The local thermomechanical conditions and the fracture behaviour of an injection moulded poly(Oxymethylene)

Abstract

This work explores the fracture behavior of an injection-molded engineering thermoplastic, poly(oxymethylene), POM. Lateral gated discs of 120 mm diameter and 1.5 mm thick were molded with variations of the melt temperature and injection flow rate. The local thermomechanical environment was characterized by computer simulations of mould filling phase. This also allows the computation of two thermomechanical indices related to the morphological state of the moldings. From the molded discs, double edge notched tensile (DENT) specimens were meticulously cut at different angles with respect to the flow direction and with distinct ligament lengths. The fracture tensile tests were performed at 2 mm/min at controlled room temperature. As the test configuration (specimen geometry) imposes a plane stress state, the stress intensity factor, K, was calculated as function of the processing conditions and orientation angle with respect to the flow direction. The K, values and their degree of anisotropy are dependent upon processing conditions. Finally, the dependences of K-1 values upon the thermomechanical indices (and therefore on the expected morphological state of the moldings) are established.