
Finally, mills are now available which can impart enormous rotational torques on powders, on the order of 2.0×107 rpm. Such forces cause grains to disintegrate into yet finer particles.
[edit] Powder production in space-based manufacturingPowders prepared in the vacuum of space will largely avoid this problem, and the availability of zero-g may suggest alternative techniques for the production of spherical or unusually shaped grains.
Two powdering techniques which appear especially applicable to space manufacturing are atomization and centrifugal disintegration. Direct Solar energy can be used to melt the working materials, so the most energy-intensive portion of the operation requires a minimum of capital equipment mass per unit of output rate since low-mass solar collectors can be employed either on the Moon or in space. The two major energy input stages - powder manufacturing and sintering - require 19 MJ/kg and 17 MJ/kg, respectively. At a mean energy cost of $0.007/MJ, this corresponds to about $0.13/kg. Major savings might be possible in space using solar energy.
[edit] Powder pressingAlthough many products such as pills and tablets for medical use are cold-pressed directly from powdered materials, normally the resulting compact is only strong enough to allow subsequent heating and sintering. Release of the compact from its mold is usually accompanied by small volume increase called "spring-back."
In the typical powder pressing process a powder compaction press is employed with tools and dies. Normally, a die cavity that is closed on one end (vertical die, bottom end closed by a punch tool) is filled with powder. The powder is then compacted into a shape and then ejected from the die cavity. Various components can be formed with the powder compaction process. Some examples of these parts are bearings, bushings, gears, pistons, levers, and brackets. When pressing these shapes, very good dimensional and weight control are maintained. In a number of these applications the parts may require very little additional work for their intended use; making for very cost efficient manufacturing.
In some pressing operations (such as hot isostatic pressing) compact formation and sintering occur simultaneously. This procedure, together with explosion-driven compressive techniques, is used extensively in the production of high-temperature and high-strength parts such as turbine blades for jet engines. In most applications of powder metallurgy the compact is hot-pressed, heated to a temperature above which the materials cannot remain work-hardened. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. Also it permits better dimensional control of the product, lessened sensitivity to physical characteristics of starting materials, and allows powder to be driven to higher densities than with cold pressing, resulting in higher strength. Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres during forming and cooling stages.
[edit] SinteringMain article: sinteringSolid State Sintering is the process of taking metal in the form of a powder and placing it into a mold or die. Once compacted into the mold the material is placed under a high heat for a long period of time. Under heat bonding takes place between the porous aggregate particles takes and once cooled the powder has bonded to form a solid piece.
Sintering can be considered to proceed in three stages. During the first, neck growth proceeds rapidly but powder particles remain discrete. During the second, most densification occurs, the structure recrystallizes and particles diffuse into each other. During the third, isolated pores tend to become spheroidal and densification continues at a much lower rate. The words Solid State in Solid State Sintering simply refer to the state the material is in when it bonds, solid meaning the material was not turned molten to bond together as alloys are formed.[2]
One recently developed technique for high-speed sintering involves passing high electrical current through a powder to preferentially heat the asperities. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders.
[edit] Continuous powder processingThe phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however, the term refers to processes whose products are much longer in one physical dimension than in the other two. Compression, rolling, and extrusion are the most common examples.
In a simple compression process, powder flows from a bin onto a two-walled channel and is repeatedly compressed vertically by a horizontally stationary punch. After stripping the compress from the conveyor the compact is introduced into a sintering furnace. An even easier approach is to spray powder onto a moving belt and sinter it without compression. Good methods for stripping cold-pressed materials from moving belts are hard to find. One alternative that avoids the belt-stripping difficulty altogether is the manufacture of metal sheets using opposed hydraulic rams, although weakness lines across the sheet may arise during successive press operations.
Powders can also be rolled to produce sheets. The powdered metal is fed into a two-high rolling mill and is compacted into strip at up to 100 feet per minute. [3] The strip is then sintered and subjected to another rolling and sintering.[4] Rolling is commonly used to produce sheet metal for electrical and electronic components as well as coins. [5]Considerable work also has been done on rolling multiple layers of different materials simultaneously into sheets.
Extrusion processes are of two general types. In one type, the powder is mixed with a binder or plasticizer at room temperature; in the other, the powder is extruded at elevated temperatures without fortification. Extrusions with binders are used extensively in the preparation of tungsten-carbide composites. Tubes, complex sections, and spiral drill shapes are manufactured in extended lengths and diameters varying from 0.05-30 cm. Hard metal wires 0.01 cm diam have been drawn from powder stock. At the opposite extreme, large extrusions on a tonnage basis may be feasible.
There appears to be no limitation to the variety of metals and alloys that can be extruded, provided the temperatures and pressures involved are within the capabilities of die materials. Extrusion lengths may range from 3 to 30 m and diameters from 0.2 to 1 m. Modern presses are largely automatic and operate at high speeds (on the order of m/s).
