MXPA01008535A - Compression molding of optical lenses - Google Patents

Abstract

A lens molding method and system which uses glass molds (204, 206) held by floating cannisters (200, 202) with a cylindrical dam (210) to define the edge of a moldingcavity (99) into which a preheated lens blank is inserted. Once between the molds (204, 206), the lens blank is additionally surface heated, molded and cooled under pressure.

Description

MOLDING BY COMPRESSION OF OPTICAL LENSES Background Polycarbonate is a popular material for making ophthalmic lenses. Polycarbonate lenses are often produced by injection molding and compression of a liquid resin. Conventional operation causes the lenses to exhibit residual stress effects. ^ These residual stress effects can affect the dimensional stability of the lenses, and lead to other problems. For example, extremely hard polycarbonate lenses may buckle when stored or used. The edges of such lenses may have variable optical powers. A phenomenon known as cold drag can also cause lens problems. Compendium The system described herein teaches a new method and apparatus for forming low effort polycarbonate lenses. The machine and process that are described herein produce finished ophthalmic lenses by simultaneously molding the concave as well as convex surfaces of these lenses under controlled pressure and uniform heating. A preferred mode presses both the top and the lower part at the same time, and at substantially equal pressures. Another mode uses a mold made of glass to match the coefficient of expansion of the mold more closely with the initial polycarbonate material. The mold is heated and cooled using a micro-granular solid that is pressed against the mold. This provides uniform heating and diffusion of heat. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention and their embodiments will now be described in detail with respect to the accompanying drawings, in which: Figure 1 shows a flow chart of a general scheme of the process used in accordance with the present system; Figure 2 shows a diagram of the molding machine used to mold the low effort polycarbonate lenses; Figure 3 shows the machine used for a normal lens; and Figure 4 shows the machine used for a prescription lens. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present system preferably operates by starting with an initial solid polycarbonate resin material. These initial materials are commercially available. The operation follows the flow chart of figure 1. The material Initial resin is first pre-heated in step 110. The initial pre-heating brings the initial material to a uniform temperature / which is at a point below the point of deformation of the plastic. The surface epidermis of the initial polycarbonate resin material is further softened in step 112 by increasing the heating to a point above the glass transition point of the plastic. In particularly preferred embodiments, the pre-heating brings the initial material to about 270 ° F, and the supplementary heating is carried out at 305 ° F. The internal temperature of the polycarbonate can therefore be different from its external temperature in this The initial material / with its softened surfaces is then thermoformed in step 114. This is done by placing the heated initial material in a mold cavity 99 which is defined between two glass molds and a cylindrical cavity around the molds. The details of the arrangement or arrangement of the structure are shown and described with reference to Figure 2. The thermoformed process brings the heated external surfaces of the initial material into contact with pre-heated glass molds. glass have been heated to a temperature higher than the external surface temperature of the initial material, an exemplary temperature for the molds is , for example, 390"F.

