Because the chemical structure of thermoset plastics is completely different from the structure of thermoplastics, there are also some differences in their production processes. Whichever method is used, the processing of thermoplastics is as follows: plastic becomes fluid through pressure and heat, this liquid plastic gets the shape of the mould, the mould is cooled and finally the final product is get. In thermoset plastics, again through heat and pressure the plastic becomes liquid and then gets the shape of the mould. However, in the final stage of the production process thermoset plastics become solid by “being burned” instead of becoming solid by cooling.

This difference in the production process of thermoset plastics doesn’t pose an obstacle for their processing with plastic injection and compression moulding methods which are one of the most popular methods. You can find the most important points with regard to these two methods.

Compression Moulding

Compression moulding is a very old and common production method used for the processing of thermoset plastics. With this method, plastic raw material turned into the final product form by being compressed through heat and pressure. This method is performed with a press containing heated board or heated mould. In this method, two piece moulds are used.

In compression moulding method, the mould is heated before the moulding in order to shorten the burning time. Raw material which is put on the mould by being weighted is compressed with the press heated beforehand. Raw material which melts with heat and pressure fills the blanks in the mould. After waiting for certain time the press is opened and the plastic which has reached its final form is taken from the mould.

Although preheating, mould heating degrees and mould pressure change according to plastic raw material type that will be used, preheating in thermoset plastics is averagely 93C, mould temperature is 120C-200C and mould pressure is 7psi-14psi.

Another important issue is that extra plastic raw material should be poured into the mould. This is important because by doing this mould fills up completely. This extra material is formed around the final product as burr. This burr can be easily removed from the final product.

Thermoset gases need to be taken into consideration in compression moulding. These gases which generally occur in urea formaldehyde and melamine formaldehyde mouldings should be taken out. Otherwise, raw material can weaken and on the surface some stains can occur.

Advantages and disadvantages of compression moulding method are given below.

Injection Moulding

The second method used in the processing of thermoset plastics is injection moulding method which is also used in thermoplastics. Injection moulding method is performed by making small changes in the machine. The reason is that thermoset plastics have different burning features.

In the processing of thermoset plastics with injection moulding method, heat control is much more important. In this method, thermoset plastic granules are carefully heated in machine’s reservoir. However, this heating process is performed in such a way that raw material softens but the cross-linking shouldn’t occur until the raw material enters the mould hole. On the other hand, the mould should be heated with higher temperatures so that the cross-linking occurs and thus the mould raw material burns and becomes final product as soon as possible.

Approximate values of parameters to be used while thermoset plastics are moulded in injection machines are given below.

Melting temperature: 110 - 140C

Piston temperature: 65C - 90C

Nozzle temperature: 85C - 120C

Mould temperature: 165C - 195C

Injection pressure: 85MPA – 250MPA

Screw pressure: <7 MPA

Screw speed: 65 – 85 rev/min

Burning time: 15sec – 80sec