Abstract: A mechanism for locking two mold parts together includes a cylindrical tie rod securely mounted on one mold part and having a circumferential groove therein. A cylindrical collet sleeve is rigidly mounted on the other mold part and has a plurality of axially extending, circumferentially spaced, resiliently flexible arms at one end. The free end of each arm has an inwardly projecting locking member thereon which the arm resiliently urges radially inwardly. When the mold parts are moved to a closed position, the tie rod is received within the collet sleeve and the locking members become axially aligned with and are urged into engagement with the tie rod groove. An annular collar encircling the tie rod is then moved axially until its inner surface snugly engages the outer surface of the collet arms, thereby preventing radially outward movement of the collet arms.