Abstract: A mold base has an annular support plate secured thereto and formed with a plurality of equiangularly spaced radial slots and with an external peripheral guide groove. A support structure on the support plate carries an annular base plate mounting the lower half of a cylindrical radially inner chill whose upper half is carried by an annular closure plate. An arm on the base pivotally supports a radial arm pivotally connected to the closure plate, and a toggle linkage is connected between the base and the radial arm. An operating ring is guided in the guide groove for angular displacement by a hydraulic actuator. Radially outer chill sectors are positioned on the base plate and carried by respective die sectors guided in the radial slots of the support plate, and bent crank links connect the die sectors to the operating ring. The die sectors have beveled clamping surfaces engageable with beveled peripheries of the base and closure plates upon displacement of the operating ring in a mold closing direction.