Analysis of Dimple Formation Phenomenon of In-mold Injection Molding Using Polypropylene Decorative Sheet

Injection molding using decorative sheets has recently been drawing increasing attention from the perspective of recyclability. However, dimple defects, which differ from sink marks, occur on the decorative molded surface during the in-mold injection molding process. In this study, we analyzed dimples formed on the surface of decorative sheets by visualization experiments of the molding process. The results confirmed that the correlation between the deformation stress accumulated on the decorative sheet and the inner cavity pressure affects dimple formation. It was also found that when the cavity inner pressure becomes lower than the deformation stress before the injected resin solidifies, the deformation stress decreases, causing the surface to indent and become dimpled in the thickness direction. We also analyzed the same problem in decorative foam injection molding. The results showed that the foaming pressure affects dimple formation, and that the molded product surface will become dimpled if the foaming pressure is lower than the deformation stress of the decorative sheet. We proposed a model on this dimple formation phenomenon based on correlation between the residual pressure of injected resin and accumulated deformation stress on the sheet for both non-foaming and foaming. Based on the model, we proposed a method for reducing dimple formation during in-mold injection molding from the perspective of controlling the inner cavity pressure.