This study employed the Taguchi method, analysis of variance, and response surface methodology for plastic gear rack injection molding parameters followed by a factorial quality validation. This study was expected to reduce the time cost of mold design and injection molding by making different combinations of the molding parameters, designing an experimental method, and performing the data simulation experiment by computer-aided engineering (CAE). With the research tool of polymer (polyacetal) for plastic material, computer-aided design mold design, and CAE mold flow analysis software, a numerical analysis of plastic molding flow was conducted. Taguchi L16 (45) orthogonal array designed 16 experimental combinations including injection molding conditions of filling time, holding pressure, holding time, plastic temperature, and mold temperature. The experimental results of molding analysis of software (Moldex3D) determined the optimum molding essentials of plastic injection: filling time 0.2 s, holding pressure 98 MPa, plastic temperature 195 °C, and mold temperature 65 °C. In this study, the parameters of the response surface method were used for the actual injection verification. The CAE simulation software can greatly improve the mold design and injection molding parameter testing time to enhance the overall working efficiency and cost control.