Injection molding and extrusion

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


	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.
(Only the left- hand side of the machine is shown - the right-hand side is the same as the injection moulding machine)
Opposite are examples of the type of shapes (sections) that can be extruded using an extrusion machine.

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.

The process is similar to injection moulding and extrusion. 1. The plastic is fed in granular form into a 'hopper' that stores it. 2. A large thread is turned by a motor which feeds the granules through a heated section. 3. In this heated section the granules melt and become a liquid and the liquid is fed into a mould. 4. Air is forced into the mould which forces the plastic to the sides, giving the shape of the bottle. 5. The mould is then cooled and is removed.

Source: http://folders.sirthomasboughey.staffs.sch.uk/easylink/intranet/stbh/files/Staff1/INJECTION%20MOULDING%20AND%20EXTRUSION.doc

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Injection molding and extrusion

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