Evaluation of mold surface roughness and wettability in micro injection molding of thin cavities

Injection molding of micro components is more and more an emerging technology for its ability to manufacture low cost and high repeatable micro polymeric parts relevant to many different fields, from IT to healthcare, to medicine. In particular, micro injection molding of thin cavities is an interesting challenge due to the large surface to volume ratio that characterizes the process. This condition emphasizes the heat transfer at the polymer/mold interface, especially when the depth of the mold cavity becomes very thin. Furthermore, the boundary conditions, neglected in macro scale, affect the process and hence the mold surfaces show high influence during products filling and not only on their final quality. Thus, the mold roughness should be considered in evaluating the polymer flow behaviour in filling micro thin cavities. In this work, three inserts were designed to evaluate roughness contribution during molding and were manufactured by micro electrical discharge machining process obtaining three thin cavities (depth of 100µm and different roughness values). These inserts were used for molding polymeric micro plates and the flow lengths, reached inside these parts, were measured as process quality response. The results show that the cavity roughness affects the micro injection process favouring the filling of the thin cavities. In fact, the samples, molded with the insert having the highest roughness, show the longest flow length. Finally, wettability measurements were performed on the mold inserts and also these results sustain the occurred relation suggesting a decrease of the heat transfer in the process at the increase of mold surface roughness.