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Supplement Supplement A) Plastic lens manufacturing principles 1. “Mass” production Depending on whether the resin used is thermoset or thermoplastic, the manufacturing method differs considerably. We will consider them in turn. Thermoset resins Thermoplastic resins Take, for example, the material CR39. The monomer is supplied We will take polycarbonate as an example. The base material is by the chemical industry in liquid form and then goes through already a polymer and comes in the form of granules, the purity the following stages of manufacture: of which has been adapted for use in the optical industry. These - preparation of the monomer: filtration, degassing and addition granules are softened and melted by heating for injection into & TREATMENTS walls which are assembled, either by pressure on a circular or glass mold. An extrusion screw plasticises the material in the the lens-shaped mold. The technology consists in making the of a catalyst and additives; - assembly of the molds: these comprise two glass or metal material fluid by heating it, so that it penetrates into the metal gasket and clamping with a clip, or with adhesive tape; injection cylinder and simultaneously acts as a piston, pushing - filling: the empty space between the two parts of the mold the hot material through several ducts into the mold cavity. After is filled with the liquid monomer; injection and a cooling time, the molds are opened and the - polymerization: the filled molds are placed in ovens and lenses released. subjected to a temperature cycle over several hours – or, for The various manufacturing operations are as follows: certain materials, subjected to ultraviolet radiation for a few - preparation of the material: de-dusting and drying of the minutes – which causes a progressive hardening of the resin; granules by hot air and loading onto the press; - demolding: the gasket or tape and the walls of the mold - setting up the press: positioning of the molds, adjustment of MATERIALS are separated to release the lens. the liquid pressure, mold temperature, injection and cooling time, This procedure is also used for the mass production of “finished” heating of the material (to about 300°C); and “semi-finished” lenses; only the shape of the mold and the - injection: molding under pressure of the molten material; polymerization time are different. The overall principle is the - cooling: solidification of the material by conduction through same for the majority of thermoset plastic materials used in the molds; ophthalmic optics. - demolding: by opening the press and the mold support block. This technology allows all lens geometries to be manufactured, depending on the shape of the molds inserted in the injection press. These lenses are either “finished” and can undergo treatments as they are, or “semi-finished” and will be surfaced later on their rear face, before undergoing various surface treatments. © Essilor International © Essilor International Figure 13: Mass production of plastic lenses in thermoset resin. Figure 14: Mass production of plastic lenses in thermoplastic resin. 16 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.