Minimum "Safe Cycle Time": Selecting the Frozen Layer Thickness

Predicting the minimum safe cycle time in injection molding is complicated by the cyclic nature of the process. The mold thermal state changes with continuous molding until a quasi steady state is achieved. In the first part of these series, we presented and experimentally tested a simple approach to predict part and mold surface temperature during continuous molding. In this work, we evaluate the minimum frozen layer thickness required before the part can be demolded and how the growth of this frozen layer is affected as the mold thermal state changes with continuous molding cycles. Experiments are used to relate the minimum frozen layer thickness to dimensional stability. Based on these experiments, statistical tools are used to select an ejection criterion to obtain a minimum safe cooling time. We show that the ejection criteria should be adjusted depending on the precision required from the molded parts. POLYM. ENG. SCL, 49:2320-2328, 2009. (C) 2009 Society of Plastics Engineers