Extrusion Temperatures Of Common Metals And Alloys Metals and alloys Temperature of extrusion, K Aluminium and alloys 673-773 Magnesium and alloys 573-673 Copper 1073-1153 Brasses 923-1123 Nickel brasses 1023-1173 Cupro-nickel 1173-1273 Nickel 1383-1433 Monel 1373-1403 Inconel 1443-1473 Steels 1323-1523
[edit] Special productsMany special products are possible with powder metallurgy technology. A nonexhaustive list includes Al2O3 whiskers coated with very thin oxide layers for improved refractories; iron compacts with Al2O3 coatings for improved high-temperature creep strength; light bulb filaments made with powder technology; linings for friction brakes; metal glasses for high-strength films and ribbons; heat shields for spacecraft reentry into Earth's atmosphere; electrical contacts for handling large current flows; magnets; microwave ferrites; filters for gases; and bearings which can be infiltrated with lubricants.
Extremely thin films and tiny spheres exhibit high strength. One application of this observation is to coat brittle materials in whisker form with a submicrometre film of much softer metal (e.g., cobalt-coated tungsten). The surface strain of the thin layer places the harder metal under compression, so that when the entire composite is sintered the rupture strength increases markedly. With this method, strengths on the order of 2.8 GPa versus 550 MPa have been observed for, respectively, coated (25% Co) and uncoated tungsten carbides. It is interesting to consider whether similarly strong materials could be manufactured from aluminium films stretched thin over glass fibers (materials relatively abundant in space).
CNC machining
The abbreviation CNC stands for computer numerical control, and refers specifically to a computer "controller" that reads G-code instructions and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via a software load program.
Contents [hide]1 Historical overview 2 Production environment 3 Types of instruction 3.1 Movements 3.2 Drilling 3.3 Parametric programming 4 Tools with CNC variants 5 See also 6 External links 7 Footnotes
[edit] Historical overviewCNC was preceded by NC (Numerically Controlled) machines, which were hard wired and their operating parameters could not be changed. NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing.
Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades after 1950, until it was eventually superseded by RS232 cables, floppy disks, and now is commonly tied directly into plant networks. The files containing the G-codes to be interpreted by the controller are usually saved under the .NC extension. Most shops have their own saving format that matches their ISO certification requirements.
The introduction of CNC machines radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action have been dramatically reduced.
With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality have been achieved with no strain on the operator. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required to change the machine to produce different components.
[edit] Production environmentA series of CNC machines may be combined into one station, commonly called a "cell", to progressively machine a part requiring several operations. CNC machines today are controlled directly from files created by CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the manufactured component. In a sense, the CNC machines represent a special segment of industrial robot systems, as they are programmable to perform many kinds of machining operations (within their designed physical limits, like other robotic systems). CNC machines can run over night and over weekends without operator intervention. Error detection features have been developed, giving CNC machines the ability to call the operator's mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts it is already loaded with up to that tool and wait for the operator. The ever changing intelligence of CNC controllers has dramatically increased job shop cell production. Some machines might even make 1000 parts on a weekend with no operator, checking each part with lasers and sensors.
[edit] Types of instructionMain article: G-codeA line in a G-code file can instruct the machine tool to do one of several things.
[edit] MovementsLately, some controllers have implemented the ability to follow an arbitrary curve (NURBS), but these efforts have been met with skepticism since, unlike circular arcs, their definitions are not natural and are too complicated to set up by hand, and CAM software can already generate any motion using many short linear segments.
[edit] DrillingA tool can be used to drill holes by pecking to let the swarf out. Using an internal thread cutting tool and the ability to control the exact rotational position of the tool with the depth of cut, it can be used to cut screw threads.
A drilling cycle is used to repeat drilling or tapping operations on a workpiece. The drilling cycle accepts a list of parameters about the operation, such as depth and feed rate. To begin drilling any number of holes to the specifications configured in the cycle, the only input required is a set of coordinates for hole location. The cycle takes care of depth, feed rate, retraction, and other parameters that appear in more complex cycles. After the holes are completed, the machine is given another command to cancel the cycle, and resumes operation.
[edit] Parametric programmingA more recent advancement in CNC interpreters is support of logical commands, known as parametric programming. Parametric programs incorporate both G-code and these logical constructs to create a programming language and syntax similar to BASIC. Various manufacturers refer to parametric programming in brand-specific ways. For instance, Haas Automation refers to parametric programs as macros. GE Fanuc refers to it as Custom Macro A & B, while Okuma refers to it as User Task 2. The programmer can make if/then/else statements, loops, subprogram calls, perform various arithmetic, and manipulate variables to create a large degree of freedom within one program. An entire product line of different sizes can be programmed using logic and simple math to create and scale an entire range of parts, or create a stock part that can be scaled to any size a customer demands.
Parametric programming also enables custom machining cycles, such as fixture creation and bolt circles. If a user wishes to create additional fixture locations on a work holding device, the machine can be manually guided to the new location and the fixture subroutine called. The machine will then drill and form the patterns required to mount additional vises or clamps at that location. Parametric programs are also used to shorten long programs with incremental or stepped passes. A loop can be created with variables for step values and other parameters, and in doing so remove a large amount of repetition in the program body.
Because of these features, a parametric program is more efficient than using CAD/CAM software for large part runs. The brevity of the program allows the CNC programmer to rapidly make performance adjustments to looped commands, and tailor the program to the machine it is running on. Tool wear, breakage, and other system parameters can be accessed and changed directly in the program, allowing extensions and modifications to the functionality of a machine beyond what a manufacturer envisioned.
