Injection molded part material failure analysis

Abstract

In this study, the causes of the failure of an injection-molded polyamide part were analyzed. The working conditions of injection-molded parts are very complex depending on: pressure, temperature, friction, corrosion, etc. and, consequently, they could fail due to different reasons. Microscopic characterization, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and FTIR spectroscopy were used for fracture analysis of defected parts. Mechanical testing (flexural modulus, torque and notched Izod impact test) was also considered for part failure analysis. This analysis suggests that the parts have failed because they were overloaded during assembly operation, and their material was harder (in a transition state between ductile and brittle) than the parts from normal production (in ductile state). Our investigations revealed that, due to plastic deformation at room temperature, Nylon 6 presents an unusual phase transformation. There are similarities between austenitic stainless steel and Nylon 6. Thus, it consists in the fact that their γ forms transform into α forms, due to an applied heat treatment or at room temperature as a result of an applied pressure (plastic deformation). In both situations, material became harder and then less resistant to applied physico-mechanical actions. In this particular case, an unusual local material hardening was observed due to a combined influence of two processes (cooling process, torque test), with a failure of the molded parts.