There are eight major types of plastic moulding processes:
- Injection moulding
- Rotational moulding
- Structural foam moulding
- Gas assist moulding
- Blow moulding
- Compression moulding
- Film insert moulding
The most common type of plastic moulding process used for large scale manufacturing is the injection moulding process. Due to its widespread use, injection moulding will be the focus of this article. Injection moulding is employed to make a wide variety of products for commercial use ranging from car parts to cell phones.
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Plastic injection moulding uses pellets made of a polymer resin. The more commonly used kinds of pellets are thermoplastic pellets, which can be melted, set and melted again if necessary. The pellets are fed, unmelted, into the injection machine through a hopper which is positioned at the rear of the barrel. The barrel is heated and houses the auger, a rotating device which resembles a screw and draws the plastic pellets toward the front of the barrel. As the auger draws the pellets forward, the heated barrel that surrounds the auger melts the plastic. At the tip of the barrel is an injection nozzle, which attaches the barrel to the mould, and the molten plastic builds up behind this nozzle. As the plastic builds up, it forces the auger backward until a preset limit is met. Once the limit is achieved, the necessary amount of plastic is melted and ready for injection into the mould.
The auger is used to physically drive the molten plastic forward. The plastic is forced through the injection nozzle and driven into the mould. The auger is usually held in the forward position for a predetermined period of time to allow the pressure to equalise between the plastic in the mould and the plastic remaining in the injection barrel. When the predetermined period is over, the auger will resume drawing plastic forward. The injected plastic in the mould is given time to cool and then the mould is opened. The moulded piece is promptly ejected from the mould and the mould is returned to its original position so the process may be repeated. The moulds used for plastic injection are required to bear up beneath a significant amount of heat and pressure. They are usually made of steel, though aluminium can be used in some cases. The moulds are held closed during the physical injection using hydraulic pressure. The hydraulic pressure used to keep the moulds closed and in place during injection can be upward of 4,000 metric tons.
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