The basic extrusion process is shown below.

The material is forced through a die which has the required cross-section in the same way that toothpaste is squeezed from a toothpaste tube. Many different materials can be extruded. Metal can be extruded either hot or cold usually using copper and aluminium alloys, whilst cast iron and steel can be extruded hot. The extrusion of thermoplastics is basically the same as for metals except that the temperature must be very accurately controlled.
One of the most commonly extruded pruducts is tubing. The 2 alternative techniques for producing this are shown below.

Extrusion continued

There are also 2 different ways of generating the pressure required to force material through the die. The first is to simply use a hydraulic ram, but for a continuous process, a helical screw is used and the thermoplastic is heated as it progresses towards the die. This method is illustrated below along with a selection of the shapes that can be extruded.

Drawing

This technique is used to create rods, wires and tubes by pulling, or drawing, the material through a series of dies. For this operation the material must be ductile which limits it to copper (this is how most copper tubing is made), steel and aluminium