Manufacturing (Resistant Materials)
Methods of Production( Injection Moulding)
When identical plastic products are to be manufactured in large quantities industry tends to use Injection Moulding. This system is of course automated in industry but tends to be operated by person on the school workshop environment.
A shot of granulated or powdered thermosetting plastic which has been heated is injected under pressure into a pre-heated mould. The plastic is injected quickly to prevent the plastic hardening before it fills the mould.
Once inside the mould the pressure is maintained (dwell time) to prevent the plastic from creeping back into the screw chamber. The pressure also prevents hollows from occurring inside the plastic moulded shape.
The plastic cools and takes on the shape of the mould cavity. When the plastic component has solidified the mould is opened and the moulding ejected
image courtesy of Royal College of Art. Hodder and Stoughton.
As can be seen from the above diagram the plastic (high impact polystyrene- HIPS) or (ABS - acrylonitrile butadienestyrene) is fed into the hopper. It is forced forward under pressure by the Archimedean screw. It is heated by the surrounding heater coils. As the plastic builds up at the front of the chamber the screw moves backward. Eventually the screw is forced forward by the hydraulic ram forcing the plastic through a sprue into the mould.
Such small items as bottles, sink plugs, model kits, dustbins, bowls and milk crates etc. can be made using this system.
Even much larger items for example dinghy hulls and high precision components such as camera parts, razors and gear wheels are injection moulded.
The design and construction of the mould is essential for a high quality moulded component or product. A mould may consist of two or more parts in which the shape of the moulding has been cut.
Where large quantities of a product are required ( long production run) a hard wearing, durable mould material is required (steel).
Aluminium can be used but only if the production run is in hundreds instead of thousands of mouldings.
The cost of a mould that may make a moulding the size of a lunchbox may cost £10000 to £20000. The capital cost is of course expensive but consider that a single mould may produce tens of thousands of mouldings. Therefore the unit cost of each moulding can be very low because they are manufactured in such large quantities.
The cost of a particular mould is expensive. To solve this problem quick- change tooling systems are used. These kits offer inserts to modify existing moulds rather than making new ones every time a different type of moulding is needed. This has also reduced the setting up times of the injection moulding machine. This is called (Quick change injection moulding techniques).
In order to guarantee a quality moulding;
1. Sharp corners and sudden changes in section must be avoided. This may weaken the plastic moulding wall.
2. Large, flat surfaces should be avoided on a mould as the plastic will not come out completely flat.
3. All surfaces on the mould should be smooth and highly polished.
4. The Sprue (the hole in which the plastic is injected) needs to be carefully designed. It needs to round or half round in section. This allows ease of flow. The sprue also needs to be large enough to allow the mould to be filled very quickly.
Examples of Classic Products that have been Injection Moulded.
Exercises and Questions
The following article which may be found in the Guardian Archives examines the work of contemporary designers and inventors. Once you have read the article answer the questions in the exercises and questions section above. The artical discusses Robin Day and The ''Poyprop''chair.
The people's chair (link to article)