Fibre orientation and mechanical behaviour in reinforced thermoplastic injection mouldings

Abstract

Process variables, fibre orientation distribution and mechanical properties were inter-related for injection-moulded short-glass fibre-reinforced polypropylene and polyamide and long-glass fibre-reinforced polyamide. The properties of the reinforced grades were also contrasted with those of the base polymer. A rectangular mould with triple pin-edge gates on the same side to facilitate a single melt flow-front or double flow-front advancing adjacently, was employed. Mouldings were evaluated for fibre orientation distribution, and tensile, dynamic mechanical and fracture properties. The relative magnitudes of the shell and the core fibre-orientation persuasions depended on the melt and the mould temperatures, and the injection ram speed. An increase in G c, K c, ultimate tensile strength (UTS) and elastic moduli values and a decrease in tan δ values were observed with fibre reinforcement. The properties showed marked sensitivity to the position of the specimens in the moulding because of the associated variations in the fibre orientation. The prediction of UTS based on a rule of mixture relationship for strength and the Halpin-Tsai equations for elastic moduli had limited success. The predictions were improved by employing measured data representative of regions of high fibre orientation, e.g. knit-line. The estimation was weakened in the material systems containing significant production-induced voids. An inference of the fracture toughness values was that the composites contained flows of the order of 0.2 mm in size, which is conceivable, either as voids or as defects introduced in the machining of the specimens.