Abstract: 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.