The invention is based on an injection unit for a plastic injection molding machine having a screw, having a motion spindle as first drive element, via which the screw can be rotationally driven for the plasticization and is movable in the axial direction for the injection, having a spindle nut as second drive element, which is coupled to the motion spindle via a motion screw thread, having a large electric motor which is arranged so as to be fixed to the frame and by which, while one drive element is held against rotation, the other drive element can be rotationally driven for moving the screw during the injection, and having a device which is arranged so as to be fixed to the frame and by which a rotational speed of the one drive element can be preset during the plasticization, this rotational speed being different from the rotational speed at which the large electric motor simultaneously drives the other drive element. In a known injection unit of this type, two equally large electric motors, which can both apply a large torque, are used, by which such a counter-torque is exerted on the motion spindle during the injection operation that the motion spindle does not rotate. On the whole, the known injection unit is quite expensive on account of the two electric motors designed for a high torque. According to the invention, in order to provide a more cost-effective injection unit, the drive element held against rotation during the moving of the screw for the injection can be locked by a releasable mechanical brake.