Indirect Extrusion is also termed as Backward/ Reverse extrusion. Here the die is not stationary and moves with respect to the container. Therefore the difficulty of direct extrusion in terms of the frictional force between the container walls and the billet is eliminated. In case of indirect extrusion, the die placed at the front end of the hollow stem moves in relation to the container. However there is no relative displacement as such between the container and the billet. The indirect extrusion process is marked by the absence of friction between the billet surface and the container. Neither there is any displacement of the billet center in relation to the peripheral regions. The following image shows the indirect extrusion process. The following graph shows the variation of pressure or load with the ram travel during both indirect and direct extrusion processes.

However one limitation is the difficulty on holding the extruded part once it exits the die. The basic advantage enjoyed by the indirect extrusion process other than the absence of frictional forces include a more uniform metal flow, less power requirement and better section uniformity along the length. The following figures show the indirect extrusion process for solid and hollow cross sections.

Friction plays an important part in the aluminium extrusion process. It influences the accuracy of the shape and the surface quality of the extrusion. The following section takes a closer look at the frictional components in an extrusion process.


Comparing Friction Components of Direct and Indirect Extrusion
Friction components are completely dependent on the type of extrusions. Direct or indirect. Using the popular flat-face dies. The figures show the friction-force components in indirect and direct extrusion. It is reveled from the flow pattern that in case of indirect extrusion, a dead-metal zone exists that has a much higher zone semi angle as denoted by in comparison to that in direct extrusion process. For a same size extrusion, i > d.. i is the dead metal zone semi angle of indirect extrusion and d is the dead metal zone semi angle of direct extrusion.