The term "polymer" describes classes of molecules with large numbers of repeating structural units connected through covalent bonds. The major identifying feature distinguishing polymers from other molecules is the repetition of many similar, identical, or complementary subunits.

Most manufacturers decide upon polyethylene as the polymer of choice for the plastic moulding process because of its availability, ease of use, and suitable properties. According to recent reports 80% - 90% of all polymers used in the plastic moulding industry are polyethylene compounds (HDPE, LPDE and LLPDE). Although PVC, nylons, and polypropylene compounds are also used.

Nylons
Polyethylene
Polypropylene
PVC
A paper written by J.D. Ratzlaff of Chevron Phillips Chemical Company LP in 2004 entitled "Polyethylene: Process Sensitivity in Rotational Moulding" presents the results of a study of the impact sensitivity of polyethylene to processing conditions and discusses methods to maintain high impact standards.