A method for producing cast pieces from a synthetic material comprises the steps of preplasticizing the synthetic material while the plasticizing cylinder is in communication with the mold cavity, relaxing the preplasticized material to remove the pressure thereon by a limited relative movement between a plasticizing cylinder and a screw conveyer, lifting the plasticizing cylinder away from the mold cavity and out of communication therewith, removing the previously cast piece from the mold cavity, and injecting the material in the plasticizing cylinder through the always open nozzle and into the mold cavity by a stroke of a screw conveyer toward the mold cavity. The apparatus for carrying out this method includes a plasticizing cylinder having a screw conveyer arranged therein. The cylinder and conveyer are mounted for movement relative to one another along the injection axis. Means including a guide member and an abutment are provided for permitting a limited relative movement between the cylinder and screw conveyer.