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METAL FORMING PROCESS
- 1. RAMCO INSTITUTE OF TECHNOLOGY Mr.M.LAKSHMANAN Assistant Professor (Senior Grade) Department of Mechanical Engineering
- 3. Unit III - Syllabus Introduction Hot Working and Cold Working of Metals Forging Processes- Open, impression die forging, Closed die forging-forging operation Rolling of metals-types of rolling- Flat strip rolling-shape rolling operation - Defects in rolled parts- Principle of rod and wire drawing-tube drawing -Principle of extrusion Types-hot and cold extrusion.
- 4. Forming/Metal Forming Forming or metal forming, is the metalworking process of fashioning metal parts and objects through mechanical deformation; the workpiece is reshaped without adding or removing material, and its mass remains unchanged. Forming operates on the materials science principle of plastic deformation, where the physical shape of a material is permanently deformed.
- 5. Metal forming processes are: Forging Rolling Extrusion Drawing Pressing Metal forming can be done by Three types: 1. Hot working 2. Warm working 3. Cold working
- 6. Hot working It is defined as the plastic deformation of metal above their recrystallization temperature but below the melting point. The new crystals are to start forming when the atoms of metal reach a certain higher energy level under the action of heat and force. This is called Re-crystallization. This is important because recrystallization keeps the materials from strain hardening, which ultimately keeps the yield strength, hardness and high ductility.
- 7. Advantages: • The process is quick and economical. • Deformation occurs in less forces. • Mechanical properties such as ductility, elongation, toughness can be improved. • This process is suitable for nearly all the commercial metals. • The blow holes, cracks are eliminated.
- 8. Disadvantages: • The cost of the tooling and handling are very high. • Poor surface finish. • The close tolerances can not be achieved. • Life of the equipment is less. • Skilled labour is required to maintain the hot metals.
- 9. Warm working The material deformation carried between hot and cold working is called Warm working. Warm Forming is the process of deforming metal heated to a temperature that maximizes the material’s malleability without allowing re- crystallization, grain growth, or metallurgical fracture. Temperatures can range from 200-850 degrees C.
- 10. Advantages • The equipment works under lighter loads. • Metal ductility is more. • Less strain hardening • Less amount of heat energy is required. • Good precision is achieved. • Good surface finish.
- 11. Cold working It is defined as the plastic deformation of metal below their recrystallization temperature at room temperature. (or above room temperature).
- 12. Advantages • No heating is required. • Strength, fatigue and wear properties are improved. • Better surface finish is obtained. Disadvantages • Higher forces are required. • Less ductility is available. • More powerful equipment is required. • Metal surfaces must be clean and scale-free.
- 13. Recrystallization Recrystallization temperature is the temperature at which the first new grains appears. It requires realignment or diffusion in a materials. Recrystallization is a process by which deformed grains are replaced by a new set of defects-free grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility.
- 14. Recrystallization of a metallic material (a → b) and crystal grains growth (b → c → d).
- 15. Strain Hardening Work hardening, also known as strain hardening is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.
- 16. Comparison between hot and cold working processes SI.No Hot working Cold working 1. Processing done at temperature above recrystallization temperature. Processing done at temperature below recrystallization temperature 2. Forces required are lower Forces required are higher 3. New crystals are formed due to recrystallization Recrystallization does not occur 4. Blow holes, porosity and cracks are eliminated due to by the high working pressure. Excessive cold working can lead to production and propagation of cracks 5. Difficulty in handling; require heat resistant tools No such requirement 6 Dimensional accuracy is poor Dimensional accuracy is much better 7 Oxidation and scaling result in poor appearance. Finish obtained is much better 8. Thin gauge sheet and fine wires cannot be produced. Even very thin sheets and fine wires can be produced.
- 17. Classification of mechanical working of metals Mechanical working of metals Hot working Cold working • Forging • Rolling • Extrusion • Spinning • Drawing • Tube Piercing • Drawing • Coining • Sizing • Extrusion • Squeezing • Bending • Shearing
- 18. Hot-working operation • Forging – Open die forging – Closed die forging – Impression die forging • Rolling • Extrusion • Spinning • Drawing • Tube Piercing
- 19. Forging Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at which it is performed: • Cold forging (a type of cold working), • Warm forging, or hot forging (a type of hot working).