There are three types of variables used in CNC systems: local variable, common variable, and system variable. Local variable is used to hold data after machine off preset value. Common variable is used to hold data if machine switch off does not erase form data. The System variable this variable used system parameter this cannot use direct to convert the common variable for example tool radius, tool length, and tool height to be measured in millimeters or inches.
Contents [hide]1 Historical overview 2 Production environment 3 Types of instruction 3.1 Movements 3.2 Drilling 3.3 Parametric programming 4 Tools with CNC variants 5 See also 6 External links 7 Footnotes
[edit] Historical overviewCNC was preceded by NC (Numerically Controlled) machines, which were hard wired and their operating parameters could not be changed. NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing.
Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades after 1950, until it was eventually superseded by RS232 cables, floppy disks, and now is commonly tied directly into plant networks. The files containing the G-codes to be interpreted by the controller are usually saved under the .NC extension. Most shops have their own saving format that matches their ISO certification requirements.
The introduction of CNC machines radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action have been dramatically reduced.
With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality have been achieved with no strain on the operator. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required to change the machine to produce different components.
[edit] Production environmentA series of CNC machines may be combined into one station, commonly called a "cell", to progressively machine a part requiring several operations. CNC machines today are controlled directly from files created by CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the manufactured component. In a sense, the CNC machines represent a special segment of industrial robot systems, as they are programmable to perform many kinds of machining operations (within their designed physical limits, like other robotic systems). CNC machines can run over night and over weekends without operator intervention. Error detection features have been developed, giving CNC machines the ability to call the operator's mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts it is already loaded with up to that tool and wait for the operator. The ever changing intelligence of CNC controllers has dramatically increased job shop cell production. Some machines might even make 1000 parts on a weekend with no operator, checking each part with lasers and sensors.
[edit] Types of instructionMain article: G-codeA line in a G-code file can instruct the machine tool to do one of several things.
[edit] MovementsLately, some controllers have implemented the ability to follow an arbitrary curve (NURBS), but these efforts have been met with skepticism since, unlike circular arcs, their definitions are not natural and are too complicated to set up by hand, and CAM software can already generate any motion using many short linear segments.
[edit] DrillingA tool can be used to drill holes by pecking to let the swarf out. Using an internal thread cutting tool and the ability to control the exact rotational position of the tool with the depth of cut, it can be used to cut screw threads.
A drilling cycle is used to repeat drilling or tapping operations on a workpiece. The drilling cycle accepts a list of parameters about the operation, such as depth and feed rate. To begin drilling any number of holes to the specifications configured in the cycle, the only input required is a set of coordinates for hole location. The cycle takes care of depth, feed rate, retraction, and other parameters that appear in more complex cycles. After the holes are completed, the machine is given another command to cancel the cycle, and resumes operation.
[edit] Parametric programmingA more recent advancement in CNC interpreters is support of logical commands, known as parametric programming. Parametric programs incorporate both G-code and these logical constructs to create a programming language and syntax similar to BASIC. Various manufacturers refer to parametric programming in brand-specific ways. For instance, Haas Automation refers to parametric programs as macros. GE Fanuc refers to it as Custom Macro A & B, while Okuma refers to it as User Task 2. The programmer can make if/then/else statements, loops, subprogram calls, perform various arithmetic, and manipulate variables to create a large degree of freedom within one program. An entire product line of different sizes can be programmed using logic and simple math to create and scale an entire range of parts, or create a stock part that can be scaled to any size a customer demands.
Parametric programming also enables custom machining cycles, such as fixture creation and bolt circles. If a user wishes to create additional fixture locations on a work holding device, the machine can be manually guided to the new location and the fixture subroutine called. The machine will then drill and form the patterns required to mount additional vises or clamps at that location. Parametric programs are also used to shorten long programs with incremental or stepped passes. A loop can be created with variables for step values and other parameters, and in doing so remove a large amount of repetition in the program body.
Because of these features, a parametric program is more efficient than using CAD/CAM software for large part runs. The brevity of the program allows the CNC programmer to rapidly make performance adjustments to looped commands, and tailor the program to the machine it is running on. Tool wear, breakage, and other system parameters can be accessed and changed directly in the program, allowing extensions and modifications to the functionality of a machine beyond what a manufacturer envisioned.
There are three types of variables used in CNC systems: local variable, common variable, and system variable. Local variable is used to hold data after machine off preset value. Common variable is used to hold data if machine switch off does not erase form data. The System variable this variable used system parameter this cannot use direct to convert the common variable for example tool radius, tool length, and tool height to be measured in millimeters or inches.
Metalwork cutting tool
A cutting tool, in the context of metalworking is any tool that is used to remove metal from the workpiece by means of shear deformation. In order to last, cutting tools must be made of a material harder than the material which is to be cut, and they must be able to withstand the heat generated in the metal cutting process. They also must have a specific geometry, designed so that the cutting edge can contact the workpiece without the rest of the tool dragging on its surface. The angle of the cutting face is also important.
[edit] List of types of cutting toolsThis list may not be exhaustive.
Broaches Milling cutters Endmills Reamers Drill bits Tool bits (used in lathes, flycutters, shapers, or planers) Countersinks Counterbores Files Burrs Slitting saws
[edit] List of commonly used materialsThis list may not be exhaustive.
