An injection molding apparatus for processing thermoplastic materials has at least one plasticizer unit, at least one injection molding cylinder with associated emptying piston, and at least one injection molding mold. The injection molding cylinder is provided with a sleeve which is arranged reciprocally between a position in which communication with the injection molding mold is interrupted and a position in which communication with the injection molding mold exists. In the position in which the communication with the injection molding mold is interrupted, a passage is provided between the sleeve and the housing which is connected between the feed conduit from the plasticizer unit and a storage space defined on the inside by the sleeve. The outside diameter of the ejection piston is slightly smaller than the inside diameter of the sleeve so that remaining between the two is an annular gap through which the plastic material passes into the storage space from one end of the sleeve.