Abstract: A tripartite mold has a control bar on a fist outer plate and a locking bar parallel thereto on a second outer plate, both of them traversing respective passages in a guide block on an intermediate plate when the mold is closed. A U-shaped detent, slidable transversely to the two bars in a channel of the guide block, straddles the locking bar with clearance and is urged by spring pressure against the control bar which, in the closed position, it contacts with one leg while its other leg engages in a notch on the locking bar so as to latch the intermediate plate to the second outer plate. When the outer plates are driven apart with initial entrainment of the intermediate plate by the second outer plate, a cam on the control bar represses the detent against its spring force and disengages the other leg thereof from the locking bar whereby further separation of the outer plates lets the intermediate plate stay put while the second outer plate moves away.