The invention relates to a mold clamping unit on a plastic injection molding machine, the unit having a clamping mechanism for opening and closing an injection mold, the mechanism having a servo motor as the drive motor (21). The drive motor drives at least one spindle drive (10) which has a spindle nut (12) and spindle which work together on a bearing position (14). A cooling system with cooling channels (25) is provided for removing heat from the clamping mechanism. A more efficient cooling of the spindle drive is achieved in that the cooling channels (25) penetrate the bearing position (14) of the spindle drive (10) and/or one of the cooling plates (23) assigned to the bearing position.