Thermosetting polymer
The curing process transforms the resin into a plastic or rubber by a cross-linking process.  Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking forms a molecule with a larger molecular weight, resulting in a material with a higher melting point.  During the reaction, the polymer's molecular weight increases to the point that its melting point is higher than the surrounding ambient temperature. So, the material forms into a solid material.