Extrusion moulding is an economical method of producing plastic rods, tubes and other items of uniform cross section. Extrusion involves forcing softened plastic through a die --- a piece of steel cut in the shape of the cross section of the part to be produced --- to form a continuous piece of work.
Thermoplastic materials --- in the form of powder, pellets or beads --- are fed into the barrel of an extruder from a hopper. The barrel of the extruder and the extruder screw are heated by external elements so the plastic melts as it moves toward the die.
The extruder screw is a rotating, helical screw whose purpose is to melt, mix and convey plastic material to the die. Its design and length depend on the type of plastic being processed.
The hot, extruded form passes from the die onto a conveyor and is subsequently hardened by cooling. Once cool, it can be cut to the desired length.
Read more: How Extrusion Moulding Works | eHow.com