Abstract: In an injection molding apparatus, wherein a plasticizing chamber has a nozzle attached to one end thereof, with the nozzle having a passageway therein and being in communication with the sprue of a mold, the improvement which constitutes a compact, pneumatically-powered, reciprocable spool-type device having a passageway therein, operated by alternately actuated opposed pistons, interposed between the nozzle and the plasticizing chamber, in one position blocking the communication between the plasticizing chamber and the nozzle and in the other position allowing molten material to enter the nozzle.