The structure and mechanical properties of an injection-moulded acetal copolymer

Abstract

Processing conditions, microstructure and mechanical property correlations have been explored in injection-moulded plaques of an acetal copolymer. Barrel temperature was varied systematically between 453 and 503 K, with a constant mould temperature of 343 K. The microstructure and texture were determined by polarized light microscopy and X-ray pole-figure analysis, respectively. The overall structure of the mouldings was layered through the thickness and symetrical about the moulding centre line. At all barrel temperatures five layers were present: the outer three layers possessed significant preferred chain-axis orientation in the crystalline phase, while the two layers at the centre of the moulding were equiaxed. The texture and morphology of each layer is described and related to a model of mould filling. Increases in the barrel temperature reduced the extent of the outer oriented layers while increasing the extent of the equiaxed layers. Tensile tests were conducted on samples taken at 0° and 90° to the injection direction. Increases in barrel temperature had no influence on modulus but decreased both (α=0 and 90°) engineering yield stresses. The yield stress could be correlated with the extent of the oriented layers within mouldings. At all barrel temperatures the yield stress was greater when α=90°; this behaviour is explained in terms of a composite model.