- 20. Forging is a deformation processing of materials through compressive stress. It is carried out either hot or cold. Hot forging is done at temperatures above recrystallization temperatures, (0.6 Tm, or above) Where, Tm is melting temperature. Warm forging is done in the temperature range: 0.3 Tm to 0.5 Tm. Cold forging has advantages such as good surface finish, high strength and greater accuracy.
- 21. Applications: bolts, disks, gears, turbine disk, crank shaft, connecting rod, valve bodies, small components for hydraulic circuits etc. Advantages: • Closer dimensional accuracies achieved require very little machining after forging. • Material saving is the result. • Higher strength, greater productivity, favorable grain orientation, high degree of surface finish are other merits.
- 22. Forging Forging is a process in which the work piece is shaped by compressive forces applied through various dies and tools. Forging Process • Open die forging • Closed die forging • Impression die forging
- 23. Forging Tools • Tongs • Hot chisel • Round punch • Cold chisel • Anvil • Hammer
- 24. Forging Tools
- 25. Open die forging Open die forging is the process of deforming a piece of metal between multiple dies that do not completely enclose the material. The metal is altered as the dies “hammer” or “stamp” the material through a series of movements until the desired shape is achieved.
- 26. Open die forging
- 27. Open-die forging is a hot forging process in which metal is shaped by hammering or pressing between flat or simple contoured dies • Equipment: Hydraulic presses, hammers. • Materials: Carbon and alloy steels, aluminium alloys, copper alloys, titanium alloys, all forgeable materials.
- 29. Closed die forging process, a billet is formed (hot) in dies (usually with two halves) such that the flow of metal from the die cavity is restricted. The excess material is extruded through a restrictive narrow gap and appears as flash around the forging at the die parting line. • Equipment: Anvil and counterblow hammers, hydraulic, mechanical, and screw presses. • Materials: Carbon and alloy steels, aluminium alloys, copper alloys, magnesium alloys, beryllium, stainless steels, nickel alloys, titanium and titanium alloys, iron and nickel and cobalt super alloys.
- 30. Closed Die Forging is a forging process in which dies move towards each other and covers the workpiece in whole or in part. The heated raw material, which is approximately the shape or size of the final forged part, is placed in the bottom die. The shape of the forging is incorporated in the top or bottom die as a negative image. Coming from above, the impact of the top die on the raw material forms it into the required forged form
- 32. • In impression-die forging the heated work piece is placed between two required shaped die and is pressed or hammered. • During hammering or pressuring it takes the shape of the die. • A small amount of materials is forced outside the die impression, which is finally trimmed.
- 34. Comparison of open-Die and closed Die forging Open Die Forging Closed Die Forging Open-die forging is a hot forging process in which metal is shaped by hammering or pressing between flat or simple contoured dies. In this process, a billet is formed (hot) in dies (usually with two halves) such that the flow of metal from the die cavity is restricted It is a small mass production It is a heavy mass production low production efficiency High production efficiency high labour intensity Low labour intensity Equipment: Hydraulic presses, hammers. Equipment: Anvil and counterblow hammers, hydraulic. Materials: Carbon and alloy steels, aluminium alloys Materials. Carbon and alloy steels, copper alloys, magnesium alloys, stainless steels Application: Forging ingots, large and bulky forgings Application: automobiles, trucks, tractors, off-highway equipment, aircraft, railroad
- 35. Upsetting Upset forging increases the diameter of the work piece by compressing its length. A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.
- 37. Roll Die forging
- 38. Roll Die Forging Roll 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 through the machine.
- 40. Drop forging
- 41. Drop forging Drop forging is a forging process where a hammer is raised and then "dropped" onto the work piece to deform it according to the shape of the die.
- 42. Hot spinning
- 43. Hot spinning It is a metalworking process by which a disc or tube of metal is rotated at high speed and formed into an axially symmetric art. Spinning can be performed by hand or by a CNC lathe.
- 45. Piercing
- 46. Piercing • It can be used to create a cavity or hole in the work piece. • Piercing is a shearing process where a punch and die are used to create a hole in sheet metal or a plate. • Blanking and piercing are shearing processes in which a punch and die are used to modify webs. The tooling and processes are the same between the two, only the terminology is different: • In blanking the punched out piece is used and called a blank; • In piercing the punched out piece is scrap. The process for parts manufactured simultaneously with both techniques is often termed "pierce and blank." An alternative name of piercing is punching.
