Influence of high injection pressures on the internal stress level in injection moulded specimens

Abstract

Internal stresses in high and low density polyethylene specimens, injection moulded at pressures ranging from 100 MPa to 450 MPa have been measured by a stress relaxation method. The internal stress parameter ( σ i ), which is an average value of the internal stress distribution in the samples, changes from a negative value (compressive stresses) at normal injection pressures to a small positive value (frozen-in tensile stresses) at the highest pressures used. The yield stress increases in approximately the same way with the pressure, while the mould shrinkage in the flow direction decreases. It is suggested that the decrease in the absolute value of the internal stress parameter originates from an increase of the melting temperature with pressure, resulting in a more homogeneous solidification.