Impact of deep cores surface topography generated by micro milling on the demolding force in micro injection molding

In micro injection molding the quality of 3D complex parts is influenced by the efficiency of the ejection phase. During demolding, the forces taking place at the component-tool interface, due to adhesion and friction, need to be overcome preserving the integrity of the part. This issue is severe in the case of molds characterized by the presence of several deep cores, which are used to manufacture interconnecting through holes in multi-layer microfluidic devices. In this work, the impact of the micro milling cutting strategy on the demolding forces was investigated, using a critical cavity geometry, specifically designed to this purpose. The relation between mold micro manufacturing and the micro injection molding process was studied with the aim of optimizing the demolding phase. The topographies of micro-milled mold surfaces and the molded parts were characterized and different roughness profile parameters were taken in consideration. The results of in-line force acquisitions indicated that the effects of the micro milling strategy on the demolding force is markedly higher than those of micro injection molding process variables. Moreover, the experimental analysis indicated that a combination of worst surface finishing and low viscosity of the molding polymer can result in higher interface interlocking and thus in critical stresses applied to the part during the demolding phase.