Tool steel High speed steel Carbides Tantalum carbide Titanium carbide Tungsten carbide Titanium nitride Ceramics
[edit] List of types of cutting toolsThis list may not be exhaustive.
Broaches Milling cutters Endmills Reamers Drill bits Tool bits (used in lathes, flycutters, shapers, or planers) Countersinks Counterbores Files Burrs Slitting saws
[edit] List of commonly used materialsThis list may not be exhaustive.
Tool steel High speed steel Carbides Tantalum carbide Titanium carbide Tungsten carbide Titanium nitride Ceramics
Drilling
This article is about making holes in solid materials. For drilling in the earth, see Borehole. For data processing usage, see data drilling. For agricultural usage, see seed drill. For firearm type, see combination gun.Drilling is the process of using a drill bit in a drill to produce cylindrical holes in solid materials, such as wood or metal. Different tools and methods are used for drilling depending on the type of material, the size of the hole, the number of holes, and the time to complete the operation.
Contents [hide]1 Drilling in metal 2 Drilling in wood 3 Microdrilling 4 Drilling as a Manufacturing Process 5 References 6 See also
[edit] Drilling in metalUnder normal usage, swarf is carried up and away from the tip of the drill bit by the fluting of the drill bit. The continued production of chips from the cutting edges produces more chips which continue the movement of the chips outwards from the hole. This continues until the chips pack too tightly, either because of deeper than normal holes or insufficient backing off (removing the drill slightly or totally from the hole while drilling). Lubricants and coolants (i.e. cutting fluid) are sometimes used to ease this problem and to prolong the tools life by cooling and lubricating the tip and chip flow. Coolant is introduced via holes through the drill shank (see gun drill).
Straight fluting is used for copper or brass, as this exhibits less tendency to "dig in" or grab the material. If a helical drill (twist drill) is used then the same effect can be achieved by stoning a small flat parallel with the axis of the drill bit.
For heavy feeds and comparatively deep holes oil-hole drills can be used, with a lubricant pumped to the drill head through a small hole in the bit and flowing out along the fluting. A conventional drill press arrangement can be used in oil-hole drilling, but it is more commonly seen in automatic drilling machinery in which it is the workpiece that rotates rather than the drill bit.
[edit] Drilling in woodWood being softer than most metals, drilling in wood is considerably easier and faster than drilling in metal. Cutting fluids are not used or needed. The main issue in drilling wood is assuring clean entry and exit holes and preventing burning. Avoiding burning is a question of using sharp bits and the appropriate cutting speed. Drill bits can tear out chips of wood around the top and bottom of the hole and this is undesirable in fine woodworking applications.
The ubiquitous twist drill bits used in metalworking also work well in wood, but they tend to chip wood out at the entry and exit of the hole. In some cases, as in rough holes for carpentry, the quality of the hole does not matter, and a number of bits for fast cutting in wood exist, including spade bits and self-feeding auger bits. Many types of specialised drill bits for boring clean holes in wood have been developed, including brad-point bits, Forstner bits and hole saws. Chipping on exit can be minimized by using a piece of wood as backing behind the work piece, and the same technique is sometimes used to keep the hole entry neat.
Holes are easier to start in wood as the drill bit can be accurately positioned by pushing it into the wood and creating a dimple. The bit will thus have little tendency to wander. In metal working, an accurate position needs to be marked with a punch to avoid the bit wandering from the desired position of the hole.
[edit] MicrodrillingMicrodrilling refers to the drilling of holes less than 0.5 mm. Drilling of holes at this small diameter presents greater problems since coolant fed drills cannot be used and high spindle speeds are required.
[edit] Drilling as a Manufacturing ProcessOPERATION DEFINITION Hole making is one of the most important machining operations in the manufacturing process. Holes serve a variety of functions including but not limited to: fasteners for assembly, weight reduction, ventilation, access to other parts, or simply for aesthetics. Hole making or drilling is used in the production of almost any part conceivable and those that aren't drilled are made with machines that have been drilled.
HOLE MAKING OPERATIONS On most workpieces it is vitally important that the hole be drilled precisely in reference to the x, y, z-axes. When possible drilled holes should be located perpendicular to the workpiece surface. This is due to the large length-to-diameter ratio which causes the drill bit to be easily deflected which can cause the hole to be misplaced, or the drill bit to break or fatigue. Because there are so many types of production operations that involve making a variety of holes in countless different materials, there are many methods for hole making.
CONSIDERATION FOR DRILLING Because drilling can often be such a critical process there are a number of considerations that should be taken in order to ensure the most accurate drill hole possible.
As mentioned before the hole and drill motion should be perpendicular to the surface of the workpiece to reduce the tendency to fatigue or break the drill bit. This also helps to reduce 'walking' of the drill bit over the workpiece surface.
• 'Walk' is common when drilling small diameter holes. It is advantageous to create a centering mark or feature during the casting or forging process. Creating a centering dimple with a centering punch will also reduce the tendency to 'walk'.
• The bottoms of the hole should match the standard drill point angles. Avoid flat bottom hole or odd shapes.
• Create through holes instead of blind holes when possible.
• If a blind hole must be drilled and tapped, it should be drilled deeper than the tapped depth.
• Holes that need to be reamed must also be initially drilled deeper than the reamed hole depth.
