Effects of insert film on asymmetric mold temperature and associated part warpage during in-mold decoration injection molding of PP parts

In mold decoration (IMD) injection molding is a relatively new multi-process technique that has been used for improving the surface quality and achieving colorful cosmetic surface of molded parts. During IMD processing, the film having the same shape as mold cavity is first inserted into the mold then molten polymer is injected into the cavity. Heat transfer in the cavity surface is significantly retarded because of the low thermal conductivity of film. As a result of the asymmetric melt and mold temperature, thermal-induced part warpage easily occurs. The effects of inserted film on the asymmetric mold temperature field of IMD injection molding process for polypropylene (PP) parts were investigated. Experiments were conducted under various conditions of mold temperature, melt temperature and film thickness. The associated part warpage and crystallinity due to the asymmetric mold temperature were examined. It was found that the heat transfer retardation results in a delay in mold temperature drop at film–mold interface of the cavity surface and the maximum temperature difference compared with that of conventional injection molding without film may be as high as 10 °C. The retardation-induced mold temperature difference and part warpage increased with increasing melt temperature and film thickness, whereas they decreased with increasing mold temperature. The crystallinity of molded PP parts increased through insertion of film. The mold temperature field of the IMD process and molded part's warpage were predicted via numerical analyses. The predicted values showed reasonable agreement with the experimental results. Asymmetric cooling system design reduced part warpage when IMD processing was used.