The document describes a study that used Taguchi's design of experiments methodology to optimize the process parameters of plastic injection molding for polypropylene parts. Response surface methodology was used to analyze the effect of melt temperature, injection pressure, holding pressure, and cooling time on cycle time and tensile strength. Regression equations developed for 6 different part sizes showed that melt temperature and cooling time had the greatest influence on cycle time, while melt temperature was most influential for tensile strength. Residual plots indicated the developed models fit the data well. Optimization of parameters was performed to minimize cycle time and maximize tensile strength.