- 48. Ring Rolling
- 49. Ring rolling is a particular category of metal rolling, in which a ring of smaller diameter is rolled into a precise ring of larger diameter and a reduced cross section. This is accomplished by the use of two rollers, one driven and one idle, acting on either side of the ring's cross section.
- 50. The work will essentially retain the same volume, therefore the geometric reduction in thickness will be compensated for entirely by an increase in the ring's diameter.
- 51. Skew Rolling
- 52. Skew rolling is a metal forging process that uses two specially designed opposing rolls, that rotate continuously. Round stock is fed into the rolls, the material is forged by each of the grooves in the rolls and emerges from the end as a metal ball. The stock is fed through the rolls continuously, but each ball is produced separately, thus it is a discrete process and not a continuous one. Skew rolling, similar to roll forging, is a manufacturing process that bears qualities of both metal rolling and metal forging.
- 55. Hydraulic Presses The hydraulic press depends on Pascal's principle: the pressure throughout a closed system is constant. One part of the system is a piston acting as a pump, with a modest mechanical force acting on a small cross-sectional area; the other part is a piston with a larger area which generates a correspondingly large mechanical force. Only small-diameter tubing (which more easily resists pressure) is needed if the pump is separated from the press cylinder.
- 59. Pneumatic Power Hammer Power hammer consists of a frame, an anvil, and a reciprocating ram holding a hammer head or die. The work piece is placed on the lower anvil or die and the head or upper die strikes the work piece. The power hammer is a direct descendant of the trip hammer, differing in that the power hammer stores potential energy in an arrangement of mechanical linkages and springs, in compressed air and by the fact that it accelerates the ram on the downward stroke. This provides more force than simply allowing the weight to fall.
- 61. Ironing • Ironing is the process of smoothing and thinning the wall of a shell or cup (cold or hot) by forcing the shell through a die with a punch. • Equipment: Mechanical presses and hydraulic presses. • Materials: Carbon and alloy steels, aluminium and aluminium alloys, titanium alloys. • Applications: Shells and cups for various Machines.
- 62. Ironing Ironing is carried out by pulling the workpiece through an ironing ring with the help of a punch. The wall thickness of the hollow body is reduced in this process.
- 63. Swaging Process that reduces/increases the diameter, tapers, rods or points round bars or tubes by external hammering.
- 64. Swaging is a forging process in which the dimensions of an item are altered using dies into which the item is forced.
- 66. Cold-working operation • Bending • Shearing • Drawing • Presses • Rolling • Cold Forging • Thread Rolling • Coining
- 67. ROLLING Process used in sheets, strips, bars, and rods to obtain products that have smooth surfaces and accurate dimensions.
- 68. Rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform.
- 69. Cold Forging Cold forging deforms metal while it is below its recrystallization point. Cold forging is generally preferred when the metal is already a soft metal, like aluminum. This process is usually less expensive than hot forging and the end product requires little, if any, finishing work.
- 70. Cold Forging Process in which slugs of material are squeezed into shaped die cavities to produce finished parts of precise shape and size.
- 72. Thread rolling Thread rolling is a metal rolling process used extensively in manufacturing industry to produce screws, bolts and other fasteners. A common thread rolling process, used in industry to manufacture threaded parts, involves forming the threads into the metal of a blank by a pressing and rolling action between two die. The die surfaces hold the shape and the force of the action forms the threads into the material.
- 73. Thread rolling Process is used for making external threads; in this process, a die, which is a hardened tool with the thread profile, is pressed on to a rotating work piece.
- 74. Thread Rolling
- 75. Bending
- 76. Bending Bending is a manufacturing process that produces a V-shape, U-shape, or channel shape along a straight axis in ductile materials, most commonly sheet metal. Commonly used equipment include box and pan brakes, brake presses, and other specialized machine presses.
- 78. Shearing
- 79. Shearing Shearing, also known as die cutting, is a process which cuts stock without the formation of chips or the use of burning or melting. Strictly speaking, if the cutting blades are straight the process is called shearing; if the cutting blades are curved then they are shearing-type operations. The most commonly sheared materials are in the form of sheet metal or plates, however rods can also be sheared.
- 81. Coining Coining is a closed die forging process, in which pressure is applied on the surface of the forging in order to obtain closer tolerances, smoother surfaces and eliminate draft. Closed die forging is a process in which forging is done by placing the work piece between two shaped dies. This process may be done in hot or cold working conditions, but is predominantly a cold work process.
