A preform for making a prosthetic limb socket includes an injection molded disk of thermoplastics material, and a peripheral portion of the disk is supported by a rigid metal ring. The ring and the peripheral disk portion have an interfitting connection which permits the disk and ring to be heated in an oven. The ring and softened disk are then inverted and stretched downwardly over a positive model of a residual limb while a vacuum is introduced through holes within the model to form a plastic socket which conforms to the model. The interfitting connection may be peripherally spaced studs on the ring and projecting into corresponding holes in the disk or may be an annular rib or arcuate rib sections on the ring which project into an annular recess or arcuate cavities within the disk. After the plastic cools, the ring is separated for reuse.