Methods and devices to control polymerization

Abstract

A method and mold assembly to control the polymerization of a molded article. In one embodiment, an amorphous posterior mold comprising a non-critical surface having a controlled radius of curvature is used to produce molded articles. In an alternate embodiment, ophthalmic lenses are produced using a posterior mold in which the concave surface of the non-critical surface is filled with a liquid having a similar refractive index as the mold material. In still another embodiment, a positive lens is placed at a predetermined distance adjacent to the mold assembly to alter the irradiation path to the mold assembly. In still another embodiment, a positive lens is placed within the concave surface of the posterior lens.

Description

BACKGROUND OF THE INVENTION [0001] 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 controlled 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. [0002] It is often desirable to 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...

AI Technical Summary

Benefits of technology

[0019] The present invention is a method for photocuring cast articles such as ophthalmic lenses in which defects in the cured article are reduced. By altering the pathway by which irradiation rays reach the article to be cured, defects can be reduced. By controlling the relative intensity of radiation upon a particular portion of lens-forming material, the rate of polymerization taking place at various portions of the lens can be controlled. This method is particularly suited for use with mold materials which are amorphous.

[0021] Another embodiment of this invention comprises reshaping or removing any junctions formed between different geometries (e.g. a radius of curvature and flat surface portion) used to form the non-critical surface. The non-critical surface of the posterior mold may then be comprised of a controlled radius of curvature, eliminating any shadows or areas which the irradiation rays may not penetrate evenly. The controlled radius may be spherical or aspherical, provided that the surface is smooth and continuous.

[0023] In the preferred embodiment, the optical element is a positive lens which is placed at a predetermined distance above the posterior mold. The positive lens converges radiation rays, preferably ultraviolet (UV) radiation, passing through the mold and increases the energy available to the cured article. The distribution of irradiation rays radiates from the center of the mold. This distribution reduces the cure gradient across the lens, which reduces or removes any residual stress induced during curing. The result is a cured article such as a contact lens having an acceptable apex in the central portion of the lens. The positive optical element allows control of the illumination intensity profile reaching various sections of the contact lens. Stress developed by uneven intensity profiles can be removed or introduced.

[0026] The ophthalmic lenses formed from these methods are relatively free from defects such as dimpling and warpage.

Problems solved by technology

For instance, some materials, such as thermoplastic crystalline polymers, may diffuse the radiation, causing a scattering of the light rays.

This may result in non-uniform distribution of light intensity over the lens-forming material.

Additionally, the non-critical surface of the mold may refract the radiation from the optical source.

This may lead to non-uniform curing speed of the ultraviolet curable resin.

This stress may deteriorate the precision of the optical device face.

Additionally, since the faster curable portion receiving higher radiation intensity is cured with absorption of the surrounding uncured resin in order to compensate for the contraction of resin resulting from curing, 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.

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.

Lipscomb et al. found that if incident UV light is not uniform throughout the lens, visible distortion pattern may appear in the finished lens.

Without the filter, the reflection of the metal mold and the glass lens result in non-uniform distribution of UV light and non-uniform curing speed.

The center of the resin cures faster than the outer perimeter, causing defects such as shrinkage in the resin.

None of the above art completely solves the problems which occur when using a mold assembly in which one mold portion is made from an amorphous material and acts as an optical device.

The resultant lens made from this particular mold assembly may have defects such as dimpling and warpage.

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