The differences between residual stress formation mechanisms are studied for several important polymer processes. The most important cases are free quenching (e.g. in extrusion), constrained quenching and constrained quenching with varying applied pressure (injection moulding). A simple, but physically relevant, elastic solid model is assumed (viscous melt and purely elastic solid) yielding Struik＇s cooling stress model for the free quenching case. In the constrained quenching case, however, the residual stresses are found to vanish. This allowed us to derive a simple analytical expression for the residual stresses in injection moulded products. These stresses were found to be proportional to the maximum melt pressure and are tensile near the surfaces. The predictions compared well with the measured stress distribution. Finally the effects of residual stresses with respect to warpage and environmental stress cracking are discussed.