Prediction of mechanical properties of polyethylene mouldings based on laminate theory and thermomechanical indices

Abstract

The properties of moulded plastic products are dependent on the processing technology used in their manufacture and in particular on the structural morphology resulting from the thermomechanical environment imposed on the melt. This paper presents a unified approach to describe the behaviour of the products based on knowledge of the thermomechanical conditions imposed during processing. A linear medium density polyethylene has been processed using rotational moulding, compression moulding, and injection moulding in order to achieve different thermomechanical conditions (i.e. shear rates and cooling rates). The processing conditions used were typical of those in common use in the respective industries. The moulding parts were mechanically tested to determine the tensile, flexural, and impact properties. These measurements were performed both on samples corresponding to the entire thickness of the moulding and on slices taken from across the section of the mouldings. On the basis of these measurements, two models were developed. One is based on laminate theory, in which, from a knowledge of the mechanical properties of the individual layers through the wall thickness, it is possible to predict the tensile and flexural properties of the full thickness moulding. The other is an empirical model that predicts the tensile modulus of a plastic part as a function of two thermomechanical indices. It is shown that the type of dependence of the mechanical performance on the thermomechanical conditions imposed during processing is similar for the three moulding techniques used. A good agreement is achieved between the experimental data and those predicted by the thermomechanical model. It is also shown that via the combined use of the thermomechanical indices concept and the laminate analysis, good predictions of the mechanical behaviour of plastic mouldings with complex microstructures can be achieved. It is proposed that this approach could provide a very valuable addition to existing melt flow simulation packages. This would enable not only processing conditions to be optimised but the properties of the end product could be predicted.