Injection compression molding method and injection compression machine of lens

An injection compression molding method of a lens for molding a lens of a thermoplastic resin, comprising the steps of:

• providing a molding machine having a tie bar for mutually connecting a pair of space-retaining plates, a movable die plate movable along the tie bar, a molding die provided between a first space-retaining plate of the pair of the space-retaining plates and the movable die plate and accommodating a lens-molding cavity including a pair of cavity forming members thereinside for shaping concave and convex surface of the lens, and an advancement-retraction mechanism provided between a second space-retaining plate of the space-retaining plates and the movable die plate for advancing and retracting the movable die plate relative to the first space-retaining plate, the lens-molding cavity being reduced when the movable die plate advances toward the first space-retaining plate while the molding die is closed;
• actuating the advancement-retraction mechanism to close the molding die and to move the movable die plate to a position for the thickness of the lens-molding cavity to be thicker than a thickness of a molding article while the molding die is closed;
• injecting a thermoplastic molten resin into the lens-molding cavity defined while setting the volume of the cavity and sealing in the molten resin inside the molding die;
• actuating the advancement-retraction mechanism to advance the movable die plate toward the first space-retaining plate, keeping a constant relative position between the second space-retaining plate and the movable die plate where the extension of the tie bar becomes a predetermined value in accordance with the characteristics of the lens,
• the actuating being carried out so that the thickness of the lens-molding cavity becomes smaller than the thickness of the lens, and pressurizing the molten resin;
• cooling the molten resin for a predetermined time after completion of pressurizing the resin:
• and
• actuating the advancement-retraction mechanism in a direction that lessens the space between the second space-retaining plate and the movable die plate to reduce the pressure applied to the molten resin within the lens-molding cavity after the cooling step while keeping substantially constant relative position of the cavity forming member on the movable die and the cavity forming member of the fixed die in a primary die-release operation such that the molding cavity is made substantially equal to the size of the lens.