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.
EXTRUSION
(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.
Link to this article：INJECTION MOULDING AND EXTRUSION
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