A plastification and injection unit in injection-molding machines for processing of polymer materials, for example rubber, has a plastification screw that is disposed so as to rotate in a screw cylinder, which simultaneously represents the injection piston. The screw cylinder is disposed in an injection cylinder, in an axially displaceable manner. The injection cylinder can be connected with the sprue channel of an injection-molding die by way of a nozzle, and the injection piston has a back-flow barrier. The screw is axially displaceable in the screw cylinder by means of a separate drive. The screw tip projects out of the screw cylinder and has a closure body having a sealing surface for contact with the face surface of the screw cylinder.