The term “thermosetting polymers” refers to cross-linked or heavily branched polymers that harden during the molding process. They cannot be softened any further by heating. For example, Bakelite is made up of urea-formaldehyde resins.

“Thermoplastic polymers” are long-chain linear polymers with a few branching structures that can be softened and hardened repeatedly upon heated. Examples are polythene, polypropylene, etc.