Mechanism of bulge defect formation and its suppression at film-substrate interface in microcellular injection molding process combined with in-mold decoration

Microcellular injection molding process combined with in-mold decoration (MIM/IMD) is a crucial technique for producing lightweight polymer products. However, it faces a significant challenge to achieve strong adhesion between decorative film and polymer substrate. Gas entrapment at the film-substrate interface often results in defects such as bulges and weak interfacial bonding. To address these defects, this study examines the dynamic behavior of gas cells during MIM/IMD processing and proposes a strategy to mitigate bulges by modifying molding conditions. Specifically, we demonstrate that high melt and mold temperatures reduce the melt viscosity and slows the cooling rate. This facilitates the re-dissolution of trapped gas into the melt at the interface before solidification. Numerical simulations and molding experiments are performed to assess melt viscosity, frozen layer fraction, bulge coverage ratio ( R ), and interfacial peel strength. The results show that re-dissolution of trapped gas is essential for preventing bulges. When the melt temperature is raised from 180 °C to 240 °C and the mold temperature from 35 °C to 80 °C, cell density in the skin layer more than doubled, bulge coverage decreases to nearly zero, and peel strength improves significantly. However, excessive high temperature causes large cell sizes, small cell density, long molding cycles, and weak mechanical properties. This method effectively reduces interfacial bulges and offers a feasible approach for optimizing MIM/IMD manufacturing processes.