Surface footprint in molds micromilling and effect on part demoldability in micro injection molding

Accuracy of micromilled molds play an important role in complex process chains enabling mass production of polymer micro components, such as lab-on-chips, fabricated by micro injection molding. Surface footprint of micromilling is defined as the technological signature left by machining process on the generated mold surface. This is sensitive to selected tools and machining parameters and, when not controlled properly, can badly affect mold topography and functionality (e.g. part demoldability). In case of complex mold geometry, the impact of micromilling footprint increases, in particular during the demolding phase due to the friction generated by the polymer shrinking around cores. This work studies these effects on molds characterized by sub-millimetric cylindrical cores. A physical and statistical modeling was developed to provide deep insights about the effects of milling strategies and cutting parameters on the generated footprint on the mold cores. These effects are investigated by machining cylindrical pins whose roughness and surface form errors, caused by static deflection of tool and parts, were controlled in the range of Sa = 150–400 μm and ΔRmax = 1–10 μm (profile radial deviation), respectively. Micro injection molding experiments proved that mold topography has a relevant effect on the ejection force. The demolding force generated by a specifically developed polystyrene micro part reached the highest value with the mold machined with the most unfavorable milling conditions. Proper controlling of machine parameters and conditions led to a reduction greater than 60% of the demolding force peak, confirming the feasibility of the conjunct approach to processes optimization. The results of this work move a step forward into the integrated optimization of micro manufacturing process chains.