Flow and solidification of semi-crystalline polymer during micro-injection molding

This work investigates flow and solidification characteristics of semi-crystalline thermoplastic polymer melt during the micro injection molding process. When the melt flow enters micro-cavities and solidifies, it has been observed that micro-balls are formed on the flow front and coalesced with each other creating many micro-weld lines. Consequently, a characteristic pattern is observed by optical microscopy. This work has examined the mechanism of their creations experimentally as well as numerically. It has been hypothesized based on the experimental results that the onset of this phenomenon is initiated by nucleation in the micro-cavities. For the same micro-cavity, both injection molding and imprinting have been conducted and the shape and the morphologies of the molded parts have been analyzed. Based on observation of the morphologies, this work concludes that nucleation is due to high shear rate while entering the micro-cavity and the balls are formed by the growth of the spherulites. Moreover, the validity of the hypothesis has been verified by the numerical simulation of the ball formation process. The numerical simulation has reproduced the geometric shape found in the experiments. This result supports the proposed hypothesis.