Simulation / optimization in reactive in-mold coating

In Mold Coating (IMC) has been applied to Sheet Molding Compound (SMC) as an environmentally friendly alternative to priming to make the surface conductive; for subsequent electrostatic painting operations [1, 2]. Due to its successful application to exterior body panels made from compression molded SMC, the application of In Mold Coating for injection molded thermoplastic parts is being developed. The coating is a thermoset liquid that when injected onto the surface of the part cures and bonds to provide a smooth conductive surface. When used as part of the injection molding process, the coating is injected during the thermoplastic cooling stage and flows by compressing the thermoplastic surface. Predicting coating pressures is critical for the success of the process as if the pressures exceed the clamping pressure the coating will leak and not cover the part [3]. To model the flow of the coating, the chemical reaction can be neglected as the typical coating materials react via free radical polymerization and enough inhibitor is used to avoid reaction during flow [4]. A viscosity change with reaction time for a typical coating material is shown in Figure 1. In order to make the coating conductive, the filler used in IMC is carbon black (CB). The more CB, we use, the more conductive the coating [5], as shown in Figure 2, which improves the paint transfer efficiency. However, the coating viscosity is greatly increased with the amount of CB as shown in Figure 3. As a consequence, using more than 6%, will make coating unfeasible due to injection pressures being too high, to keep the mold fully closed and avoid coating leakage. For a simple rectangular part, the coating flow from a line injection port can be approximated as a one-dimensional flow as shown schematically in Figure 4. The relationship between the pressure gradient and the flow rate for any given location x is given by [6, 7]: ⁄