Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
1998-05-29
2001-07-17
Silverman, Stanley S. (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S401000, C065S404000, C065S403000, C065S066000, C065S126000, C065S475000
Reexamination Certificate
active
06260387
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the molding of optical glass lenses and, more particularly, to a method for the manufacture of preform elements for use in the molding of optical glass lenses.
BACKGROUND OF THE INVENTION
Because the molding of glass optical elements is done by compression rather than injection (as is utilized in plastic molding) a precursor metered amount of glass, generally referred to as a preform, is required. There are two fundamental shapes of preforms required which generally parallel the two fundamental finished lens shapes. For negative lenses, plano-plano preforms usually will be sufficient. These can be fabricated in high volume relatively inexpensively by grinding and polishing. For positive lenses, a ball (sphere) or ball-like lump of glass is needed. The basic constant when molding positive or negative lenses is that the molds must touch the softened glass at the center first and then press out to the edges to avoid wrinkles and voids in the finished lens element.
Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass optical elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Angle et al, U.S. Pat. No. 4,139,677 to Blair et al, and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of mold inserts used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the mold inserts included glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform is inserted into a mold cavity with the molds being formed out of one of the above mentioned materials. The molds reside within a chamber in which is maintained a non-oxidizing atmosphere during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the preform up to about 100° C. above the Glass Transition Temperature (T
g
) for the particular type of glass from which the preform had been made. Pressure is then applied by the mold to force the preform to conform to the shape of the mold. The mold and preform are then allowed to cool below the transition temperature of the glass. The pressure from the mold is then relieved. The temperature is lowered further and the finished molded lens is removed.
One of the major factors which greatly effects the economics and quality of precision glass molding of optical elements is the fabrication of the precursor quantity of glass referred to herein as a preform. A preform may be distinguished from a blank such as those used in the conventional grinding and polishing operations for the manufacture of a lens. A blank has glass removed to become a finished part. A preform is merely reshaped in the molding process and it is not necessary to remove glass from the preform. This requires that the preform have a very good surface quality as well as internal quality. There are two principal methods used to fabricate preforms. The first is to grind and polish the preform to lens type quality in a shape that allows it to be further reshaped to a finished lens through the molding operation. Although this is the most straight forward method, it is also the most costly. The second technique is to produce the preform through some type of molten glass gobbing process. One such process is taught in U.S. application Ser. No. 08/725,382 entitled, “Method and Apparatus for Making Optical Preforms with Controlled Peripheral Edge Wall Geometry”. A variant of this method is taught in U.S. Pat. No. 5,709,723 to Gearing.
The prior art fails to teach a method for producing continuous rods or sticks of glass having high surface and internal quality which can be cut to length to produce preforms of predetermined volumes with both good surface and internal quality. In such manner, the production of preforms becomes substantially continuous.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method for producing continuous rods of glass which can be cut in predetermined lengths for use as preforms for molding optical glass elements.
It is a further object of the present invention to provide a method for producing continuous sticks of glass comprising integrally formed, interconnected spherical beads which can be separated into individual preforms for use in molding optical glass elements.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a reading of the detailed description, claims and the drawings set forth herein. These features, objects and advantages are accomplished through the steps of forming a homogeneous melt of a predetermined optical glass, maintaining the temperature of the glass at at least the melt temperature and producing a rod of glass from the homogeneous melt, allowing the rod of glass to cool and solidify, and then cutting the rod of glass into predetermined lengths. The rod may be formed by dipping a bob into a vessel such as a crucible containing the homogeneous melt and withdrawing the bob at a predetermined pull rate thereby producing the rod of glass with a predetermined diameter. Using a constant pull rate, the rod of glass will be cylindrical in shape and can be cut into a series of individual cylindrical preforms. Using a cyclical pull rate, the resulting rod of glass will be comprised of a stick of interconnected beads of glass which can be cut into individual spherical preforms. In either case, the preforms can be inserted into a compression molding apparatus for the forming of a glass optical element therewith. Whether the preform is cylindrical in shape or spherical in shape, orientation of the preform is such that the cut planar surfaces of the preform are parallel to the direction of compression of the compression molding process. The preforms formed by the process of the present invention exhibit both good surface quality and good internal quality. Orientation of the preform with the cut surfaces parallel to the direction of compression in the final compression molding operation ensures that the optical quality surface of the preform will be the optical surface of the lens formed therewith and the cut or sawed surfaces of the preform will make up the periphery of the final lens. Rather than drawing rods of glass using a bob as described above, alternatively, rods of glass may be extruded.
REFERENCES:
patent: 3833347 (1974-09-01), Angle et al.
patent: 4060401 (1977-11-01), Maries et al.
patent: 4101925 (1978-07-01), Kelley et al.
patent: 4139677 (1979-02-01), Blair et al.
patent: 4168961 (1979-09-01), Blair
patent: 5425795 (1995-06-01), Koide et al.
patent: 5683482 (1997-11-01), Fredholm
patent: 5709723 (1998-01-01), Gearing
patent: 5766293 (1998-06-01), Gearing
Bocchetti Mark G.
Eastman Kodak Company
Ruller Jacqueline A.
Silverman Stanley S.
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