• A part should be designed such that it won't need to be repositioned or manually moved during the drilling process. This also reduces production time and overall cost.
• Drill speed should be another consideration. Some materials like plastics as well as other non-metals and some metals have a tendency to heat up enough to expand making the hole smaller than desired.
TWIST DRILL The most common type of drill is a standard-point twist drill. This type of drill is versatile and can be used on a variety of materials such as wood, plastic, masonry, ceramic, and metal. These drill bits have two spiral grooves running the length of the drill. These grooves aid in transporting cutting fluid to the drill tip and in removing the chips from the hole. These types of drill bits are held in chucks or collets on machines that are either hand-held or automated. This type of drilling can often cause burrs at both the entrance and the exit of the hole and parts will often need a subsequent deburring operation to smooth out the holes.
GUN DRILLING Another type of drilling operation is called gun drilling. This method was originally developed to drill out gun barrels and is used commonly for drilling smaller diameter deep holes. This depth-to-diameter ratio can be even more than 300:1. The key feature of gun drilling is that the bits are self-centering; this is what allows for such deep accurate holes. The bits use a rotary motion similar to a twist drill however; the bits are designed with bearing pads that slide along the surface of the hole keeping the drill bit on center. Gun drilling is usually done at high speeds and low feed rates.
PUNCHING AND TREPANNING These operations involve drilling but there are other methods such as punching and trepanning which don't necessarily use common drill bits. Punching essentially works just like a paper punch, it uses a punch that pushes the material through a die. The punch and die set can be in almost any shape. Punching can be cost effective because labor costs are low, however, the equipment costs can be high making punching most cost effective for high production parts that don't require high tolerances.
Trepanning is commonly used for creating larger diameter holes (up to 250mm or 10in) where a standard drill bit is not feasible or economical. Trepanning removes the desired diameter by cutting out a solid disk similar to the workings of a drafting compass. Trepanning is performed on flat products such as sheet metal, plates, or structural members like I-beams. Trepanning can also be useful to make grooves for inserting seals like O-rings.
GENERAL RECOMMENDATION FOR SPEEDS AND FEEDS IN DRILLING [1]
Workpiece Material Surface Speed (m/min, ft/min) Feed, mm/rev (in/rev) Feed, mm/rev (in/rev) rpm rpm 1.5 mm (0.060 in) 12.5 mm (0.5 in) 1.5 mm (0.060 in) 12.5 mm (0.5 in) Aluminum Alloys 30-120, 100-400 0.025 (0.001) 0.30 (0.012) 6,400-25,000 800-3,000 Magnesium Alloys 45-120, 150-400 0.025 (0.001) 0.30 (0.012) 9,600-25,000 1,100-3,000 Copper Alloys 15-60, 50-200 0.025 (0.001) 0.25 (0.010) 3,200-12,000 400-1,500 Steels 20-30, 60-100 0.025 (0.001) 0.30 (0.012) 4,300-6,400 500-800 Stainless Steels 10-20, 60-100 0.025 (0.001) 0.18 (0.007) 2,100-4,300 250-500 Titanium Alloys 6-20, 20-60 0.010 (0.0004) 0.15 (0.006) 1,300-4,300 150-500 Cast Irons 20-60, 60-200 0.025 (0.001) 0.30 (0.012) 4,300-12,000 500-1,500 Thermoplastics 30-60, 100-200 0.025 (0.001) 0.13 (0.005) 6,400-12,000 800-1,500 Thermosets 20-60, 60-200 0.025 (0.001) 0.10 (0.004) 4300-1,2000 500-1,500
-Manufacturing Engineering and Technology, Kalpakjian, Schmid, 2006
HOW THIS VALUE-ADDING OPERATION HELPS ACHIEVE WORKPIECE FUNCTION Drilling and hole making is an indispensable step in the manufacturing process. Many other steps in the process can be done using a variety of methods. For example forming a part can be done forging, casting, machining, it can be shaped in a die or by other methods. In order to determine the best and most profitable method some considerations need to be taken. Drilling however isn’t something that is optional in most cases. Sometimes a hole may be built into a die or a casting mold but these features can’t usually meet tight tolerances and can’t be very complex. Anytime a fastener needs to be used a hole must be drilled and tapped. There are many instances when drilling is the only option for making hole and hollow features. These include gun barrels, fastener holes, and small precise venting holes.
Contents [hide]1 Drilling in metal 2 Drilling in wood 3 Microdrilling 4 Drilling as a Manufacturing Process 5 References 6 See also
[edit] Drilling in metalUnder normal usage, swarf is carried up and away from the tip of the drill bit by the fluting of the drill bit. The continued production of chips from the cutting edges produces more chips which continue the movement of the chips outwards from the hole. This continues until the chips pack too tightly, either because of deeper than normal holes or insufficient backing off (removing the drill slightly or totally from the hole while drilling). Lubricants and coolants (i.e. cutting fluid) are sometimes used to ease this problem and to prolong the tools life by cooling and lubricating the tip and chip flow. Coolant is introduced via holes through the drill shank (see gun drill).
Straight fluting is used for copper or brass, as this exhibits less tendency to "dig in" or grab the material. If a helical drill (twist drill) is used then the same effect can be achieved by stoning a small flat parallel with the axis of the drill bit.
