There are two types of materials (resins) which can be used in the injection molding industry; Thermoplastic and Thermoset.
A thermoplastic, or thermosoftening plastic, is a plastic material, a polymer, that becomes pliable or moldable above a specific temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight.
A thermoset also called a thermosetting plastic, is a plastic that is irreversibly cured of a soft solid or viscous liquid, prepolymer or resin.
Figure source: http://www.bbc.co.uk/guides/z9tysg8
Resin selection is a very critical step for the injection molding. There are many material suppliers offering quality materials for the injection molding.
They can formulate a plastic to meet the consumer needs properly. For example, the applications of the product play a key role in the choosing material properties. For High voltage connector, the resin has to provide chemical resistance, excellent tracking defense and heat& humidity resilience. For a Fluid Engineering Components, the resin has to have a good hydrolytic stability, excellent resistance to water and FDA approved. The formulated resin has the processability and consistent properties after molding process and in the final use.