Patent ID: 7559761

Claim:
An injection molding machine for forming a plastic mold product having an inner through hole by injecting a melted resin into a cavity defined by a first mold and a second mold, comprising: a movable portion provided in one of the first and second molds; a sprue bush provided in the other of the first and second molds; a portion, provided on at least one of the movable portion and the sprue bush, having a peripheral surface for forming an inner surface of the inner through hole in the plastic mold product; a gas passage provided in at least one of the first and second molds so as to communicate with the cavity, the gas passage having an outlet at an end thereof; a valve provided in a gas pipe communicating with the gas passage; and a controller, wherein the movable portion is adapted to be movable between a normal position for injection of the melted resin and a projected position projecting into the cavity so that a clearance remains between the movable portion, its counterpart mold, and the portion having the peripheral surface for forming the inner surface of the inner through hole, the clearance being in communication with a first space defined by the peripheral surface within the cavity for forming the plastic mold product in the cavity and a second space between the movable portion and the sprue bush for accommodating a sprue separated from the plastic mold product to form the inner through hole in the plastic mold product, the outlet of the gas passage being arranged so as to supply gas into the clearance at a position that is closer to a center of the through hole than the peripheral surface is to the center of the through hole, and wherein the controller is operable to control a movement of the movable portion and an opening and closing of the valve in conjunction with the injection of the resin so as to open the valve and supply gas into the clearance before solidification of the melted resin in the first space and the second space when the movable portion is in the projected position.