Materials such as polystyrene, nylon, polypropylene and polythene can be used in a process called injection moulding. These are thermoplastics – this means when they are heated and then pressured in a mould they can be formed into different shapes. A simple diagram of an injection moulding machine is shown below.
The mould on this machine has been made to form plastic into a sphere
1. Granules of plastic powder (note the plastics listed above) are poured or fed into a hopper which stores it until it is needed.
2. A heater heats up the tube and when it reaches a high temperature a screw thread starts turning.
3. A motor turns a thread which pushes the granules along the heater section which melts then into a liquid.
4. The liquid is forced into a mould where it cools into the shape (in this case a sphere).
5. The mould then opens and the sphere is removed.
(Only the left- hand side of the machine is shown – the right-hand side is the same as the injection moulding machine)
A machine used to extrude materials is very similar to the injection moulding machine above. A motor turns a thread which feeds granules of plastic through a heater. The granules melt into a liquid which is forced through a die, forming a long ‘tube like’ shape. The extrusion is then cooled and forms a solid shape. The shape of the die determines the shape of the tube.
Opposite are examples of the type of shapes (sections) that can be extruded using an extrusion machine.