MXPA99010526A - Method of molding contact lenses - Google Patents

Abstract

An improved method for casting lenses, especially contact lenses, employs mold sections injection molded from an amorphous copolymer of ethylene anda cyclic olefin. Other contact lens applications of this copolymer are also disclosed.

Description

METHOD OF MOLDING OF CONTACT LENSES BACKGROUND OF THE INVENTION The present invention relates to improvements in the casting of contact lenses. In general, a known method for the fabrication of contact lenses is static cast molding. Casting contact lens casting "involves depositing a hardenable mixture of polymerizable monomers in a mold cavity formed by two sections of the mold, hardening the monomer mixture, and removing the mold assembly and removing the lenses. It forms the anterior surface of the lens (anterior section of the mold), and the other section of the mold forms the posterior surface of the lens (posterior section of the mold) Another known method is referred to as spin casting, which involves loading the monomers in a section of the mold configured to form the front surface of the lens and rotate the mold in a controlled manner at the same time as the monomer mixture is exposed to light, in each of the above methods, before molding the contact lens, each of the sections of the mold is formed.The conventional methods involve the injection molding of the mold sections from of a resin in the cavity of an injection molding apparatus. Typically, sections of the mold are used only once to melt a lens and then discarded. Consequently, the sub-process of the sections of the mold forms an important part of the entire lens manufacturing system. A wide variety of materials are known for use as molding materials for the casting of contact lenses, especially thermoplastic crystalline polymers, such as polypropylene and amorphous thermoplastic polymers, such as polyvinyl chloride and polystyrene. However, it would be desirable to improve the manufacture of contact lenses with molding materials better adapted for such manufacturing processes.

COMPENDIUM OF THE INVENTION This invention relates to improvements in the casting of contact lenses, wherein a lens forming mixture is cured in a mold having a molding surface that forms one of the surfaces of the lens. The improvement comprises the injection molding of the molding section from an amorphous copolymer of ethylene and a cyclic olefin. It has been found that these types of resins are compatible with conventional contact lens materials, and offer several improvements in contact lens casting. The invention additionally identifies other contact lens applications of the copolymer in question.

DESCRIPTION OF PREFERRED EMBODIMENTS A conventional way of melting contact lenses involves melting the lens in a mold assembly, which includes a back section of the mold, having a posterior optical surface (which forms the back surface of the cast lens), and an anterior mold having an anterior optical surface (forming the anterior surface of the molded lens). A mixture of monomers forming the desired lens is added to one of the sections of the mold, typically the anterior section of the mold, and the sections of the mold are assembled to contain the monomer mixture in a mold cavity formed between the two surfaces of the mold. definition of the sections of the mold and which corresponds substantially to the "desired configuration of the contact lens." After assembly of the sections of the mold, the monomer mixture is polymerized, such as by exposure to UV light and / or heat, followed by the disassembly of the set of molds and the removal therefrom of the molded lens.Other processing steps can be included after the casting, such as the inspection of the lens, the sterilization of the lens and the packaging of the lens. it is mentioned, before the molding of the contact lens, each of the sections of the mold is formed, conventionally, by injection molding the sections of the mold to starting from a resin in the cavity of an injection molding apparatus. According to the invention, at least one of the anterior and posterior sections of the mold is injection molded from an amorphous copolymer of ethylene and cyclic olefin. The following random copolymers arereferred: where R is hydrogen or C1-C4 alkyl, preferably hydrogen; each R 'is independently hydrogen or C 1 -C 4 alkyl, preferably hydrogen or methyl; And x and y are at least 1. These materials are available from Hoechst Celanese Corporation, Summit, New Jersey, USA. The sections of the mold can be injection molded from the amorphous copolymers in question by means generally known in the art. The tools for injection molding are typically made of brass, stainless steel or nickel or some combination thereof. A preferred material for use with this invention e-r brass galvanized with nickel. A desired surface is machined and polished on the tools to achieve a precision surface quality, so that surface imperfections are not transferred to the section-- of the mold being molded by injection. The Applicant finds that this type of copolymer offers several advantages over the conventional materials used in the manufacturing processes of the contact lens. First, this copolymer has better chemical compatibility with the monomer mixtures from which the contact lenses are melted. For example, some monomer mixtures have a tendency to chemically interact with PVC, resulting in surface defects in the PVC molding surface that result in surface defects in the surface of the lens fused in this section of the mold. Polypropylene is typically provided with additives that can migrate to the molding surface of the mold sections, said additives interacting with mixtures of molten monomers on this surface, giving rise either to surface defects in the lens or to the presence of unwanted additives in the the surface of the fused lens. Polystyrene can interact with certain mixtures of monomers, similar to PVC, giving rise to surface defects; furthermore, the polystyrene can be difficult to moisten when the monomer mixture is added thereto, ie, that the monomer mixture tends not to uniformly moisten the "molding surface" of the polystyrene molds, giving rise to voids on the surface of the cast lenses. Secondly, this copolymer shows a relatively low shrinkage during and after the injection molding, which is important to ensure the predictability in the molding process. Indicated differently, other plastic systems, especially polypropylene, show noticeable shrinkage during and after injection molding. When a contact lens is melted in the mold section, requiring precision casting to ensure the desired optical properties, then it is necessary to use the molds in the contact lens casting operation within a certain window of time, to Avoid casting lenses in molds that have shrunk -excessively. On the other hand, the copolymer in question shows a sufficiently low contraction so that it is not necessary to control the time in which the sections of the mold are stored before casting the lens. Additionally, while polyethylene is sensitive to relatively small process changes, the copolymer in question is less sensitive to molding tolerances, which is important for multi-cavity injection molding processes.

