Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Changing mold size or shape during molding or with shrinkage...
Reexamination Certificate
1999-03-19
2001-01-09
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Changing mold size or shape during molding or with shrinkage...
C264S002600, C264S334000
Reexamination Certificate
active
06171529
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a method of releasing from a mold transparent polymer material articles such as ophthalmic lenses (in particular spectacle lenses) made by pouring and polymerizing a polymerizable liquid substance into the cavity of a two-part mold.
2. Description of the Prior Art
Transparent polymer material (organic glass) ophthalmic lenses are conventionally made by pouring and polymerizing a polymerizable liquid substance into the cavity formed between two parts of a mineral glass mold, the assembly being held together by fixing means such as clips. The assembly is commonly called a “sandwich”.
After polymerization the fixing means are removed and the mold is dismantled to recover the ophthalmic lens.
The polymerizable liquid substances used vary widely, depending on the required refractive index of the ophthalmic lens.
Many of these substances, for example substances comprising a monomer such as diethylene glycol bis(allylcarbonate) (CR39
200
), shrink significantly on polymerization, by an amount in the order of 7% or more, generally 10% or more, creating tensile stresses in the ophthalmic lens within the sandwich.
Some ophthalmic lenses, in particular lenses with a positive optical power, can have random defects that are visible when they are subsequently colored, for example by dipping them into an aqueous dispersion of pigment. Because of their shape these defects are commonly called “ferns”.
The defects are probably due to sudden relieving of stresses during mold release, especially in the case of substances which shrink greatly on polymerization and positive optical power ophthalmic lenses.
U.S. Pat. No. 3,171,869 describes a method of releasing from the mold ophthalmic lenses obtained by polymerizing a liquid substance, in particular one which shrinks greatly on polymerization, which consists in heating the sandwich to a temperature in the range from 125° C. to 235° C. just before polymerization is completed to soften the material of the lens and simultaneously relieve the polymerization stress. During this heating the polymerized material separates from the surface of the mold.
The mold release method of U.S. Pat. No. 3,171,869 reduces the risk of breaking molded lenses when releasing them from the mold. However, the molded lens inevitably separates from the surface of the mold on heating or shortly after cooling and this induces stresses that can lead to the formation of ferns.
Document WO 97/39880 describes a method of making lenses including a step of transferring heat by conduction from the surface of the mold in order to increase the degree of polymerization of the surface of the lens to obtain a more homogeneous distribution of color over the surface of the lens during subsequent coloring.
Because heating is effected by conduction of heat from a surface of the sandwich, the sandwich is not heated uniformly and this can induce stresses in the polymerized lens.
It would therefore seem desirable to develop a method of releasing from the mold transparent polymer material articles such as lenses obtained by polymerizing a polymerizable liquid substance in the cavity of a two-part mold that significantly reduces the risk of ferns and other defects occurring during subsequent coloring of the lenses.
SUMMARY OF THE INVENTION
In accordance with the invention, the above aim is achieved by a method of releasing from the mold a transparent polymer material article formed in a cavity of a two-part mold by polymerizing a liquid polymerizable substance, including:
a) uniformly heating the article in the mold to a mold release temperature in the range from Tg+20° C. to Tg+80° C., preferably in the range from +30° C. to +60° C., where Tg is the glass transition temperature of the transparent polymer material, without separating the article and the mold parts, and
b) applying an external mechanical force to separate at least one of the mold parts from the article while it is at the mold release temperature.
The mold release temperature is an important feature of the method of the invention. If the temperature is too low the stresses are suddenly relieved on release from the mold and fern type defects appear in the articles and if the temperature is too high there is a high risk of the article breaking and/or yellowing. Generally speaking, the upper limit on the mold release temperature is in the order of 160° C. For example, in making ORMA® lenses by polymerizing a liquid substance comprising a diethylene glycol diallyl carbonate monomer, the mold release temperature is in the order of 120° C.
The article is uniformly heated for at least 30 minutes and preferably at least 40 minutes.
The sandwich and consequently the molded article can be heated to the mold release temperature by any appropriate means, such as pure convection or infrared radiation, but heating is preferably effected by means of a combination of infrared radiation and convection. Heating must be as uniform as possible to eliminate excessive temperature gradients and to prevent premature mold release due to thermal shock, leading to articles with many ferns. This applies in particular to ophthalmic lenses of positive optical power and especially high positive optical power, for example greater than +3.00 diopters.
Generally speaking, after polymerization, and before the method of the present invention is applied, the temperature of the mold assembly is reduced to a temperature in the range from 20° C. to 60° C.
Although in the method of the invention only one part of the mold is generally separated from the molded article on application of the external mechanical force at the mold release temperature, the other part being separated subsequently, it is possible to separate both parts of the mold from the molded article simultaneously by applying the external mechanical force at the mold release temperature.
The external mechanical force for separating the parts of the mold from the molded article can conveniently be applied by means of a tool operating in the manner of a lever between the two mold parts.
REFERENCES:
patent: 3171869 (1965-03-01), Weinberg et al.
patent: 4909969 (1990-03-01), Wood
patent: 5358672 (1994-10-01), Blum
patent: 5750156 (1998-05-01), Maus et al.
patent: 0 710 540 A2 (1995-10-01), None
patent: WO 97/39880 (1997-10-01), None
de Marignan Gil
DeRozier Gaston
Essilor International Compagnie General d'Optique
O'Keefe, Egan & Peterman
Vargot Mathieu D.
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