A thermoplastic is a plastic material that becomes soft when heated. A thermoplastic has long molecular chains, so-called polymers. Molecules lie tightly together with a solid form of plastic. When the thermoplastic is heated at approx. 165 °C, the molecules become more mobile. It is therefore a material that deforms and liquefies when heated. At a cooling point of approx. 120 °C, the thermoplastic becomes solid again. The cooling down of a thermoplastic depends on the Tg temperature, which is different for each thermoplastic. You can read about the difference in thermoplastics here.
Thermoplastics are almost always used forinjection moulding. This is because they are fusible polymers, which can be moulded into a mould by means of injection moulding.
An advantage of thermoplastics? The material gives the possibility to reuse, also called recycling. By reheating, the material can be brought into other desired shapes. This can also be helpful for Design For Disassembly (DFD). Think of thermoplastics like foil, bottles, clothing, Styrofoam and many other products
A thermoset is a material that, compared to the thermoplastic, can only form once, after which it remains hard when heated. The cause? The cross-links; cross-links, between the individual chains. The polymers form into a network polymer and often decompose before they melt. Whereas with a thermoplastic the material can still be transformed after processing, the thermoset after processing is not. The dense network structure keeps the polymer chains together. The dense network structure ensures a different processing than with a thermoplastic. Think of products such as power sockets or switches, you want them to remain solid under all circumstances.
The thermoset does not melt, but decomposes without becoming liquid, which makes thermosets difficult to reuse. Thermosets are used less often than thermoplastics because they are harder to produce by injection moulding and are difficult to recycle.