Two categories of plastics are identified. Those are: thermosetting plastics and thermoplastics. Plastics generally cannot stand higher temperatures. They become lose and soft their shapes on higher temperatures. Thermoplastics become raising soft or their capability to deform plastically raises as the temperature rises. They can be specified any shape by moulding any number of times. Conversely thermosetting plastics do not soften at higher temperature but may eventually burn or disintegrate and lose their shape. Hence thermosetting plastics can be moulded merely once. Both the plastics can be cut or machined.
Structure of both plastics being polymeric; have of long chain molecules that are tangled as illustrated in following figure. Thermoplastics either carry extremely week cross linking bonds or no cross linkages among chains. Alternatively thermosetting plastics carry strong cross linking bonds and several unfinished polymeric reaction. At higher temperatures such unfinished polymeric reactions are finished and cross linking assists in distortion quite than melting.
Figure: Two Types of Plastics