These glass molds then press against the initial material to effect the thermoforming. The pressing or pressing is carried out at relatively low pressures, for example between 8.5 and 140 psi. A special cylindrical cavity is used to control the thickness of the lens and the flow of resin. After the thermoforming, the stress orientation is carried out in step 116. The pressure is increased, for example doubled, preferably at a pressure level between 272 and 425 psi. The thermoformed lens is allowed to cool under this upper pressure, until the lens reaches a relatively stable temperature, for example 235"F. Then, the pressure is released to leave the finished lens.The heating and cooling of the molds of Preferably they are carried out using a liquefied solid heat transfer medium, for example a non-metallic micro-pulverized solid.The heat transfer medium can be used to exert a uniformly distributed elevated pressure of up to 1,350 psi over the set of glass molds and retention boxes, this helps to avoid plastic flow cuts during the thermoforming process.The space between a concave mold and a convex mold forms the molding cavity.The thickness of the lens and the The viscous flow of the resin is defined by the cylindrical cavity of variable dimensions, describing this in greater detail, the initial material which forms the initial product can be, for example, an initial solid resin material prepared by injection molding of polycarbonate plastic. These initial materials are commercially available. The initial resin material is typically disk-shaped, and has two flattened surfaces that are substantially parallel to each other. The thickness of the initial material should be sufficient to allow a sufficient volume of resin to completely fill the mold cavity, and at the time not be thick enough to provide too much resin loss. The initial material is also preferably smaller in diameter than the final lens. For example, an 85mm lens can be molded from an initial 75mm diameter resin material. The solid initial material is pre-heated in a normal oven, at a temperature below the initial material deformation point. Since the preferred polycarbonate material has a deformation point at 270"F, the preferred temperature for pre-heating is 260 * F. Once the initial material is uniformly heated, it is transferred to an oven at a higher temperature. which heats at least part of the initial material on the transition point to the glass of the plastic.The transition point to the glass of the polycarbonate is 305"F. An adequate amount of time for the surface heating step 112 is, for example. 9 minutes to 370 'F. After this time, it is believed that the material initial is not heated completely. That is, the surfaces of the initial material are more heated than the interior of the initial material. Step 114 represents the thermoformed. The pre-heated solid initial material has softened surfaces. This pre-heated initial material is transferred to the mold cavity described herein, which has glass molds that are pre-heated to 390 'F, slightly more than the supplemental heating temperature. The heat in the molds is cut substantially immediately upon the occurrence of the transfer. The pressure on the mold cavity is increased both from its upper side and from its lower side. The pressure increase is substantially such that the pressure on the upper one equals the pressure on the lower one. Floating boxes are preferably used to balance the load on top and bottom. The pressure is gradually increased from an initial pressure of about 10 psi to a final pressure of about 100 psi. During the thermoforming, the resin mass flows viscously into the mold cavity. After the thermoforming of the lens is finished, the lens is oriented in stress in step 116. Orientation in stress involves increasing the level of pressure, for example to double the thermoformed level at the end of the viscous flow. Once this pressure level is reached, cooling starts. The level of pressure preferred for orientation in effort is between 272 and 425 psi. Whichever upper limit is used, the heat source is removed after reaching the upper limit, and the lenses are cooled below their glass transition point, one point below their deformation temperature. The preferred level is 235"F. At this point, the finished lens is removed from the mold cavity.The thermo-formed combined cycle and stress orientation takes between 22 and 23 minutes. The previous systems suggested moving away from glass molds, for example due to the high temperatures and pressures involved.The lower temperatures and pressures of the present system allow the use of glass molds. it has been found that certain disadvantages, including excessive shrinkage, are avoided when such a glass mold is used.The inventor postulated that this it is due to the closest thermal expansion coefficient of the glass-polycarbonate system compared to the steel-polycarbonate system used in the state of the art. A solid liquid / micro-pulverized medium preferably presses or presses again the glass mold. This also provides an improved controlled heating / cooling transfer technique. Another important aspect is that the pressure exerted by the glass mold on the resin mass can be uniform and sustained throughout the cooling period. This causes stress relaxation as well as stress / cooling orientation between 320 and 235"F. Polycarbonate lenses have been produced by this process and exhibit excellent geometric curves and optical power, improved dimensional stability / low stress patterns, and superior impact resistance The comparison is given below / with the lenses produced by the present process being labeled as "stress oriented polycarbonate": Notes: (1) Commercial polycarbonate ophthalmic lenses made by Gentex and Orcolite. (2) The stress orientation process is supported by evidence of excellent geometric curves, dimensional stability and impact resistance of the lenses made by the process of this invention. Additional analytical tests have been planned for stress orientation test. It is planned to run comparative evaluations of polycarbonate films molded by injection vs. oriented in effort with regard to material resistance (to stress, modulus and shear), diffraction patterns of infra-red circularly polarized spectra and laser / Raman and x-ray. (3) Apparent buckling of samples stored at room temperature for a period of one year. (4) Readings of the Barber-Colman surface printing tester. (5) Successive strikes of a steel hammer of 2 lbs. A preferred value for molding a finished ophthalmic lens is between 1.5 and 5 mm at its center. However, unlike this typical value, the system formed by the present disclosure can produce finished polycarbonate ophthalmic lenses, which are 0.4 mm, or even possibly less, in thickness. Such devices can also have 1.0 mm