Why cannot thermosetting plastics be remolded after they have been heated?

Thermosetting plastics cannot be remolded after they have been heated because of their unique molecular structure. During their initial processing, these materials undergo a chemical reaction that creates a permanent, infusible, and insoluble network.12

Unlike thermoplastics, which consist of separate molecules that can flow past one another when heated, thermosets are characterized by cross-linking—a process where polymer chains are chemically tied together to form one large, interconnected network. This network structure prevents the molecules from moving independently, which precludes the possibility of flow. Consequently, thermosets do not melt upon reheating; instead, the application of further heat typically causes the material to undergo chemical degradation and break down.1345