Static molds – Container-type molding device – Of named component
Reexamination Certificate
2001-03-01
2004-08-10
Mackey, James P. (Department: 1722)
Static molds
Container-type molding device
Of named component
C428S800000, C264S001380
Reexamination Certificate
active
06772988
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed toward controlled curing of devices requiring optical cure. More specifically, the present invention provides a method for curing optical devices such that the devices undergo a more uniform polymerization, resulting in a reduction in defects such as dimpling and warpage in the cured device. In particular, the optical devices include ophthalmic lenses including contact lenses, intraocular lenses, spectacle lenses, corneal onlays and corneal inlays. More particularly, this method provides for a method to produce contact lenses having a controlled cure profile.
It is often desirable to direct-mold optical devices such as contact lenses and intraocular lenses, rather than form the lenses by machining operations. In general, molded lenses are formed by depositing a curable liquid such as a polymerizable monomer into a mold cavity, curing the liquid into a solid state, opening the mold cavity and removing the lens. In particular, the mold cavity may be formed by a mold assembly comprised of a posterior mold portion and an anterior mold portion, each having a lens-forming surface. When the posterior mold portion and anterior mold portion are mated, the lens-forming surface of the posterior mold portion and the lens-forming surface of the anterior mold portion form the lens-forming cavity. The non-lens-forming surfaces of both mold portions, herein referred to as non-critical surfaces, are generally molded to have a similar radius (or radii) of curvature as that of the lens-forming surfaces. While the lens-forming surfaces are of optical quality, each having a central optical zone and typically, at least one peripheral carrier zone, the only requirement of the non-critical surface generally is a smooth surface.
Polymerization is typically carried out by thermal means, irradiation or combinations thereof. Traditionally, conventional thermo-casting techniques require fairly long curing times and are used when the resultant object is thick. Rods from which rigid gas permeable lenses are lathed from or thicker lenses are often thermally cured. Curing of lenses by irradiation, in particular, ultraviolet (UV) irradiation, frequently offers shorter curing times. The monomer is poured into a transparent mold having a desired optical surface, and thereafter the UV energy irradiates the monomer through the transparent mold to cure the photosetting monomer.
A common material used as a mold material is polypropylene, which is disclosed in U.S. Pat. No. 5,271,875 (Appleton et al., assigned to Bausch & Lomb Incorporated, the entire contents herein incorporated by reference). The process disclosed in Appleton et al., may be used to produce lenses with predictable and repeatable characteristics.
The use of polypropylene may be desired with certain lens-forming materials. Other lens-forming materials, however, may cast just as well or better in other mold materials. As disclosed in U.S. Ser. No. 09/312105 (Ruscio et al. and assigned to Bausch & Lomb Incorporated, the entire contents herein incorporated by reference), polyvinyl chloride absent any UV stabilizer provides a suitable material for the posterior mold.
While the irradiation of the optical device from the light source may be conducted in a uniform and parallel manner, the material chosen for the mold portions may affect the pathways of the light rays. For instance, some materials, such as thermoplastic crystalline polymers, may diffuse the radiation, causing a scattering of the light rays. Polypropylene is such a material. Other materials such as polyvinyl chloride and polystyrene are thermoplastic amorphous polymers, which permit an unhindered pathway for the light rays during curing.
The radiation may also be reflected off the surface of the glass or plastic mold materials. This may result in non-uniform distribution of light intensity over the lens-forming material.
This invention recognized that the non-critical surface of the posterior mold may act as an optical device, reflecting and/or refracting the radiation in a non-uniform pathway through the mold portion. In particular, the non-critical surface of the mold may refract the radiation from the optical source. This may lead to non-uniform curing rates of ultraviolet polymerizable materials including resins and monomers. As a result, since the curing is completed faster and more completely in a portion receiving a high radiation intensity (in this instance, the periphery portion of the lens) and slower in a portion receiving a low radiation intensity (the central portion), stress is generated in the cured resin or monomer layer. This stress may deteriorate the precision of the optical device face. Additionally, the faster curable portion receiving higher radiation intensity is cured with absorption of the surrounding uncured resin or monomer in order to compensate for the contraction of resin or monomer resulting from the curing process. As a result, the slower curable portion (which receives lower radiation intensity) may show defects such as shrinkage. In particular, in the case of contact lenses and spectacle lenses, this can produce lenses with unacceptable optical aberrations caused by uneven curing and stress. “Dimpling” or warpage of the contact lens is a common problem caused by uneven curing. In dimpling, the apex of the lens is flattened or slightly concave in shape. Warpage is generally seen as the inability of the edge of a lens to have continuous contact with the molding surface upon which it contacts. Other drawbacks seen with plastic spectacle lenses include “striations”, which are caused by uneven curing and stress. Thermal gradients form in the gel-state, which produce convection lines (“striations”) that become frozen in place and cannot be dispersed.
U.S. Pat. No. 4,166,088 (Neefe) discloses controlling the polymerization of cast optical (plastic or contact) lenses. The mold section on the bottom is a lens which focuses UV light to the center of the cavity. The bottom mold must have a thickness which corresponds to the focal length of the refractive surface so that the UV light rays converge at the center of the monomer being cured. Neefe also requires an aluminum reflector on the outer surface of the top mold to reflect light back through the monomer.
U.S. Pat. No. 4,534,915 (Neefe) discloses the use of a convex positive refractive power cylinder lens to provide a band of actinic light to a rotating lens monomer. The center of the spin cast lens receives the most radiation, the area adjacent to the center receives less while the periphery receives still less radiation. This allows for the outer portion of the spin cast lens to migrate inward as the lens shrinks during the curing process. A fresnel lens or a Maddox rod may also be used to provide the narrow high energy line of actinic light.
U.S. Pat. No. 4,879,318 (Lipscomb et al.) discloses the use of mold members formed from any suitable material that will permit UV light rays to pass through. To aid in the even distribution of the UV light, the surfaces of the molds are frosted. In one embodiment, a Pyrex glass plate is used to filter out UV light below a certain wavelength. Lipscomb et al. found that if incident UV light is not uniform throughout the lens, visible distortion pattern may appear in the finished lens. Lipscomb et al. solved this problem by including additives in the lens forming composition to reduce the distortions. The ophthalmic lenses are formed from plastic.
U.S. Pat. No. 4,919,850 (Blum et al.) discloses a method for making plastic lenses in which the liquid lens material is dispensed into the mold cavity and put into a heated bath for a partial thermal curing. After a period of time, the mold (while still in the liquid bath) is subjected to UV light for an additional period of time. The liquid bath disperses the UV light sufficiently to avoid stresses and other adverse effects on the lens ultimately formed that may be caused by uneven exposure to the UV light. The mold may also be rotated while in the bath or the bath may includ
Bausch & Lomb Incorporated
Heckenberg Donald
Mackey James P.
McGuire Katherine
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