Indirect extrusion or backwards extrusion is a process that involves the billet and container moving together while the die remains stationary. A "stem", which is longer than the length of the container, is used to hold the die in place. As in this process, the billet moves right along with the container, it results in frictional forces being eliminated, leading to a number of advantages like:
25 to 30% reduction in friction, allows larger billets to be used in the extrusion, which inturn helps increase the speed of the process as well as providing the ability to extrude smaller cross-sections
The tendency for any extrusions to crack is significantly reduces because of the lack of heat from friction
The container liner suffers less wear and hence, lasts longer
Uniformity in the use of the billet is increased and so the chances for defects due to extrusion or coarse grained peripheral zones are reduced.
The disadvantages of this method are: In case there are any impurities or defects on the billetâ€™s surface the surface of the extrusion is also affected. These defects have the tendency of ruining the produced piece in cases where the finished product needs to be anodized or if the aesthetics are particularly important. To get around this disadvantage however, the billets can be chemically cleaned, machined or before being used to ensure there are no impurities or defects.
The cross-section area is limited by the maximum size of the stem, which makes this process much less versatile as compared to the direct extrusion method.