A tread belt is molded and cured in a mold having a base plate and an inverted cup-shaped top plate. A plurality of inner segments are disposed in the mold. When the mold is closed outer surfaces of the inner segments form a cylindrical surface which is in contact with an inner surface of the tread belt. A first portion of the inner segments are follower segments, a second portion of the inner segments are leader segments, and means are provided for controlling radial movement of the inner segments. A plurality of outer segments are disposed in the mold. When the mold is closed the inner surfaces of the outer segments form a cylindrical surface which is in contact with an outer surface of the tread belt, and means are provided for controlling movement of the outer segments.