The Extrusion Process
Extrusion is a plastic deformation process in
which a block of metal (billet) is forced to flow by comparison
through the die opening of a smaller cross - sectional area than
that of the original billet. Extrusion is an indirect -
compression process. Indirect - compressive forces are developed
by the reaction of the work piece (billet) with the container and
die results in high values. The reaction of the billet with the
container and die results in high compressive stresses that many
breakdowns from the billet. Extrusion is the best method because
the billet is subjected to compressive forces only. Extrusion can
be cold or hot, depending on the alloy and the method used. In hot
extrusion, the billet is preheated to facilitate plastic
deformation. Conventional Direct Extrusion: The most important and
common method used on aluminium extrusion is the direct process.
In this process, the principle of direct extrusion, the billet is
placed in the container and pushed through the die by the ram
pressure. Direct extrusion finds application in the manufacture of
aluminium solid rods,aluminium bars, hollow tubes, and hollow and solid sections
according to the design and shape of the die. In the same
direction as ram travel. During this process, the billet slides
relative to the walls of the container. The resulting frictional
force increases the ram pressure considerably. During the direct
extrusion, the load or pressure - displacement curve most
commonly. Traditionally, the process has been described as having
three distinct regions:
- The billet is upset, and pressure rises rapidly to its peak value.
- The pressure decreases, and what is termed "steady state" extrusion proceeds.
- The pressure reaches its minimum value followed by a sharp rise as the "discard" is completed.