Residual stress distributions in injection mouldings

Abstract

Residual stress distributions in injection moulded polystyrene plaques have been computed using various calculation methods based on procedures from the literature. Some of the mathematical procedures have been extended to provide improved analysis of the process. The results have been compared with measured distributions obtained using the layer removal technique. The purpose of this work was to resolve some of the disagreements between the measured residual stress distributions in injection moulded parts and those predicted by computations made in the literature. The calculations are made using the general purpose software "Mathcad". Various temperature, time and pressure dependent material models have been used to calculate the residual stress and they are compared. Special attention has been paid to choosing boundary conditions that match the moulding parameters used in the manufacture of the injection mouldings on which the measurements were made. Similarly, care has been taken to choose boundary conditions that correspond with the different actual storage times before analysis for the samples and also boundary conditions that correspond with the post-moulding conditioning. Measurements of residual stresses distributions were made on mouldings produced under conditions chosen to simplify the modelling requirements. The sensitivity of the calculations to the materials property data and to the boundary conditions used have been examined. The experimental verification includes examination of the postmoulding changes. The predicted residual stress distributions over the entire moulding and post-moulding history have been found to be in generally good agreement with the corresponding experimental results under various processing conditions and post-moulding changes. In particular, kinematic boundary conditions for the moulding conditions and the postmoulding conditions, due to different temperaturesa nd relaxation times of the polymer, have been found to be critical ingredients in the calculation of the residual stress distributions.