Polymers that get rigid due to curing are called thermosets.
In this process, molecules will crosslink (purple) themselves and form one single makro-molecule.
Because of very strong covalent bonds, the makro-molecule can not be melted again.
Even if high heat is applied, the only result will be degradation.
In contrast to a thermoplast material, the mold for thermoset materials is heated up.
Typical thermoset polymers are phenolics, epoxies and resins.
![Thermoset in Plastic Industry Thermoset in Plastic Industry](https://www.plexpert.ca/wp-content/uploads/2018/12/thermoset.jpg)
Plastic: Thermoset materials build very strong covalent bonds.
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This glossary of plastic industry is provided by PLEXPERT Canada Inc.