For heavy feeds and comparatively deep holes oil-hole drills can be used, with a lubricant pumped to the drill head through a small hole in the bit and flowing out along the fluting. A conventional drill press arrangement can be used in oil-hole drilling, but it is more commonly seen in automatic drilling machinery in which it is the workpiece that rotates rather than the drill bit.
[edit] Drilling in woodWood being softer than most metals, drilling in wood is considerably easier and faster than drilling in metal. Cutting fluids are not used or needed. The main issue in drilling wood is assuring clean entry and exit holes and preventing burning. Avoiding burning is a question of using sharp bits and the appropriate cutting speed. Drill bits can tear out chips of wood around the top and bottom of the hole and this is undesirable in fine woodworking applications.
The ubiquitous twist drill bits used in metalworking also work well in wood, but they tend to chip wood out at the entry and exit of the hole. In some cases, as in rough holes for carpentry, the quality of the hole does not matter, and a number of bits for fast cutting in wood exist, including spade bits and self-feeding auger bits. Many types of specialised drill bits for boring clean holes in wood have been developed, including brad-point bits, Forstner bits and hole saws. Chipping on exit can be minimized by using a piece of wood as backing behind the work piece, and the same technique is sometimes used to keep the hole entry neat.
Holes are easier to start in wood as the drill bit can be accurately positioned by pushing it into the wood and creating a dimple. The bit will thus have little tendency to wander. In metal working, an accurate position needs to be marked with a punch to avoid the bit wandering from the desired position of the hole.
[edit] MicrodrillingMicrodrilling refers to the drilling of holes less than 0.5 mm. Drilling of holes at this small diameter presents greater problems since coolant fed drills cannot be used and high spindle speeds are required.
[edit] Drilling as a Manufacturing ProcessOPERATION DEFINITION Hole making is one of the most important machining operations in the manufacturing process. Holes serve a variety of functions including but not limited to: fasteners for assembly, weight reduction, ventilation, access to other parts, or simply for aesthetics. Hole making or drilling is used in the production of almost any part conceivable and those that aren't drilled are made with machines that have been drilled.
HOLE MAKING OPERATIONS On most workpieces it is vitally important that the hole be drilled precisely in reference to the x, y, z-axes. When possible drilled holes should be located perpendicular to the workpiece surface. This is due to the large length-to-diameter ratio which causes the drill bit to be easily deflected which can cause the hole to be misplaced, or the drill bit to break or fatigue. Because there are so many types of production operations that involve making a variety of holes in countless different materials, there are many methods for hole making.
CONSIDERATION FOR DRILLING Because drilling can often be such a critical process there are a number of considerations that should be taken in order to ensure the most accurate drill hole possible.
As mentioned before the hole and drill motion should be perpendicular to the surface of the workpiece to reduce the tendency to fatigue or break the drill bit. This also helps to reduce 'walking' of the drill bit over the workpiece surface.
• 'Walk' is common when drilling small diameter holes. It is advantageous to create a centering mark or feature during the casting or forging process. Creating a centering dimple with a centering punch will also reduce the tendency to 'walk'.
• The bottoms of the hole should match the standard drill point angles. Avoid flat bottom hole or odd shapes.
• Create through holes instead of blind holes when possible.
• If a blind hole must be drilled and tapped, it should be drilled deeper than the tapped depth.
• Holes that need to be reamed must also be initially drilled deeper than the reamed hole depth.
• A part should be designed such that it won't need to be repositioned or manually moved during the drilling process. This also reduces production time and overall cost.
• Drill speed should be another consideration. Some materials like plastics as well as other non-metals and some metals have a tendency to heat up enough to expand making the hole smaller than desired.
TWIST DRILL The most common type of drill is a standard-point twist drill. This type of drill is versatile and can be used on a variety of materials such as wood, plastic, masonry, ceramic, and metal. These drill bits have two spiral grooves running the length of the drill. These grooves aid in transporting cutting fluid to the drill tip and in removing the chips from the hole. These types of drill bits are held in chucks or collets on machines that are either hand-held or automated. This type of drilling can often cause burrs at both the entrance and the exit of the hole and parts will often need a subsequent deburring operation to smooth out the holes.
GUN DRILLING Another type of drilling operation is called gun drilling. This method was originally developed to drill out gun barrels and is used commonly for drilling smaller diameter deep holes. This depth-to-diameter ratio can be even more than 300:1. The key feature of gun drilling is that the bits are self-centering; this is what allows for such deep accurate holes. The bits use a rotary motion similar to a twist drill however; the bits are designed with bearing pads that slide along the surface of the hole keeping the drill bit on center. Gun drilling is usually done at high speeds and low feed rates.
PUNCHING AND TREPANNING These operations involve drilling but there are other methods such as punching and trepanning which don't necessarily use common drill bits. Punching essentially works just like a paper punch, it uses a punch that pushes the material through a die. The punch and die set can be in almost any shape. Punching can be cost effective because labor costs are low, however, the equipment costs can be high making punching most cost effective for high production parts that don't require high tolerances.
Trepanning is commonly used for creating larger diameter holes (up to 250mm or 10in) where a standard drill bit is not feasible or economical. Trepanning removes the desired diameter by cutting out a solid disk similar to the workings of a drafting compass. Trepanning is performed on flat products such as sheet metal, plates, or structural members like I-beams. Trepanning can also be useful to make grooves for inserting seals like O-rings.
