an injection assembly for injection molding machines for plastics material comprises a fixed frame , a plastication cylinder integral with the fixed frame , a plastication screw acting inside the plastication cylinder , making a forward translational movement during the stage of injection of the plastics material into a cavity of a mold and a rotatory movement around its own axis and a rotatory translational movement of rotation around its own axis and retraction during the plastics material plastication stage , the plastication screw being mounted integral in translation with a movable plate mounted slidably on said fixed frame , there being provided drive means for translation of the movable plate , and drive means for rotation of the plastication screw .