Cold Extrusion of Steel: Part One

Cold extrusion of metals is one of the most important manufacturing processes today and effectively involves forcing the material through the die at room temperature to create a continuous product of consistent cross-section.
Although there are minor variations, the 3 main extrusion process can be covered by forward extrusion, backward extrusion and upsetting which differ basically by the direction of the metal flow during the extrusion process.

Extrusion, though one of the most important manufacturing processes today, is a relatively young metalworking process. The cold extrusion process involves forcing a billet of material through a die at room or slightly elevated temperatures, producing a continuous product of constant cross-section. Numerous metals are suitable for cold extrusion, including lead, tin, aluminum alloys, copper, titanium, molybdenum, vanadium, and steel.

A wide variety of parts can be produced, including collapsible tubes, aluminum cans, cylinders and gear blanks. Cold extruded parts do not suffer from oxidation and often have improved mechanical properties due to severe cold working, as long as the billet temperature remains below the re-crystallization temperature.

Cold extrusion technology permits the forming of a part to the desired size and shape by moving the metal at room temperature into the die. Sufficient force is required to exceed the yield strength of the stainless steel and ensure plastic deformation results, enabling the metal to fill out the die cavities to extremely close tolerances. Although there are many different cold extrusion operations, all are variations of one or more of the following:

Forward extrusion forces the metal to flow in the same direction as the descending punch and through a hole in the die to form the required shape and dimensions, as shown in Figure 1a). Forward extrusion is especially useful in the production of bolts and screws, stepped shafts, and cylinders.

Backward extrusion forces the metal to flow upwards around the descending punch, as illustrated in Figure 1b). Extrusion pressures are generally higher and slug preparation is more critical.

Upsetting is the gathering of metal in certain sections along the length of a bar, rod or wire as shown in Figure 1c). The metal is forced to flow at right angles to the motion of the tolling. Upsetting is often performed in conjunction with backward or forward extrusions.

Figure 1: a) Forward extrusion b) Backward extrusion c) Upsetting

Figure 2: Examples of cold extrusion design

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