Precision injection moulding of micro components: Determination of heat transfer coefficient and precision process simulation
Quanliang Su, Nan Zhang, Michael D. Gilchrist
Abstract
In polymer melt processing, the heat transfer coefficient (HTC) determines heat flux across the polymer/mould interface for both conventional and micro injection moulding (µIM), and is a key parameter in mould design and predictive computational simulations. However, values typically used in commercial software for injection moulding are usually obtained from large-scale experiments, which may be not appropriate for µIM. The present work introduces a novel experimental method for precisely determining the actual averaged HTC values in µIM. This method uses changeable mould inserts equipped with temperature and pressure sensors, positioned exactly at the same mould cavity location rather than at adjacent positions. The calculated HTC values obtained through this method are used subsequently in the simulation of µIM instead of software default values, following optimization of boundary conditions and mesh. This constitutes a significant addition to current knowledge in the field. The influence of injection velocity, packing pressure and mould temperature on HTC is studied. Experimentally calculated HTC values for the packing and cooling stages are approximately 8500W/m2K and 6300W/m2K, respectively. Based on process monitoring from short shot experiments, subsequent simulation of a 600µm thick part is predicted successfully using multi-scale finite element modelling. Wall slip and venting parameters are optimized using a one-factor-at-a-time method; a friction coefficient of 0 and no venting are found to be the optimal parameters for modelling wall slip and venting. Both are critical in simulations, although wall slip has a greater influence than venting on the simulation accuracy. The default and calculated HTC values are used in the simulations for comparison, while the simulations using the calculated HTC values were validated as being effective and accurate.