In plastic injection molding, molding is performed while maintaining a constant temperature in the mold by passing cooling water into the mold. If the cooling water does not flow evenly in the mold and cannot maintain a constant temperature, the temperature of the mold will increase, and it will be impossible to mold under constant conditions. In addition, the temperature is kept constant by connecting a tube to the mold, piping, and flowing a cooling medium, but the heat retention temperature differs depending on the material of the molded product. Even if the same aqueous medium is used, if the resin is polycarbonate (PC), the temperature is controlled at a high temperature of 110 degrees. For polyethylene (PE) or polypropylene (PP), the temperature below room temperature is optimal. Thus, molding may be performed under various temperature conditions. These conditions vary depending on the mold cooling structure and mold releasability.