Extrusion of Metal

Extrusion
Extrusion is a compression process in which the work metal is forced to fl ow through
a die opening to produce a desired cross-sectional shape. The process can be likened
to squeezing toothpaste out of a toothpaste tube. Extrusion dates from around 1800
(Historical Note 18.3). There are several advantages of the modern process: (1) a
variety of shapes are possible, especially with hot extrusion; (2) grain structure and
strength properties are enhanced in cold and warm extrusion; (3) fairly close tolerances
are possible, especially in cold extrusion; and (4) in some extrusion operations,
little or no wasted material is created. However, a limitation is that the cross section
of the extruded part must be uniform throughout its length.
TYPES OF EXTRUSION
Extrusion is carried out in various ways. One important distinction is between direct
extrusion and indirect extrusion. Another classifi cation is by working temperature:
cold, warm, or hot extrusion. Finally, extrusion is performed as either a continuous
process or a discrete process.
Direct versus Indirect Extrusion Direct extrusion (also called forward extrusion)
is illustrated in Figure 18.30. A metal billet is loaded into a container, and a ram
compresses the material, forcing it to fl ow through one or more openings in a die at
the opposite end of the container. As the ram approaches the die, a small portion of
the billet remains that cannot be forced through the die opening. This extra portion,
called the butt,