Third, the copolymer in question is well adapted for curing processes. The hardening of the monomer mixtures typically involves the hardening of the monomer mixture by exposure to UV or visible light, where the radiation is directed through the mold to harden the monomer mixture contained therein, or by thermal hardening, where the monomer mixture contained in the mold is subjected to high temperature to effect hardening. As a first example, this copolymer has a very high light transmission capacity, especially compared with polypropylene. Therefore, for curing operations employing light induced polymerization, the increased light transmission capacity of the mold material as a more efficient hardening process. As a second example, this copolymer has a relatively high vitreous transmission temperature, especially compared to polystyrene. Therefore, for hardening operations employing thermal induced polymerization, higher hardening temperatures may be employed, if desired. The copolymer is also useful in molds adapted for other casting operations, such as spin casting which, as mentioned previously, involves loading the mixture of monomers into a mold section configured to form the front surface of the lens, and rotating the mold in a controlled manner at the same time as the monomer mixture is exposed to light. As another example, the "semi-finished" lenses may be fused to a section of the mold having either an anterior or a posterior molding surface to form the respective anterior or posterior surface of the lens, "foed by the lathe cutting of this. semi-finished lens to form the other anterior or posterior surface of the lens Due to the rigidity of the copolymer in question, the sections of the mold made of this copolymer are more suitable for assembly, together with the semi-finished lens, in the lathe collar, the stiffness of the mold section avoids the distortion caused by the pressure exerted by the collar Finally, the described amorphous cyclic olefin copolymers also show extremely low water absorption properties and excellent properties as a water vapor barrier Therefore, these materials also tend themselves to use in contact lens packages, such packages, often referred to as "pa Bubble quetes "include a flange with" a depression for supporting a contact lens and a saline solution. A sheet sheet is sealed to the flange around the periphery of the depression to hermetically seal the contact lens therein. Such bubble packages are typically made of polypropylene, but because of the improved water absorption properties and the water vapor barrier properties of the copolymers in question, these copolymers offer advantages over conventional contact lens packaging materials. Although several preferred embodiments of the invention have been described, it is to be understood that the invention is not limited to them and would be apparent -some modifications and variations for a person skilled in the art.

Claims (5)

Claims

1. In a method of casting a lens comprising hardening a lens forming mixture in a mold having a molding surface, the improvement where the mold is injection molded from an amorphous ethylene copolymer and a cyclic olefin.

2. The method of claim 1, wherein the cyclic olefin is a copolymer of ethylene and norbornane.

3. The method of claim 1, wherein the lens is a contact lens.

4. The method of claim 1, wherein the lens is melted between the molding surfaces of the first and second mold sections of a mold assembly, a molding surface being configured to form a "front surface of the lens". , and the other section of the mold forms the back surface of the lens, at least one of said mold sections being molded from said amorphous copolymer.

5. In a method of packaging a "contact lens in a package having a flange with a depression" for supporting a contact lens, comprising placing the contact lens in the depression and hermetically enclosing the lens in the depression with a foil sheet, the improvement where the package is constructed of an amorphous ethylene copolymer and a cyclic olefin.