Introduction to Extrusion

Introduction to Extrusion

Extrusion and drawing have numerous applications in the manufacture of continu¬ous as well as discrete products from a wide variety of metals and alloys. In extrusion, a cylindrical billet is forced through a die (Fig) in a manner similar to squeezing toothpaste from a tube or extruding Play-Doh in various cross-sec¬tions in a toy press.

A wide variety of solid or hollow cross-sections may be pro¬duced by extrusion, which essentially are semifinished parts. A characteristic of extrusion (from the Latin extntdere, meaning "to force out"} is that large deforma¬tions can take place without fracture, because the material is under high triaxial compression during extrusion. Since the die geometry remains unchanged throughout the operation, extruded products typically have a constant cross-section. Lead pipes were made by extrusion in the 1700s. Plastics are extruded extensively..

Typical products made by extrusion are railings for sliding doors, window frames, tubing having various cross-sections, aluminum ladders, and numerous struc¬tural and architectural shapes. Extrusions can be cut into desired lengths, which then become discrete parts, such as brackets, gears, and coat hangers (Fig). Commonly

extruded materials are aluminum, copper, steel, magnesium, and lead; other metals and alloys also can be extruded, with various levels of difficulty.

Because a chamber is involved, each billet is extruded individually, thus extru¬sion is a batch or semi continuous process. Extrusion can be economical for large production runs as well as for short ones. Tool costs generally are low, particularly for producing simple, solid cross-sections.
Depending on the ductility of the material, extrusion may be carried out at room or elevated temperatures. Extrusion at room temperature often is combined with forging operations, in which case it generally is known as cold extrusion. It has numerous important applications, including fasteners and components for automobiles, bicycles, motorcycles, heavy machinery, and trans¬portation equipment.

Drawing is an operation developed between 1000 and 1500 A.D., in which the cross-section of solid rod, wire, or tubing is reduced or changed in shape by pulling it through a die. Drawn rods are used for shafts, spindles, and small pistons and as the raw material for fasteners (such as rivets, bolts, and screws). In addition to round rods, various profiles also can be drawn. The term drawing also is used to refer to making cup-shaped parts by sheet-metal forming operations.

The distinction between the terms rod and wire is somewhat arbitrary, rod being larger in cross-section than wire. In industry, wire generally is defined as a rod that has been drawn through a die at least once. Wire drawing involves smaller di¬ameters than rod drawing with sizes down to 0.01 mm (0.0005 in.) for magnet wire and even smaller for use in very low-current fuses.