Why can thermoplastics be heated and reshaped repeatedly?

Thermoplastics can be heated and reshaped repeatedly because of their unique molecular structure. Unlike thermosets, which form an infusible, insoluble network through chemical cross-linking during initial processing, thermoplastics consist of individual polymer molecules that remain separate from one another.1

This structure allows the molecules to flow past each other when heated above their glass transition temperature or melting temperature, granting the material mobility and the ability to be molded or extruded. Because these molecules do not form a permanent, interconnected network, the material softens upon reheating and hardens again upon cooling, enabling it to be reprocessed into new forms multiple times.213