Thermosetting polymers: Thermosetting polymers are permanently setting polymers. These polymers are cross linked or heavily branched molecules which on heating in a mould, get hardened and set and cannot be softened again. This hardening on heating is due to cross-linking between different polymer chains to give a three dimensional network solid. Its example includes bakelite.

Thermoplastic polymers: Thermoplastics are linear or slightly branched polymers. They can be repeatedly softened on heating and hardened on cooling and hence can be used again and again without any change in chemical composition and mechanical strength. Its examples include polythene and polypropene.