a method and apparatus are disclosed for the production of injection molded products from synthetic resinous materials , such as nylon , which tend to be degraded when exposed to oxygen while they are heated to the temperatures required for injection molding . the apparatus includes a barrel having a discharge opening , a screw receiving bore and a vent opening between the feed opening and the discharge opening . a two stage rotatable reciprocable screw is mounted in the bore and includes a vented portion in registry with the vent opening . water vapor and volatiles are generated during plasticization in the first stage of the screw , and evolve as the material enters the vent section of the second stage . the vapors are exhausted through the vent opening . the vent opening is provided with a check valve which readily permits the exhaust products to pass from the screw receiving bore . however , the check valve effectively prohibits the entrance of atmospheric air through the vent opening to the vent section .