Effect of injection pressure and crazing on internal stresses in injection-moulded polystyrene

Abstract

The influence of injection pressure on the internal stress distribution in injection-moulded polystyrene bars has been found to be rather small using the layer removal method. On the other hand, substantial differences in the stress relaxation behaviour have been discovered in specimens moulded at pressures in the range 37–143 MPa. The Kubát and Rigdahl internal stress parameter for all sets of specimens was numerically fairly small, but changed from negative to positive on increasing the injection pressure. The parameter most sensitive to variations in injection pressure appears to be the index in the power law expression used to describe the stress relaxation behaviour. Specimens were surface-crazed by bending around a cylindrical former and tested by the same techniques. A further increase in the magnitude of stress at all positions was indicated by the layer removal procedure while the gradients of the Kubát and Rigdahl plots from stress relaxation data were unchanged.