An injection drive unit includes a hollow cylindrical casing rotatably supported by a housing, the cylindrical casing having an axis and a front end and a back end spaced apart along the axis; a piston in the cylindrical casing, the piston axially slidable relative to the cylindrical casing along the axis between advanced and retracted positions, and the piston rotationally locked with the cylindrical casing to rotate therewith; and a cylinder cap generally closing off the back end of the cylindrical casing and providing a first pressure chamber between the piston and the cylinder cap, the cylinder cap including a stationary part affixed to the housing and a rotary part affixed to the cylindrical casing, at least the stationary part providing a stationary end face opposed to the piston and against which fluid in the first pressure chamber bears when pressurized.