Crystallinity degree of parts manufactured in multicavity injection mould

Purpose: Crystallinity degree of injection moulded parts made from polyoxymethylene was tested. The parts came from a mulitcavity injection mould with geometrically balanced runners. The purpose of research was to check if there are differences in crystallinity degree between parts from different cavities. Methodology DSC (Dynamic Scanning Calorimetry) tests were made and according to the obtained DSC curves crystallinity degree value was calculated for injection moulded parts. Findings: The results of research made show that the parts from particular mould cavities differ in crystallinity degree and the values are affected by thermal conditions which are different across the mould and, of course, for each cavity too. Research limitations: In this research only one multicavity injection mould was tested. It has a specific layout of runners and cavities and cooling channels that determine the polymer flow in the mould and other conditions like mould temperature distribution. It is usually supposed that the parts placed in areas of higher temperature in the mould will have higher crystallinity degree values. However, each mould requires separate analysis as it is built in another way and the extrapolation of the results of this research to other injection moulds can lead to errors. Practical implications: The results of this investigation can be useful for mould designers. The occurrence of different properties of parts from multicavity injection moulds is especially important in case of precision injection moulding, particularly for small parts. The way to minimize the problem with unequal part properties like crystallinity degree (which results in different mechanical properties) is minimizing the temperature differences in the mould. Originality The research made in last years by other scientists show that even when using geometrically balanced runners in the mould the problems with unequally filling cavities can occur, especially due to shearing phenomena in runners. This leads to differences in weight of parts. The work was mostly focused on the problems with filling imbalance and phenomena in runners or showing different weight of the parts. In this paper it was shown other important consequences of filling imbalance in the mould.