Study of ejection forces in micro injection molding as a function of mold surface finish obtainable by micro milling

The development and manufacturing of ejection systems in the micro injection molding process can be a complex task, particularly when the parts have several micro holes, as in the case of multi layer microfluidic devices. During the injection of such parts, the shrinkage is constrained by the mold. Since the moldings shrink against the core, inserts or pins, it is necessary to overcome the frictional forces resulting from the shrinkage during ejection. Decreasing the ejection forces significantly contributes to improve ejection systems designs and guarantees the structural integrity of the moldings, especially for micro parts, which are more prone to damage during ejection. In this paper, the relationship between the texture characteristics of the micro milled molds and the ejection forces is analyzed. A particular mold geometry was designed and studied for this purpose, and its effectiveness was experimentally evaluated. The relation between the micro milling process parameters, such as the feed per tooth and the depth of cut, and the achieved surface texture characteristics is pointed out providing a direct link between quality of micro milling process and quality of micro injection process that is not easy to find in the specific literature domain. Some preliminary testing that involved the use of an ultra high precision milling machine, a focus variation optical microscope and a micro injection molding system has been used to assess how the finishing quality of the milled mold affects the ejection forces.