Process for molding thermoplastic lenses and, steeply curved...

Details Process for molding thermoplastic lenses and, steeply curved... Process for molding thermoplastic lenses and, steeply curved...

2000-09-08

2002-08-27

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...

C264S328700, C264S328800

Reexamination Certificate

active

06440335

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to novel molding methods for the manufacture of thermoplastic lenses and is particularly adaptable for the manufacture of steeply curved ophthalmic lenses and ophthalmic lenses with thin centers. The invention includes the resultant lenses.
Lens Molding
Lenses are used for a variety of purposes, for example in optical devices such as microscopes and eye glasses. Over the past few years, the use of thermoplastic material to prepare ophthalmic lenses for such uses as in vision corrective and in prescriptive (R
x
) spectacle lenses as opposed to traditional glass lenses has increased dramatically because thermoplastic lenses offer several advantages over glass. For example, plastic is lighter than glass and hence spectacles with plastic lens are more comfortable to wear especially since the nominal lens thickness is typically 2.0-2.2 mm. Other factors for increased demand for thermoplastic lenses are that these lenses can be made scratch and abrasion resistant, they come in a wide range of fashionable colors, and because the production techniques have improved so that they can now be manufactured at higher rates and in a more automated fashion.
Of the thermoplastic lenses, the use of polycarbonate thermoplastic is becoming very attractive as compared to, for example, lenses made from individual casting and thermoset-peroxide curing allylic resins. Factors favoring polycarbonate thermoplastic lenses include lower density and higher refractive index than cast-thermoset plastic. Hence, thinner lenses in the range of 1.5-2.0 mm thickness can be made. In addition, polycarbonate lenses of the same nominal thickness as thermoset-peroxide cured allylic resins will be of lighter weight, due to lower density, and therefore will impart greater wearer comfort. Furthermore, polycarbonate thermoplastic lenses have far greater impact and breakage resistance than any other optical grade polymeric material.
Heretofore, thermoplastic, injection-molded lenses have been manufactured by injection molding with or without any compression. Injection molding without any compression typically involves the use of a mold cavity having fixed surfaces throughout the molding cycle. Such molding processes employ very long molding cycles, high mold-surface temperatures, higher than average plastication and melt temperatures for that given resin, and slow controlled fill rates followed by very high packing pressures which are held until gate freeze-off is complete.
Fixed cavity processes of the type described above, employ larger than normal gating and runner systems to permit maximum packing pressure and delivered material before gate freeze-off occurs, at which time no further transfer of molten polymer occurs between the runner system or plasticating unit and the cavity. Gate freeze-off in a fixed cavity injection machine presents a problem, given that powered lenses have differing front and back radii of curvature, prescription lenses must therefore have differing cross-sectional thicknesses which in turn leads to non-uniform shrinkage during part formation in the mold cavity and cooling-down which can cause poor optics and/or distortion. In addition, the thickest sections of the lens are subject to slight sink marks or depressions which in turn cause a break in the otherwise uniform radius of curvature of the lens surface. This break results in a localized aberration or deviation in the light bending character of the lens at that area of sink.
Thus, although great care is taken to see that the injected polymer mass conforms perfectly to the fixed lens mold cavity surface, contour, and dimensions, once gate freeze-off prevents additional packing pressure and material transfer, differential shrinkage begins to occur within the polymer melt and the polymer skin begins to pull away from the mold surfaces accordingly. This pre-release detrimentally affects optical quality since the molded lens contour and surface no longer can be forced by intimate contact to exactly replicate the precision optical mold surfaces and cure contours. Also, a fixed cavity molding process is limited in how thin the lens center can be. Below about 2 mm, the molten plastic preferentially flows around the thick edge leaving a void and/or knit line which extends into the central zone of the to-be-formed lens.
To address these problems with fixed cavity molding processes, compression molding techniques have been used. The injection/compression molding techniques are divided into two types (i) the clamp-end injection/compression and (ii) the auxiliary component injection/compression. In the clamp-end injection/compression method, the molten polymer is injected into a mold space formed by moving the mold platens and mold halves to a predetermined position. After or during injection, the molten polymer mass is allowed to cool for a predetermined time interval and the injection molding machine commences a closing motion of the movable platen. This clamping-up motion compensates for shrinkage occurring during freezing of the molten polymer. Under this clamp-induced compressive force, the mold cavity's contents continue cooling and solidifying, eventually reaching a temperature sufficiently below the glass-transition temperature, or freezing point, of the injected polymer that the molded article may be safely ejected without risking optical distortion. However, in view of the high clamp pressure, thin centered lenses may be subjected to crushing of the frozen center portion while the remaining areas of the mold retain molten polymer.
This method however, has severe limitations. First, it is critical to carefully control the injection pressure and fill rate, along with the timing interval. For example, the injected melt must be allowed to form a surface skin and partially solidify to prevent molten polymer from spilling outside the desired runner-mold-cavity configurations, necessitating costly and laborious trimming operations on the molded part. Second, if the melt solidifies to too great an extent, compression at ultimate clamping pressures can cause hobbing or deformation of the mating plats at the parting line, thus damaging the mold set. Third, if compression is delayed too long, too much polymer solidification will have occurred when the compressive force through final clamp-up is initiated, resulting in forcible reorientation of the polymer and cold working of the plastic, which, in turn, produces birefringence and undesirable molded-in stress levels, with resulting localized nonuniform light-bending characteristics.
In the auxiliary component injection/compression method the compressive pressure is applied to the opposing optical surfaces via auxiliary springs, cylinders or the like which are either internal to the mold itself or as peripheral apparatus thereto. Early thermoplastic lens molding of this type employed simple spring-loaded, movable optical dies within the mold set. Johnson, et al., “Compressor Unit”, U.S. Pat. No. 2,443,826, issued Jun. 22, 1948. Such apparatus created a variable volume lens mold cavity thereby, but relied upon high internal polymer melt pressure to spread the movable dies against the resisting spring pressure. In order to apply sufficiently great compressive forces upon the solidifying mold contents, these spring forces were great. However, the greater the spring force, the greater the injection pressure that must be used to compress the springs during variable cavity fill. The greater the injection pressure required, the greater the degree of molded-in stresses and optically unsatisfactory birefringence. The greater the optical power for the molded lens, the greater the dissimilarity between the front and back curves and thus the greater the cross-sectional thickness variation. Therefore, this process is limited to production of weakly powered lenses with minimal diameter and minimal thickness variations.
Another auxiliary component process is represented by the patents to Weber: “Apparatus for Injection Molding Lenses”, U.S.

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