MOLD FOR INJECTION MOLDING

PURPOSE:To improve the quality of a molded product without deteriorating breaking strength of resin, by a method wherein a shutter expanding and contracting a cross-sectional area of a molten resin path through an advance or retreat movement is provided, moved forward directly before completion of injection and moved backward prior to mold break after the curing of the resin. CONSTITUTION:As a shutter 26 has been retracted from a sprue 15 and a cross-sectional area of the sprue 15 is expanded, impact strength of a molding does not deteriorate. When injection and filling of a material into a cavity 9 has attained a time directly before completion, a piston rod 25 is moved forward by actuating a hydraulic cylinder 24 with a signal from a controller, the tip part of the shutter 26 is entered into a sprue 15 and the cross-sectional area of the sprue is contracted. Although the curing of the material is begun directly after that, a curing hour of the material of the sprue part turns short down to the same level as that of the molding due to expansion of a heat transfer surface and a reduction in thickness and a molding cycle does not extend. As the shutter 26 has been retracted from the sprue 15 and the time when a product is taken out of the cavity 9 by demolding, taking-out of the product is not hindered.