GENERAL RECOMMENDATION FOR SPEEDS AND FEEDS IN DRILLING [1]
Workpiece Material Surface Speed (m/min, ft/min) Feed, mm/rev (in/rev) Feed, mm/rev (in/rev) rpm rpm 1.5 mm (0.060 in) 12.5 mm (0.5 in) 1.5 mm (0.060 in) 12.5 mm (0.5 in) Aluminum Alloys 30-120, 100-400 0.025 (0.001) 0.30 (0.012) 6,400-25,000 800-3,000 Magnesium Alloys 45-120, 150-400 0.025 (0.001) 0.30 (0.012) 9,600-25,000 1,100-3,000 Copper Alloys 15-60, 50-200 0.025 (0.001) 0.25 (0.010) 3,200-12,000 400-1,500 Steels 20-30, 60-100 0.025 (0.001) 0.30 (0.012) 4,300-6,400 500-800 Stainless Steels 10-20, 60-100 0.025 (0.001) 0.18 (0.007) 2,100-4,300 250-500 Titanium Alloys 6-20, 20-60 0.010 (0.0004) 0.15 (0.006) 1,300-4,300 150-500 Cast Irons 20-60, 60-200 0.025 (0.001) 0.30 (0.012) 4,300-12,000 500-1,500 Thermoplastics 30-60, 100-200 0.025 (0.001) 0.13 (0.005) 6,400-12,000 800-1,500 Thermosets 20-60, 60-200 0.025 (0.001) 0.10 (0.004) 4300-1,2000 500-1,500
-Manufacturing Engineering and Technology, Kalpakjian, Schmid, 2006
HOW THIS VALUE-ADDING OPERATION HELPS ACHIEVE WORKPIECE FUNCTION Drilling and hole making is an indispensable step in the manufacturing process. Many other steps in the process can be done using a variety of methods. For example forming a part can be done forging, casting, machining, it can be shaped in a die or by other methods. In order to determine the best and most profitable method some considerations need to be taken. Drilling however isn’t something that is optional in most cases. Sometimes a hole may be built into a die or a casting mold but these features can’t usually meet tight tolerances and can’t be very complex. Anytime a fastener needs to be used a hole must be drilled and tapped. There are many instances when drilling is the only option for making hole and hollow features. These include gun barrels, fastener holes, and small precise venting holes.
Fabrication (metal)
Fabrication, when used as an industrial term, applies to the building of machines, structures, or process equipment for the chemical or fertilizer sector, by cutting, shaping and assembling components made from raw materials. Small businesses that specialize in metal are called fab shops.
Steel fabrication shops and machine shops have overlapping capabilities, but fabrication shops generally concentrate on the metal preparation, welding and assembly aspect while the machine shop is more concerned with the machining of parts.
Contents [hide]1 Metal fabrication 1.1 Engineering 1.2 Raw materials 1.3 Cutting and burning 1.4 Forming 1.5 Machining 1.6 Welding 1.7 Final assembly 2 Specialties 3 See also 4 External Links
[edit] Metal fabricationMetal fabrication is a value added process that involves the construction of machines and structures from various raw materials. A fab shop will bid on a job, usually based on the engineering drawings, and if awarded the contract will build the product.
Fabrication shops are employed by contractors, OEM's and VAR's. Typical projects include; loose parts, structural frames for buildings and heavy equipment, and hand railings and stairs for buildings.
[edit] EngineeringThe fabricator may employ or contract out steel detailers to prepare shop drawings, if not provided by the customer, which the fabricating shop will use for manufacturing. Manufacturing engineers will program CNC machines as needed.
[edit] Raw materialsStandard raw materials used by metal fabricators are;
plate metal formed and expanded metal tube stock, CDSM square stock sectional metals (I beams, W beams, C-channel...) welding wire hardware castings fittings
[edit] Cutting and burningThe raw material has to be cut to size. This is done with a variety of tools;
Special band saws designed for cutting metal have hardened blades and a feed mechanism for even cutting. Abrasive cut-off saws, also known as chop saws, are similar to miter saws but with a steel cutting abrasive disk. Cutting torches can cut very large sections of steel with little effort.
Burn tables are CNC cutting torches, usually natural gas powered. Plasma and Laser burn tables are also common. Plate steel is loaded on a table and the parts are cut out as programmed. The support table is made of a grid of bars that can be replaced. Some very expensive burn tables also include CNC punch capability, with a carousel of different punches and taps.
[edit] FormingHydraulic brakes with v-dies are the most common method of forming metal. The cut plate is placed in the press and a v-shaped die is pressed a predetermined distance to bend the plate to the desired angle. Wing brakes and hand powered brakes are sometimes used.
Tube bending machines have specially shaped dies and mandrels to bend tubular sections without kinking them.
Rolling machines are used to form plate steel into a round section.
English Wheel or Wheeling Machines are used to form complex double curvature shapes using sheet metal.
[edit] MachiningMain article: machiningFab shops will generally have a limited machining capability including; metal lathes, mills, magnetic based drills along with other portable metal working tools.
