Method for reducing required mold-cavity clamping force and controlling injection-molded-product wall thickness

摘要

A method for injection molding a hollow, plastic product having an end wall and a cylindrical or approximately cylindrical side wall, wherein the end wall includes a fringe region bordering the end wall. A system for performing the method includes a mold-cavity-forming combination of a core section and a cavity section, wherein the core section includes a protractable and retractable ram-shaped contact element, with the end of the contact element that faces the mold cavity including a contact surface that contacts at least a significant part of the fringe region of the end-wall-defining portion of the cavity section. Both the end-wall defining portion and the side-wall defining portion of the core section consist of the contact element. When the contact element is protracted to a forward position so that the contact surface contacts the cavity section, the area of the mold cavity as projected on a plane perpendicular to the mold opening direction is reduced, which thus reduces the clamping force required during the injection of plastic material. At least part of the contact surface is non-perpendicular to the longitudinal axis of the cavity section and contacts at least part of the fringe region of the end-wall-defining portion of the cavity section when the contact element is protracted to impede lateral movement of the core section in relation to the longitudinal axis of the cavity section. This feature controls the wall thickness of the cavity. A first quantity of plastic material is injected into the mold cavity while the contact element is held in the forward position. The contact element is then retracted to a rear position where the contact surface ceases to contact the cavity section, while the injected first quantity of plastic material at least partly solidifies to further reduce the required clamping force and to further prevent relative movement between the core section and the cavity section when a second quantity of plastic material is injected into the mold cavity to complete formation of the plastic product.