Morphology of microinjection moulded polyoxymethylene

Abstract

A microinjection moulding machine was used to obtain micromouldings of polyoxymethylene, in order to study morphology development during the process. The method of design of experiments was used to investigate statistically the effects of processing variables on the microstructural features of the mouldings. The morphological features were identified by microtoming the samples in both transverse and longitudinal (flow) directions and observing the microtomed sections under a polarised light microscope. Morphology evolution along the flow direction was followed by microtoming the specimens along the centre plane longitudinally and sequentially. A five-layer skin core structure was identified for micromoulded polyoxymethylene. The development of the structure was explained, based on mechanisms which were similar to those proposed for conventional injection moulding. Injection speed was found to be the most significant factor affecting morphological features of the final moulding. Moreover, the average plunger velocity, which is directly related to the cavity filling flow rate, was found to have good correlation with skin layer thickness. The distributions of crystalline polymorphs were observed and explained, in light of the distributions of the flow and thermal patterns in the mould. Morphology evolution along the flow direction reflected the distribution of pressure, temperature and velocity of the polymer melt during the microinjection moulding process. The results provided some indications regarding micromoulding mould design.