Numerical simulation of the in-mold coating process for injection molded thermoplastic parts

During In-Mold Coating (IMC), a liquid material is injected while the thermoplastic substrate is still in the mold. IMC can be used as a topcoat to either protect the part from outdoor exposure, or to improve its appearance to automotive levels. It can also be used as primer for subsequent painting. This paper first presents a one-dimensional mathematical model for the flow of IMC. The corresponding computer code has been developed and the numerical results have been compared with analytical solutions obtained by assuming a linear relationship between the substrate deformation (coating thickness) and the coating injection pressure. Secondly, a two-dimensional mathematical model for IMC flow, using the Hele-Shaw model and assuming the coating to be a Newtonian fluid, is presented. A computer code based on the CV/FEM method has been developed. The compressibility of the substrate is taken into account by introducing the PVT relationship of the substrate. The predictions of the two-dimensional model are verified with the analytical solution for the case of one-dimensional flow. A limited comparison to experimentally obtained short shots is presented.