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.
