Abstract: An improved method and apparatus for high-pressure injection or compression molding of optical thermoplastic parts such as lenses or optical disks, wherein the optical surfaces of a molded part are formed by intimate contact with suitably contoured or profiled and lapped surfaces of one or more optical mold insert elements. The optical surface of such mold element is of a different composition metallurgically than is the bulk composition of the rest of the mold element, thus allowing for optimum selection for materials-of-construction parameters (such as heat transfer, load-bearing quality, and ease of fabrication) in the bulk properties of the substrate metal, without necessarily trading off against optimum optical surface characteristics (such as imperviousness to chemical oxidation or attack, resistance to scratching and abrasion, ease of generating optical surface therein).