A thermosetting resin is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing is induced by the action of heat or suitable radiation, often under high pressure. A cured thermosetting resin is called a thermoset or a thermosetting plastic/ polymer.
The curing process transforms the resin into a plastic or rubber by cross-linking individual chains of the polymer. The cross-linking is facilitated by energy and catalysts at chemically active sites, which may be unsaturated sites or epoxy sites, for example, linking into a rigid, three-dimensional structure. This yields molecules with a large molecular weight, resulting in a material that usually decomposes before melting. Therefore, a thermoset cannot be melted and re-shaped after it is cured. This implies that thermosets cannot be recycled for the same purpose, except as filler material.
Some methods of molding thermosets are:
- Reactive injection moulding (used for objects such as milk bottle crates)
- Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)
- Compression molding (used to shape most thermosetting plastics)
- Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)
Thermosetting plastics are generally stronger than thermoplastic materials due to the three-dimensional network of bonds (cross-linking), and are also better suited to high-temperature applications up to the decomposition temperature. However, they are more brittle.
- Polyester fiberglass systems: sheet molding compounds and bulk molding compounds
- Polyurethanes: insulating foams, mattresses, coatings, adhesives, car parts, print rollers, shoe soles, flooring, synthetic fibers, etc. Polyurethane polymers are formed by combining two bi- or higher functional monomers/oligomers. This common type of thermoset material has also recently shown to have transient properties and can thus be reprocessed or recycled.
- Vulcanized rubber
- Bakelite, a phenol-formaldehyde resin used in electrical insulators and plasticware
- Duroplast, light but strong material, similar to Bakelite used for making car parts
- Urea-formaldehyde foam used in plywood, particleboard and medium-density fiberboard
- Melamine resin used on worktop surfaces
- Diallyl-phthalate (DAP) used in high temperature and mil-spec electrical connectors and other components. Usually glass filled.
- Epoxy resin  used as the matrix component in many fiber reinforced plastics such as glass-reinforced plastic and graphite-reinforced plastic
- Polyimides used in printed circuit boards and in body parts of modern aircraft
- Cyanate esters or polycyanurates for electronics applications with need for dielectric properties and high glass temperature requirements in composites
- Mold or mold runners (the black plastic part in integrated circuits or semiconductors)
- Polyester resins
- The Open University (UK), 2000. T838 Design and Manufacture with Polymers: Introduction to Polymers, page 9. Milton Keynes: The Open University
- Fortman, David J.; Jacob P. Brutman; Christopher J. Cramer; Marc A. Hillmyer; William R. Dichtel (2015). "Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers". Journal of the American Chemical Society. doi:10.1021/jacs.5b08084.
- Roberto C. Dante, Diego A. Santamaría and Jesús Martín Gil (2009). "Crosslinking and thermal stability of thermosets based on novolak and melamine". Journal of Applied Polymer Science. 114 (6): 4059–4065. doi:10.1002/app.31114.
- Enrique Guzman; Joël Cugnoni; Thomas Gmür (2014). "Multi-factorial models of a carbon fibre/epoxy composite subjected to accelerated environmental ageing". Composite Structures. 111 (4): 179–192. doi:10.1016/j.compstruct.2013.12.028.