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INJECTION MOULDING AND EXTRUSION
A/ INJECTION MOULDING
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
B/ EXTRUSION MOULDING
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
BLOW MOULDING
Blow moulding a shape is a common industrial process. The example shown below is of the production of a plastic bottle. The plastics normally used in this process are; polythene, PVC and polypropylene
VACUUM FORMING
Vacuum forming is a technique that is used to shape a variety of plastics. In school it is used to form/shape thin plastic, usually plastics such as; polythene and perspex. Vacuum forming is used when an unusual shape like a ‘dish’ or a box-like shape is needed. Below you can see the stages involved in vacuum forming
MOULDING THROUGH COMPRESSION
Moulding through the force of compression is another very common industrial process. The materials used are melamine formaldehyde, phenol and urea. These materials can be formed into different shapes through applying both heat and pressure, other plastics do no have these properties
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