In-mold coating is a reactive fluid designed to improve the surface quality of injection molded thermoplastic substrate in functional and cosmetic properties. In this study, a mixing head for in-mold coating was designed. Based on flow simulation, the design parameters including cleaning piston location and nozzle alignment angle were optimized. The uniform mixing was achieved by the turbulence created by the high velocity impingement of the two component polyurethane inside the head. It was found that the mixing uniformity had a maximum at the cleaning piston location of 1.2mm and the nozzle alignment angle of 120°.