- 82. Coining
- 83. Coining coining is used to form indentations and raised sections in the Part. During the process, metal is intentionally thinned or thickened to achieve the required indentations or raised sections.
- 85. Advantages of Coining • Produces finer and detailed surface finish • Provides accurate bends in a consistent manner • Does not require expensive machinery • The work hardened surface can resist impact and abrasion • Eliminates the need for complex finishing processes • Plastic flow reduces the surface grain size and work hardens the surface.
- 86. Industrial Applications • Minting of coins and medallions • Making of jewelry • Making badges, buttons, precision-energy springs • Making complex electronic parts • Making precision parts that require finer polished surface finishes.
- 88. Drawing Drawing is a metalworking process which uses tensile forces to stretch metal. As the metal is drawn (pulled), it stretches thinner, into a desired shape and thickness. It is broken up into two types: • Sheet metal drawing • Wire, bar, and tube drawing.
- 91. Press forging Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop- hammer forging. The amount of time the dies are in contact with the work piece is measured in seconds (as compared to the milliseconds of drop- hammer forges). The press forging operation can be done either cold or hot.
- 94. Press forging
- 96. Forging Defects
- 99. ROLLING Process used in sheets, strips, bars, and rods to obtain products that have smooth surfaces and accurate dimensions. The metal is subjected to high compressive stresses as a result of the friction between the rolls and the metal surface.
- 100. ROLLING Rolling process is most common industrial process used for making large length cross section like sheets and plates of steel and aluminum for structure and other works. Rolling mills are used to perform rolling process. These machines are available in different shapes and size according to requirement of the process and due to technical issues. Every rolling mill consist minimum two rolls. These numbers can extend according to the process requirement.
- 102. Types of Rolling Mill
- 103. Types of rolling mills • Two High Rolling Mill • Three High Rolling Mill • Four High Rolling Mill • Tandem Rolling Mill • Cluster Rolling Mill • Planetary Rolling Mill
- 104. Two High Rolling Mill This mill consist two rollers arranged as shown in figure. Both the rollers rotate in opposite direction for desire movement of work piece. Work piece is feed between the rollers which apply a compressive force and tends to plastically deform work piece and convert it into desire shape.
- 106. This machine can further classified into two types The first one is two high Non reversible machine in which the rollers can rotate in only one direction (Either clockwise or anticlockwise) and thus work piece can feed only in one direction (Either left to right or right to left). One big problem arises in this machine is that every time the work piece carried back over the front side of mill to feed again for reduction in thickness.
- 108. The other one is two high reversible machine in which both the rollers can be rotate in both directions. In this machine no need to carry work piece at front side. We have to just change the direction of rotation of rollers to again pass the work piece through rollers for decrease thickness.
- 109. Three High Rolling Mill This type of rolling mill consist three rollers arrange parallel to each other. In this machine the middle roller rotate in opposite direction of other two rollers. This machine is used to reduce two times thickness in one pass. In this machine, the work piece is feed between bottom and middle roller in one direction and top and middle rollers in opposite direction. This machine can handle two work pieces in single run.
- 112. Four High Rolling Mill These machine consist four rollers (two small and two big). These rollers are arranged as shown in figure. Small rollers are in direct contact with work piece and rotate in opposite direction. Big rollers works as backup rolls and they also rotates in opposite direction with each other and also with contact roller. This machine is used for cold rolling where high rigidity required.
- 115. Tandem Rolling Mill Tandem rolling is also known as continuous rolling in which two or more set of four high rolling mill arranged in a series in which each set is called stand. These stands are arranged so they can roll the work piece in decreasing cross section. It is used for rolling thick work piece in single pass.
- 118. Cluster Rolling Mill In this type of rolling mill, each of working roller is backup by two or more of larger backup roller. These rollers are arrange as shown in figure. This machine is used for rolling hard material.
- 120. Planetary Rolling Mill In this type of rolling machine, a large backup roller is surrounded by various numbers of planetary working rollers as shown in figure. Each planetary roll gives constant reduction. It is used to reduce large thickness in single pass of steel strip. Its rolling capacity is more than cluster machine but less than tandem rolling machine.
