Extrusion moulding, or extrusion molding, is the molding of plastic or metals into a pre-defined shape. During the process, metal or plastic is melted and pushed through a die to create a linear form. The final shape, commonly called a rod, although it is not necessarily cylindrical, is then cooled and can then be cut to various lengths, depending upon the application.
If you slice a rod of material, the shape you see is called the cross-section. The shape that emerges from an extrusion moulding machine is identical to this cross-section piece. It is uniform throughout the item until the material or the die is changed.
An extrusion moulding machine varies in size, but is typically at least the size of an automobile. It is equipped with some method of inserting the material into the machine and melting it to a liquid state. Then it can be pushed through a die to create its shape.
The assemblies for plastic and metal extrusion moulding are similar in effect, but different in operation. For plastic extrusion moulding, a large funnel, called a hopper, is placed on top of the machine. Into it are poured small pellets of material, for example high density polyethylene, or plastic. Those pellets are easily melted into a liquid state and pushed through a die to create a plastic tube or rod of some shape. For metal extrusion moulding, pre-heated heavy metal rods called billets are inserted into the machine and then pressed to force it through a die to create a metal tube or rod.
As with injection moulding, there are many variables to consider when extruding materials. The temperature of the material, the pressure at which the material is forced through the die, and the time of extrusion are all factors that can be adjusted to create ideal material conditions. Extrusion moulding is similar to injection moulding in that it melts and forms plastic or metal into pre-defined shapes. However, with injection moulding, the plastic or metal is not continuously extruded. The mold or die fills with material, the material cools, and then it opens to drop pre-formed shapes into a container.
When designing a die for an extrusion moulding machine, one must take into consideration factors that affect the finished material after it cools. For example, plastics and metals have a shrink rate, which is a percentage of the size that is lost after material cools. The shrink rate of various materials must be considered when designing a die.