VENTED INJECTION MOLDING MACHINE

A vented injection molding machine adapted to handle a wide variety of synthetic resinous materials without pre-drying is disclosed. The molding machine includes a two-stage screw rotatably and reciprocably mounted within a barrel. The barrel includes a pair of ends, a feed opening at one end and a vent opening intermediate the pair of ends. A first stage of the screw essentially completes material plastication. The first stage has a protruding helical screw flight with a first pitch and includes a feed section communicating with the feed opening, a transition section, and a pumping section. A second stage of the screw includes a deep flighted vent section which communicates with the vent opening and the first stage pumping section, a short transition section and a relatively short second pumping section. The second stage also includes a protruding helical screw flight having a second pitch which is greater than the first pitch of the first stage. In addition, a radially protruding slotted blister ring may be provided on the screw to separate the first and second stages while allowing fluid communication therebetween. An operating assembly is connected to the screw to rotate and advance the screw and may cause retraction of the screw from the barrel during the accumulation of a quantity of plasticated material.