Explanation:

A thermoset is a material (such as rubber) that cures or hardens into a given shape, generally through the application of heat.
Curing is an irreversible chemical reaction in which permanent connections (known as cross- links) are made between the material’s molecular chains.
These cross-links give the cured polymer a three-dimensional structure, as well as a higher degree of rigidity than it possessed prior to curing.
A cured thermoset material will not remelt or otherwise regain the processibility it had before being cured. Curing changes the material forever. Thermoset polymers outperform thermoplastics in a number of areas, including mechanical properties, chemical resistance, thermal stability, and overall durability. For these reasons, thermoset parts tend to make more effective seals.
A thermoplastic material softens (becomes pliable and plastic) when heated, but it does not cure or set. A thermoplastic often begins in pellet form, then becomes softer and more fluid as heat increases. This fluidity allows it to be injected under pressure from a heated cavity into a cool mold.
As it cools, the thermoplastic will harden in the shape of the mold, but there is no chemical curing at work. No cross-links are formed as with a thermoset material.
The changes seen in the thermoplastic are purely physical and, with the reapplication of heat, wholly reversible.