Automotive industry faced the intense competition have been tried to lower product prices as well as satisfy performance in order to secure demands. Various ways to reduce manufacturing cost in this market environment have been approached, especially shortening manufacturing time has been mainly proposed. Because traditionally shortening manufacturing time activities took place in production sites, so they were not systematical and limited so the effect was less than expected. To overcome this, these activities should extend to development stage of product. Recently many plastic products have been widely used in automobile parts for light weight to improve fuel efficiency. The manufacturing process of such as plastic parts consists of injection molding and assembling parts. The process of injection molding process takes longer than other assembling if the product is big or complicated. So reducing the injection molding cycle time is could be the best way to reduce the manufacturing time. Reducing the cycle time is affected by the machine performance such as injection machine and takeout robot but they are related to investment so we exclude in this study. Therefore we should focus on the injection process itself from flow to cooling and optimize the process condition. The optimization of injection molding could be effective before mold tooling so we introduced CAE for pre-validation. This study shows which factors affect cycle time and the optimization process ad result and then compare between CAE and real of manufacturing process and production.