Influence of thermomechanical conditions on structure development and mechanical properties of polyethylene mouldings produced using different moulding methods

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 investigates these phenomena, through the moulding of polyethylene using different methods and the subsequent testing of the mechanical properties of the products. A unified approach is presented to describe the behaviour of the products based on knowledge of the thermomechanical conditions imposed during processing. A linear medium density polyethylene was 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 moulded parts were mechanically tested in order 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. The mechanical tests were complemented with density measurements, to assess the crystallinity across the wall thickness, and polarised light microscopy, to characterise the mouldings' microstructure. 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.