[edit] WeldingMain article: weldingWelding is the main focus of steel fabrication. The formed and machined parts will be assembled and tack welded into place then re-checked for accuracy. A fixture may be used to locate parts for welding if multiple weldments have been ordered.
The welder then completes welding per the engineering drawings, if welding is detailed, or per his own judgment if no welding details are provided.
Special precautions may be needed to prevent warping of the weldment due to heat. These may include; welding in a staggered fashion, using a stout fixture, covering the weldment in sand during cooling, and straightening operations.
Straightening of warped steel weldments is done with an Oxy-acetylene torch and is somewhat of an art. Heat is selectively applied to the steel in a slow, linear sweep. The steel will have a net contraction, upon cooling, in the direction of the sweep. A highly skilled welder can remove significant warpage using this technique.
Steel weldments are occasionally annealed in a low temperature oven to relieve residual stresses.
[edit] Final assemblyAfter the weldment has cooled it is generally sand blasted, primed and painted. Any additional manufacturing specified by the customer is then completed. The finished product is then inspected and shipped.
[edit] SpecialtiesMany fab shops have specialty processes which they develop or invest in, based on their customers needs and their expertise;
brazing casting chipping drawing extrusion forging heat treatment hydroforming oven soldering plastic fabrication powder coating powder metallurgy punching shearing spinning English wheeling welding And higher-level specializations such as:
electrical hydraulics prototyping/machine design/technical drawing sub-contract manufacturing
Steel fabrication shops and machine shops have overlapping capabilities, but fabrication shops generally concentrate on the metal preparation, welding and assembly aspect while the machine shop is more concerned with the machining of parts.
Contents [hide]1 Metal fabrication 1.1 Engineering 1.2 Raw materials 1.3 Cutting and burning 1.4 Forming 1.5 Machining 1.6 Welding 1.7 Final assembly 2 Specialties 3 See also 4 External Links
[edit] Metal fabricationMetal fabrication is a value added process that involves the construction of machines and structures from various raw materials. A fab shop will bid on a job, usually based on the engineering drawings, and if awarded the contract will build the product.
Fabrication shops are employed by contractors, OEM's and VAR's. Typical projects include; loose parts, structural frames for buildings and heavy equipment, and hand railings and stairs for buildings.
[edit] EngineeringThe fabricator may employ or contract out steel detailers to prepare shop drawings, if not provided by the customer, which the fabricating shop will use for manufacturing. Manufacturing engineers will program CNC machines as needed.
[edit] Raw materialsStandard raw materials used by metal fabricators are;
plate metal formed and expanded metal tube stock, CDSM square stock sectional metals (I beams, W beams, C-channel...) welding wire hardware castings fittings
[edit] Cutting and burningThe raw material has to be cut to size. This is done with a variety of tools;
Special band saws designed for cutting metal have hardened blades and a feed mechanism for even cutting. Abrasive cut-off saws, also known as chop saws, are similar to miter saws but with a steel cutting abrasive disk. Cutting torches can cut very large sections of steel with little effort.
Burn tables are CNC cutting torches, usually natural gas powered. Plasma and Laser burn tables are also common. Plate steel is loaded on a table and the parts are cut out as programmed. The support table is made of a grid of bars that can be replaced. Some very expensive burn tables also include CNC punch capability, with a carousel of different punches and taps.
[edit] FormingHydraulic brakes with v-dies are the most common method of forming metal. The cut plate is placed in the press and a v-shaped die is pressed a predetermined distance to bend the plate to the desired angle. Wing brakes and hand powered brakes are sometimes used.
Tube bending machines have specially shaped dies and mandrels to bend tubular sections without kinking them.
Rolling machines are used to form plate steel into a round section.
English Wheel or Wheeling Machines are used to form complex double curvature shapes using sheet metal.
[edit] MachiningMain article: machiningFab shops will generally have a limited machining capability including; metal lathes, mills, magnetic based drills along with other portable metal working tools.
[edit] WeldingMain article: weldingWelding is the main focus of steel fabrication. The formed and machined parts will be assembled and tack welded into place then re-checked for accuracy. A fixture may be used to locate parts for welding if multiple weldments have been ordered.
The welder then completes welding per the engineering drawings, if welding is detailed, or per his own judgment if no welding details are provided.
Special precautions may be needed to prevent warping of the weldment due to heat. These may include; welding in a staggered fashion, using a stout fixture, covering the weldment in sand during cooling, and straightening operations.
Straightening of warped steel weldments is done with an Oxy-acetylene torch and is somewhat of an art. Heat is selectively applied to the steel in a slow, linear sweep. The steel will have a net contraction, upon cooling, in the direction of the sweep. A highly skilled welder can remove significant warpage using this technique.
Steel weldments are occasionally annealed in a low temperature oven to relieve residual stresses.
[edit] Final assemblyAfter the weldment has cooled it is generally sand blasted, primed and painted. Any additional manufacturing specified by the customer is then completed. The finished product is then inspected and shipped.
[edit] SpecialtiesMany fab shops have specialty processes which they develop or invest in, based on their customers needs and their expertise;
brazing casting chipping drawing extrusion forging heat treatment hydroforming oven soldering plastic fabrication powder coating powder metallurgy punching shearing spinning English wheeling welding And higher-level specializations such as:
electrical hydraulics prototyping/machine design/technical drawing sub-contract manufacturing
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