An injection molding apparatus including a stack mold tool and a ball screw centering device. The stack mold tool including a stationary mold block, a center mold block movable relative to the stationary mold block, and a distal mold block movable relative to the center mold block. The ball screw centering device includes a center bearing housing connected to the center mold block, a rotatable shaft having a first side ball track and a second side ball track, a first nut coupled to the stationary mold block, the first nut including a track portion cooperating with the first side ball track to define a passage for a first set of ball bearings, and a second nut coupled to the distal mold block, the first nut including a track portion cooperating with the second side ball track to define a passage for a second set of ball bearings.