A thermosetting polymer is conventionally made from a resin which acts as a type of prepolymer that is in a viscous state. Such a plastic resin can be altered irreversibly through a process of curing into a durable and insoluble polymer which means that it is much used in moulding applications. Essentially, the resin is heated under high pressure and goes hard. Sometimes referred to as a thermoset, this type of heat-induced polymer setting is a mainstay of the plastics industry.
During the curing process, the individual chains of a resin prepolymer are chain-linked at a molecular level to make long polymers. It is the heat energy that allows this process to take place, but some plastic manufacturing plants also make use of chemical catalysts to assist the procedure. Forming a three-dimensional structure, the polymer chains of thermosets will take on the shape of any mould they have been poured or injected into. However, once cured they cannot be melted again to be reused in another mould.
Reactive Injection Moulding
Used to make objects like storage crates and milk containers, reactive injection moulding is a plastics process that typically makes use of thermosetting polymers. Under this system, the prepolymer is injected into the mould by an impinging mixer which forces it in under high pressure. The liquid material is then simply allowed to sit in the mould to cure. With reactive injection moulding, the material will usually expand to fill up the mould as it polymerises. Nylon 6, polyurethane and polyester are all common materials that are used in this type of injection moulding.
This moulding application tends to use thermosetting resins that have been partially cured already, usually in the form of putty or granules. The material tends to be poured into an open mould under heat, rather than high pressure and is used to make things like rubber boots. Only when the heat has reached a sufficient level in all parts of the mould will the material have cured properly. Unlike reactive injection moulding, the mould must be designed to cool evenly or the product may be defective. Furthermore, sections of material which are much thinner or more intricate than others in the mould may heat and cool at different rates, making some types of moulds unsuited to this particular process.
Thermosetting polymers are also used in some plastic extrusion processes to make things like pipes. They can also be put to use with certain spin casting methods, for example, to manufacture items like toys and figurines.