Abstract: An injection molding system (10) generally includes a molding device (100), a mold controller (200), and a negative pressure apparatus (300). The molding device defines a molding cavity (160) and a plurality of cooling channels (110) therein and has a plurality of heating elements (120). The heating elements are used for heating the molding cavity to a determined temperature. A cooling medium is supplied in the cooling channels to cool the molding cavity. The negative pressure apparatus is used for keeping the cooling channels in a negative pressure state, thereby improving the fluidity of the cooling medium during heat removal and avoiding leaving a portion of the cooling medium in the cooling channels during heating. Accordingly, the negative pressure apparatus can effectively decrease the heating and cooling terms/lengths. A method for using this system to manufacture a product made from a thermoplastic material is also provided.