edges with excellent geometric curves. However, any central thickness between 1 and 4 mm of agreement with this process. Lenses with high diopters may require thick edges in relation to the centers. The cylindrical cavities / as described herein, define the molding cavity. These higher diopter lenses may require insulation to prevent heat loss at the edges of the lens. The specific equipment used to carry out the above-described process is shown in Figure 2. This system uses two floating boxes 200, 202. The floating boxes are effectively pistons that can be controlled separately. A glass mold 204 is held by the floating box 200, and another glass mold 206 is held by the other floating box 202, respectively, at the end of the floating box. The glass molds 204 and 206 can therefore be moved towards one another and pressurized relative to each other. The cylindrical cavity 210 defines the edge of the molding cavity of the lower case 202. The cylindrical cavity 210 rests on a spine 212 to hold the molded lens in place. According to a specifically preferred embodiment, the pressures exerted by the respective platform pistons 220 and 222 on the floating convex mold 206, and the floating concave mold 204, are equal. The thrust cylinders, for example 224, transmit the pressure of the pistons of 220 associated platform. The walls of the boxes 228 are preferably formed of stainless steel. Similarly, push cylinders 224 preferably include stainless steel parts or other heat conductor. These walls and cylinders allow the transmission of heat through the conductive material. The pressure plates 234, 236 are preferably flattened plates which are driven respectively but symmetrically by the pressures P! and P2. The plates, for example 234, press against a liquefied heat conducting means 240 to press or press that medium against the molds. In a particularly preferred mode, the heat conducting medium is a micro-sprayed salt, corundum, or other non-metallic, free-flowing solid, which has adequate heat conduction and compression capacity. The material must be maintained within the chamber between the glass mold 204 and the pressure plate 234. An O-ring 242 rests between the pressure plate 234 and the walls 228 and seals the edges of the chamber. The upper glass mold 204 is held by a retaining lip 244. A lower lip 246 can be provided to hold the lower glass mold in place. However, while the resin mass is flowing viscously, the resin mass can be accommodated under the retaining lips 244, 246. In order to avoid this, the solid medium 240 it needs to be pressurized against the glass molds 204, 206. A spring is preferably used to maintain this constant pressure. For example, a Bellevue spring 250 may be pre-tensioned in place to provide a constant pressure, for example of 1,350 psi, on the pressure plate 234. The spring may be held in place relative to the threaded rod 224, by screwing a screw plate 252 on the threaded rod 224 and screwing to the point where it presses against the spring of the floating boxes. The cylindrical cavity 210 controls the viscous flow rate and hence controls the thickness of the lens. The molding cavity 99 is therefore defined by external surfaces of the glass molds 204, 206, and the internal surfaces of the cylindrical cavity 210. A cross section of the cylindrical cavity is preferably biased to approximately 35 'in order to facilitate the lens removal finally molded. The glass molds 204, 206 are preferably formed of tempered glass. This improves the resistance of the mold to temperature and pressure. In operation, the solid initial material is first pre-heated, and then heated in addition, as noted above. The initial material is placed in the molding cavity 99. Pressure is applied substantially equally through the upper and lower platform pistons 234, 236. The pressure applied to the top equals substantially the pressure applied to the lower part. These pressures are applied at an equal rate and from opposite directions. Once the lens is thermoformed in this way, the pressure is increased, and the temperature is reduced. The pressure 5 remains until the lens cools sufficiently to be removed from the mold. The preferred system uses glass molds that are formed from tempered glass for improved operation. Although glass molds are preferred, molds of other materials can alternatively be used. Preferred materials for the molds include those materials that have a coefficient of expansion and contraction similar to that of the material being molded. Another preferred material for the mold is nickel. Nickel can be formed electrolytically. Other metals may alternatively be used if the liquefied solid medium can be kept sufficiently in heat transfer contact with the boxes 200, 202, such that differences in the resulting shrinkage control at the metal-polycarbonate interface can be minimized. Therefore, this alternative system prevents stresses in the molded lens, even with a metal mold. The edges of the metal molds are preferably suitably tensioned so as to exclude cuts from the viscous flow of the resin mask. An alternative embodiment accommodates certain prescription lenses, in the present Rx lenses, by the formation of prisms in ophthalmic lenses. The formation of prisms represents the act of moving the optical centers of the lens to a non-central location. This is typically necessary because of the anatomy of the user of the lenses. Prism-shaped lenses have an optical center that is no longer their geometric center. The proper geometrical centering of commercial lenses can be achieved by using an initial finished lens material. The overall size of the initial lens material allows the geometric center to be placed to meet the prescription while cutting and mounting the lens in a selected frame. However, the formation of prisms needs to be accomplished by additional carving and polishing typically. The preferred system of this embodiment can accommodate additional prism formation by changing the position of the thrust cylinder. Figures 3 and 4 show the operation of prism formation of the thrust cylinder and the cylindrical cavity. Figure 3 shows the thrust cylinder with and without prism formation, and Figure 4 shows the cylinder with and without prism formation. The molding can be subject to the formation of prisms for prescription lenses. One half of the optical device is inclined relative to the other half, thereby forming a finished polycarbonate lens that does not require cutting and polishing the lenses. Although they have been described in detail previously only a few embodiments / those skilled in the art will recognize that many modifications are intended and that they are predictable from the disclosed embodiments. For example, it is intended that all such modifications be encompassed within the following claims.