- 122. The initial breakdown of ingots into blooms & billets is generally done by hot-rolling. And then cold-rolling is to be done to get good surface finish and close control over dimension. HOT ROLLING: occurs above the recrystallization temperature of the material
- 123. • Hot rolling is used mainly to produce sheet metal or simple cross sections, such as rail tracks. • It will reduce the average grain size of metal, this improves the strength of material.
- 124. COLD ROLLING • Used to produce sheet & strip with superior surface finish and dimensional tolerance. • Also, the strain hardening results from the cold reduction may be used to give increased strength. • Total reduction achieved will vary from 50 to 90 percent. • The lowest %age reduction is taken in the last pass to permit better control of flatness, gage and surface finish. • Done usually at room temperature.
- 125. • Four-high or cluster mills are used. (Al, Cu alloys) • Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass. • This process increases strength upto 20% via strain hardening.
- 126. Hmax = μ2R Hmax - Maximum thickness reduction in one pass μ – Static friction coefficient between rolls and metal R – Radius of rolls
- 127. HOT ROLLING 1. Blowholes and porosity eliminates by welding together. 2. This will results into better ductility and toughness. 3. Surface decarburization of steels. 4. Not good surface finish. 5. Dimensional tolerance due to expansion/ contraction of metal. COLD ROLLING 1. Results in increased strength or hardness. 2. Better dimensional control. 3. Good surface finish. 4. Ductility decreases. 5. Yield point phenomenon (results in inhomogeneous deformation), occur in annealed steel sheet.
- 128. TYPES OF ROLLING PROCESSES • CONTINOUS ROLLING • TRANSVERSE ROLLING • SECTION OR SHAPE ROLLING • RING ROLLING • THREAD ROLLING
- 129. CONTINOUS ROLLING The objective is to decrease the thickness of the metal with an increase in length and with little increase in width.
- 130. TRANSVERSE ROLLING • Using circular wedge rolls. • Heated bar is cropped to length and fed in transversely between rolls. • Rolls are revolved in one direction.
- 131. SECTION OR SHAPE ROLLING • Flat slap is progressively bent into complex shapes. • Suitable for producing moulded sections such as irregular shaped channels and trim.
- 132. RING ROLLING • A specialized type of hot rolling that increases the diameter of a ring • The resulting grain structure is circumferential, which gives better mechanical properties
- 133. THREAD ROLLING • Rolled threads are produced in a single pass at speeds far in excess of those used to cut threads. • Dies are pressed against the surface of cylindrical blank. As the blank rolls against the in-feeding die faces, the material is displaced to form the roots of the thread, and the displaced material flows radially outward to form the thread's crest.
- 134. TERMINOLOGY Bloom is the product of first breakdown of ingot (cross sectional area > 230 cm2). Billet is the product obtained from a further reduction by hot rolling. (cross sectional area > 40x40 mm2). Slab is the hot rolled ingot (cross sectional area > 100 cm2 and with a width ≥ 2 x thickness).
- 135. Plate is the product with a thickness > 6 mm. Sheet is the product with a thickness < 6 mm and width > 600 mm. Strip is the product with a thickness < 6 mm and width < 600 mm.
- 136. Shape Rolling Operation Structural shape rolling, also known as shape rolling and profile rolling, is the rolling and roll forming of structural shapes by passing them through a rolling mill to bend or deform the workpiece to a desired shape while maintaining a constant cross- section. Structural shapes that can be made with this metal forming process include: I-beams, H-beams, T-beams, U-beams, angle iron, channels, bar stock, and railroad rails.
- 142. Principle of wire Drawing
- 143. Wire drawing is a metalworking process used to reduce the cross-section of a wire by pulling the wire through a single, or series of, drawing die(s). There are many applications for wire drawing, including electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes for wheels, and stringed musical instruments. Drawing is different from extrusion, because in drawing the wire is pulled, rather than pushed, through the die. Drawing is usually performed at room temperature, thus classified as a cold working process, but it may be performed at elevated temperatures for large wires to reduce forces.
- 144. Tube Drawing
- 145. • Tube drawing is a metalworking process to size a tube by shrinking a large diameter tube into a smaller one, by drawing the tube through a die. • This process produces high-quality tubing with precise dimensions, good surface finish, and the added strength of cold working. Because it is so versatile, tube drawing is suitable for both large- and small-scale production.
