Extrusion is a process in which a heated billet or slug of metal is forced by high pressure through an orifice that is shaped to provide the desired form to the finished part. A everyday analogy is the squeezing of toothpaste from a collapsible tube. Because of the large forces required in extrusion, most metals are extruded hot under conditions where the deformation resistance of the metal is low. How ever, cold extrusion is possible for many metals and is rapidly taking an important commercial position. Most commercial metals and their alloys, such as steel, copper, aluminum, magnesium, and nickel, are directly extruded at elevated temperatures. Rods, tubes, molding trim, structural shapes, brass cartridges, lead - converted cables, Aircraft parts, flooring strips, and many hardware items such as window sash, door trim, etc., are typical products of extrusion.
The reaction of the extrusion billet with the container and die results in high compressive stresses which are effective in reducing the cracking of the materials during primary breakdown from the ingot. This is an important reason for greater use of extrusion in working of metals difficult to form, e.g., stainless steels, nickel - base alloys, and molybdenum.
Most hot extrusion is done on horizontal hydraulic presses especially constructed for this purpose. Common sizes are rated from 250 to 5500 tons. Temperatures of billets are 350*C to 425*C for magnesium, 425*C to 475*C for aluminum, 650*C to 1300*C for copper alloys, and 1200*C to 1300*C for steel. Pressure normally vary from 4998 to 7038 kg-f per sq cm (69*10^7 N/m2). Lubrication of the extrusion chamber, die, and ram is necessary and is ordinarily achieved by mopping with oil supported graphite. Vegetable oils are better than petroleum oils for this purpose. The extrusion of steel at high temperatures is most successfully done by using glass which, at the temperatures involved, acts as a lubricant.
Metal is extruded in a number of basic ways as follows :
Direct or forward Extrusion : Direct Extrusion employs a press-operated ram and a cylinder or container into which the workpiece is placed for confinement. A dummy block is used between the ram and the hot metal. With application of ram pressure, the metal first plastically fills the cylindrical shape, and it is then forced out through the die opening until a small amount remains in the container. It is then sawed off next to the die and the butt end removed.
Indirect or Backward Extrusion : Indirect Extrusion is similar to direct extrusion except that the extruded part is forced through the hollow ram. It involves no friction between the metal billet and container walls, because the billet does not move the container. Compare with direct extrusion, less total force is required, but the equipment used is mechanically more complicated in order to accommodate the passage of extruded shape through the center of the hollow man.
Tube Extrusion : It is a form of direct extrusion but uses a mandrel to shape the inside of the tube. After the heated billet is placed inside the container , the die containing the mandrel is pushed through the billet. The ram then advances and extrudes the metal through die and around the mandrel.
Impact Extrusion : Extrusions are also made by striking slugs of metal and forming them by high impact.
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