18 - Extrusion

Summary

As previously described in Ch. 2, extrusion is done by placing a piece of metal of good formability, called a billet, in a hardened steel container. In direct extrusion, a ram is then pushed from the back end of the container, so the billet is forced against a die placed at the other end of the container. The die has a hole in its middle, and as the billet is pushed forward, the front part of it starts to flow into the hole, and then out into the space at the exit side of the hole. Here it appears as a continuous extrusion with cross section approximately that of the hole. The longitudinal shape obtained in this process is generally called an extruded profile, or an extrudate. This metal forming technique is hence used to manufacture products with constant cross-sectional shape along their length, either as massive or hollow profiles, or as rod, tube, wire, or strip.

Because the flow stress is reduced when metals are heated, and workability also most commonly increases, extrusion is usually conducted as hot forming. Extrusion can either be done as forward (direct) or backward (indirect) extrusion; see Ch. 2.2.5. In forward extrusion, the material is pushed through the container and the die by means of the ram, whereas in backward extrusion, the die is placed in front of the hollow ram, whereafter the die is pushed into the billet.