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

The properties of moulded plastic products are dependent on the processing technology used in their manufacture and in particular on the structural mo rphology resulting from the thermomechanical environment imposed on the mel t. This paper investigates these phenomena, through the moulding of polyeth ylene 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 condi tions imposed during processing. A linear medium density polyethylene was p rocessed using rotational moulding, compression moulding, and injection mou lding in order to achieve different thermomechanical conditions (i.e. shear rates and cooling rates). The processing conditions used were typical of t hose in common use in the respective industries. The moulded parts were mec hanically tested in order to determine the tensile, flexural, and impact pr operties. These measurements were performed both on samples corresponding t o the entire thickness of the moulding and on slices taken from across the section of the mouldings. The mechanical tests were complemented with densi ty 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 t he thermomechanical conditions imposed during processing is similar For the three moulding techniques used.