Method for expansion injection molding

摘要

An injection-foaming molding process for manufacturing a formed article by injecting a molten resin containing a foaming agent into a cavity of a mold being retained in an openable state and then expanding the cavity by opening the mold to allow the resin to foam includes: i) a step of discontinuing a cavity of a mold from a resin-reserving section located at front end of a screw immediately after the completion of injection of a molten resin containing an expandable resin into the mold cavity; and ii) a step of keeping the resin-reserving section under pressure at least equal to back pressure for charging during a period from the completion of injection to start of charging the molten resin containing the expandable resin for next shot, decreasing pressure in the resin-reserving section to back pressure for charging, and then starting the charging. According to the process, a molding cycle can be shortened and a lightweight formed article by foaming having high rigidity and good appearance can be obtained.