PLASTICS PROCESSING

Injection moulding is a process by which the plastics is melted and injected into a mould cavity. Once the melted material is in the mould, it cools to a shape that reflects the form of the cavity.

The resulting form is a finished part requiring no additional work before
assembly and, in most cases, is the finished product. Many details, such as bosses, ribs, and screw threads can be formed during the one-step injection moulding process.

3.1.1 Thermoplastics

Thermoplastics only undergo a physical change when exposed to heat. As these plastics are heated, they melt.

Plastics like polyethylene, polypropylene, ABS, Styrene, and Polycarbonate soften as they are heated. There is a wide temperature range in which these materials will melt without degrading.

Nylon, PBT, PET, and Acetal do not soften as they are heated and experience little or no change as they reach their melting temperature. However, once they reach their melting point, they melt very quickly. Therefore they are said to have a narrow melting range, and degradation may occur faster. As any plastics reaches the high end of its melting range it degrades.

The greatest benefit of thermoplastics is that they can be re-melted many times without much degradation.


3.1.2 Thermosets

Another category of plastics called thermosets will change both physically and chemically when heated. As a chemical change has also taken place, a thermoset cannot be re-softened by reheating and therefore cannot be reused. Thermosets are used for applications requiring high heat and are usually long lived products such as pot handles, ash trays and electrical fuse box holders.