- 147. There are five types of tube drawing: • Tube sinking • Mandrel drawing/ Rod drawing • Stationary mandrel/ Fixed plug drawing • Floating mandrel/ Floating plug drawing A mandrel is used in many of the types to prevent buckling or wrinkling in the work piece.
- 149. Tube sinking Tube sinking, also known as free tube drawing, reduces the diameter of the tube without a mandrel inside the tube. Especially on thick- walled tubes and tubes smaller than 12 mm (0.47 in) in diameter.
- 150. Rod drawing/Mandrel drawing Rod drawing is the process that draws the tube with a mandrel inside the tube; the mandrel is drawn with the tube. The advantage to this process is that the mandrel defines the ID and the surface finish and has a quick setup time for short runs. The disadvantages are that lengths are limited by the length of the mandrel, usually no more than 100 feet (30 m), and that a second operation is required to remove the mandrel, called reeling. This type of process is usually used on heavy walled or small ID tubes.
- 151. Fixed plug drawing/ Stationary mandrel Fixed plug drawing, also known as stationary mandrel drawing, uses a mandrel at the end of the die to shape the ID of the tube. This process is slow and the area reductions are limited, but it gives the best inner surface finish of any of the processes. This is the oldest tube drawing method.
- 152. Floating plug drawing/ Floating mandrel Floating plug drawing, also known as floating mandrel drawing, uses a mandrel that is not anchored whatsoever to shape the ID of the tube. The mandrel is held in by the friction forces between the mandrel and the tube. This axial force is given by friction and pressure. The greatest advantage of this is that it can be used on extremely long lengths, sometimes up to 1,000 feet (300 m). The disadvantage is it requires a precise design otherwise it will give inadequate results. This process is often used for oil-well tubing.
- 155. Extrusion Extrusion is a process of forcing the metal to flow through a shape- forming die. The metal gets deformed due to compressive and shear force applied at inlet end. Main advantage of extrusion is that high compressive and shear forces are set up in the material without any tensile forces, which makes deformation possible without much crack formation. Extrusion is a hot working process which makes use of good deform ability of the material for shaping them. The difference between the extrusion and drawing processes lie in the fact that in drawing operation, the metal is forced through the die by compressive forces applied at the entry side.
- 156. Principle of Extrusion • It is a primary shaping process in which a metal kept in a cylinder is forced to pass through the opening of a die. • The metal is subjected to plastic deformation and it undergoes reduction and elongation during extrusion Types • Hot Extrusion • Cold Extrusion
- 157. Hot extrusion: It is carried out at high temperatures, about 50-70% of the melting point of the metal. cold extrusion: It is carried out at room temperature or slightly above room temperature. Lead, tin, aluminium, etc can be easily extruded.
- 158. Hot Extrusion • Forward Extrusion/ Direct Extrusion • Backward Extrusion/ Indirect Extrusion Forward Extrusion
- 159. • Direct extrusion, also known as forward extrusion, is the most common extrusion process. It works by placing the billet in a heavy walled container. • The billet is pushed through the die by a ram or screw. There is a reusable dummy block between the ram and the billet to keep them separated. • The major disadvantage of this process is that the force required to extrude the billet is greater than that needed in the indirect extrusion process because of the frictional forces introduced by the need for the billet to travel the entire length of the container. Forward Extrusion
- 160. Because of this the greatest force required is at the beginning of process and slowly decreases as the billet is used up. At the end of the billet the force greatly increases because the billet is thin and the material must flow radially to exit the die. The end of the billet (called the butt end) is not used for this reason.
- 162. Backward Extrusion
- 163. Backward Extrusion • In indirect extrusion, also known as backwards extrusion, the billet and container move together while the die is stationary. • The die is held in place by a "stem" which has to be longer than the container length. • The maximum length of the extrusion is ultimately dictated by the column strength of the stem. Because the billet moves with the container the frictional forces are eliminated
- 166. Cold Extrusion
- 167. Impact Extrusion This method is chiefly used for making small workpieces from ductile materials. The material is placed in the position into a blind die and a ram with clearance is forced into the die making the metal to flow plastically around the punch. With the help of this process, collapsible medicine tube and tooth paste tubes are produced. This method is restricted to soft metals as lead, aluminium, copper, tin, etc.
- 168. Impact Extrusion
- 169. Hydrostatic Extrusion In the hydrostatic extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The process must be carried out in a sealed cylinder to contain the hydrostatic medium.