Investigation of the influence of vacuum venting on mould surface temperature in micro injection moulding

The application of vacuum venting for the removal of air from mould cavity has been introduced in injection moulding with the intent to enhance micro/nano features replication and definition. The technique is adopted to remove air pockets trapped in the micro-features, which are out of reach for conventional venting technologies and can create considerable resistance to the melt filling flow. Nonetheless, several studies have revealed a negative effect on replication that could possibly arise from the application of vacuum venting. Although the incomplete filling of micro-scale features has often been attributed to poor venting, the limited research examining the application of vacuum venting has produced mixed results. In this work, the effect of air evacuation was experimentally investigated, monitoring mould and polymer temperature evolution during the micro injection moulding process by means of a high-speed infrared camera and a sapphire window, which forms part of the mould wall. The results show that air evacuation removes a mould surface heating effect caused by rapid compression of the air ahead of the flow front and subsequent conduction of that heat into the mould surface. Hence, with the increase of the surface-to-volume ratio in micro-cavities, air evacuation has a detrimental effect on the cavity filling with polymers that are sensitive to changes of the mould temperature.