Patent ID: 7147814

Claim:
An injection molding method comprising: injecting a molten resin material into a cavity defined by a fixed die and a movable die via a runner provided in the fixed die and a gate provided in the fixed die, wherein when the molten resin material is being charged into the cavity through the resin reservoir from the gate, a cut punch, which is provided in the side of the movable die that confronts the gate through a resin reservoir formed by recessing the fixed die toward the gate and which is movably provided so that the cut punch is inserted into the resin reservoir so as to be in slidable contact with the resin reservoir, has a distal end thereof extending in a moving direction thereof that is toward the resin reservoir of the cut punch, and the distal end is located between the resin reservoir and the cavity at such a position as to open a communicating portion that allows the resin reservoir and the cavity to communicate with each other so that the molten resin material is introduced into the cavity via the resin reservoir; wherein when an inner portion of the resin material that is present in the resin reservoir is still molten and a portion of the resin material thereof that is in direct contact with the cooled dies is gradually solidified after the molten resin material has been charged into the cavity and the resin reservoir, the cut punch moves toward the gate so that the cut punch is inserted into the resin reservoir, whereby the cut punch not only closes the communicating portion while forcibly pushing the still molten resin material present in the resin reservoir back into the gate, but also cuts the resin material at the communicating portion so that a resin molded product formed in the cavity is separated from a resin solidified portion formed in the resin reservoir, and wherein an undercut portion provided at a periphery of the distal end of the cut punch, the undercut portion disposed closer to an edge portion of the distal end than a center portion of the distal end, retains the resin solidified portion in the resin reservoir.