Abstract: A mold clamping unit includes a mold height adjustment system having a length-adjustable threaded connection, comprised of a threaded spindle and a spindle nut which supports a platen of a molding tool, for stroke transmission and transmitting a locking force in clamping position. A clamping mechanism, which locks the spindle nut in relation to the threaded spindle and is selectively releasable for length adjustment of the threaded connection, includes a clamping nut, connected in fixed rotative engagement with and coupled for slight axial displacement in relation to the spindle nut, and an actuator, switchable between an active position, in which the spindle nut is braced with the clamping nut by a tightening force so that the platen is urged against the spindle nut, and an idle position, in which the actuator is released and which can be implemented irrespective of a stroke position of the drive mechanism.