In backward extrusion die is placed in the piston, so out going material will come out in the opposite direction to that of the piston. In this process billet does not move inside the chamber So there is no friction between billet and drum. So less force is required in this method in comparison to the direct extrusion. A more complicated type of equipment is required because the plunger becomes weak due to the reduction in its effective area of cross section and ductility is experienced in supporting the overhanging extruded part.
The sequence of operations for the backward extrusion of a solid section is as follows :
- The die is inserted into the press.
- The billet is loaded into the container.
- The billet is extruded, leaving a butt.
- The die and the butt are separated from the section
Backward extrusion offers a number of advantages, as follows:
- A 25 to 30% reduction in maximum load relative to direct extrusion.
- Extrusion pressure is not a function of billet length, because there is no relative displacement between the billet and the container. Therefore, billet length is not limited by the load required for this displacement but only by the length and stability of the hollow stem needed for a given container length.
- No heat is produced by friction between the billet and the container; consequently, no temperature increase occurs at the billet surface toward the end of extrusion, as is typical in the direct extrusion of aluminum alloys. Therefore, in backward extrusion, there is a lesser tendency toward cracking of the surfaces and edges, and extrusion speeds can be significantly higher.
- The service life of the tooling is increased, especially that of the inner liner, because of reduced friction and temperatures.
The disadvantage of backward extrusion is that impurities or defects on the billet surface affect the surface of the extrusion and are not automatically retained as a shell or discard in the container. As a result, machined billets are used in many cases. In addition, the cross-sectional area of the extrusion is limited by the size of the hollow stem.