Analysis of the effect of draft angle and surface roughness on ejection forces in micro injection molding

Minimizing ejection forces is a crucial challenge in micro injection molding to prevent component damage during demolding. This research investigates the effects and interactions of draft angle, surface roughness, mold temperature, and holding pressure on the ejection force in a small, box-shaped component made from polypropylene (PP) and cyclic olefin copolymer (COC). A piezoelectric force sensor, integrated into the ejection tray of the mold, was used to measure the peak ejection force. The results indicate a significant influence of polymer type on ejection force, with PP exhibiting a 26% lower peak ejection force than COC. The draft angle consistently reduced demolding forces. Strong interactions were observed between mold temperature, surface roughness, and polymer type. Specifically, an increase in mold temperature led to an 88% increase in ejection force for COC, while resulting in a 63% decrease for PP. For PP, the optimal ejection force was measured at a surface roughness (Sa) of 0.095 mu m, while for COC, a continuous decrease in ejection force was measured with decreasing surface roughness.