Claims (14)

1. CLAIMS 1. A method of forming a polycarbonate lens, comprising: obtaining an initial polycarbonate material to be molded into a lens; pre-heating the initial material to a specific temperature below a deformation point of the polycarbonate material for an effective time to uniformly heat the polycarbonate material; raising the temperature of only the outer surfaces of the polycarbonate material to a second higher temperature, but below the transition point to the glass of the polycarbonate material, to soften the surface of the polycarbonate material; pre-heating the upper and lower molds to a third temperature, higher than said second temperature, to form pre-heated molds; pressing or pressing the initial polycarbonate material heated between respective first surfaces of said pre-heated molds, using substantially the same pressure to press or press from the bottom as that used to press or press from the top.
2. 2. A method as in claim 1, further comprising spring biasing a heating means against a second surface of each of said molds, said second surface being opposite to the first surface pressing or pressing against the initial polycarbonate material.
3. 3. A method as in claim 2, wherein said heating medium is a micro-granular solid.
4. 4. A method as in claim 3, wherein said pressing is carried out along a straight axial line.
5. 5. A method as in claim 3, wherein said pressing is carried out along a bent axial line.
6. 6. A method as in claim 1, further comprising, after pressing or pressing the lens, allowing the lens to be cooled under pressure.
7. 7. A method as in claim 1, further comprising, after pressing or pressing the lens, increasing the pressure to between 272 and 450 psi, allowing the lens to cool under 15 said pressure, and removing the lens after the lens has cooled.
8. 8. A method as in claim 6, wherein said pressures are effective to compress the lens without causing strain on the lens.
9. 9. A method as in claim 1, wherein said molds are glass molds.
10. 10. A polycarbonate lens molding apparatus, comprising: a first glass mold, having a convex molding surface and having a second surface facing away of said convex molding surface; a second glass mold having a concave lens molding surface and having a second surface facing away from said concave molding surface; a micro-granulated heating means in contact with said second surfaces of said first and second molds; a spring element, which presses or presses said micro-granulated heating means against said second surfaces, and a pressure-applying device, which operates to apply a substantially constant pressure to said first and second molds for pressing or pressing a lens of polycarbonate between said convex surface and said concave surface.
11. An apparatus as in claim 10, further comprising a mold retainer, positioned at least in a position surrounding said mold to hold the mold in place, and further comprising a spring-loaded element for pressing or pressing the mold. heating material against said mold to press or press the mold edges against the mold retainer.
12. 12. An apparatus as in claim 10, wherein said micro-granulated heating material is a micro-pulverized, liquefied solid.
13. 13. An apparatus as in claim 10, further comprising a cylindrical cavity / surrounding a molding area.
14. 14. A polycarbonate lens-forming device, which comprises: an oven, which is programmed to first heat an initial polycarbonate material to a first specified temperature below a deformation point of the polycarbonate material and then to heat an effective amount to raise the temperature of only one surface external of the polycarbonate material at a second higher temperature, above the first temperature but below a transition point to the glass of the polycarbonate material; and a pressurized, heated mold, which is heated to a third temperature greater than said second temperature, and after said outer surface of said polycarbonate material is heated to said second temperature, pressing or pressing against surfaces of said polycarbonate material at a pressure less than 450 pounds per square inch.