Extrusion is a process used to create objects of a fixed cross-sectional profile. It is a method
of forming in which metals or plastics are forced through a die or series of dies,resulting in a specific
shape of constant cross section. With the proper tooling, extrusions may be tapered or stepped. Extrusions
can be either very thick in cross section or very thin and be either solid or hollow. The extruded stock,
which can be 100 feet in length or longer, is then cut to a convenient stock size and used as specific
products,assembly components, or as raw stock material for further processing. Extrusion size is expressed
as a circle size which relates to the smallest circle diameter which can enclose an extrusion’scross
Metal extrusion processes may be performed hot, warm, or cold. Each method has its own unique operating
Hot Extrusion uses heated feedstock, called a billet, that ranges in temperature from 200° to
2,300°Fahrenheit, or 90° to 1,260° Celsius depending on the material.
Aluminum is the most common hot extruded material, with billet temperatures ranging from 575° to
1,100°Fahrenheit, or 300° to 600° Celsius. Hot extrusion is always performed at temperatures much higher
than the recrystallization temperature of the material to be extruded. The heated billet is confined in a
container, force is applied and the billet is extruded through a die or dies.
Hot extrusion is used to produce close tolerance dimensions as well as smooth, fine surfaces.
Additionally,and depending on the metal used, improved microstructures are obtained. The process is also
very economical in that most of the metal extruded is usable.
Hot extrusion presses are rated in force capacity which relates to available ram pressure on the billet. Ram
pressure requirements are based upon:
- • Billet material and temper
- • Cross section dimensions
- • Complexity of the extrusion
- • Extrusion length and temperature
Another factor in determining ram force requirements is the extrusion ratio. This is determined by dividing
the cross sectional area of the container liner by the cross sectional area of the die openings.
Warm and Cold Extrusion Processes
Warm extrusion refers to the extruding of feedstock or billet while it is above room temperature, but well
below the recrystallization temperatures used in hot extrusion. Cold extrusion refers to extrusion at room
temperatures. Because the feedstock is at lower temperatures, no micro-structural changes occur during
Warm and cold extrusion processes increase the strength and hardness of the finished extrusion. Reduced heat
also lowers pollution concerns and eliminates costly high temperature tooling. While virtually all metals
may be warm and cold extruded, those having the highest ductility are more suited for processing.