Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2002-06-18
2004-06-22
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S079000
Reexamination Certificate
active
06753281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical glass of a lanthanum borosilicate system having a low glass transformation temperature (Tg) and excellent resistance to devitrification property, the optical glass having optical constants which are a refractive index (nd) in a range of 1.70 to 1.75 and an Abbe number (&ngr;d) in a range of 45.0 to 54.0, the optical glass being suitable for forming a glass preform material used for precision press molding, and being suitable for precision press molding.
2. Description of Related Art
Precision press molding of a glass is a technique to obtain a glass molded product having a shape of a final product or a shape extremely close to the final product and surface accuracy by performing press molding to a glass preform material, which is soften by heating, under a high temperature by using a molding die having a cavity with a predetermined shape. According to the precision press molding, it is possible to manufacture a molded product with a desired shape on the basis of high productivity without performing grinding and polishing, or hardly performing grinding and polishing after molding. Therefore, at present, glass molded products, such as spherical lenses, aspherical lenses and the like, have been manufactured by precision press molding.
Recently, miniaturizing and lightening of optical devices have been remarkably progressing. Aspherical lenses are used so as to decrease the number of lenses that construct an optical system of the optical devices. Since it is extremely difficult to manufacture an aspherical lens in large quantities and inexpensively by a method according to grinding and polishing in earlier technology, the above-described precision press molding is the most suitable molding method for manufacturing particularly an aspherical lens.
In order to obtain a glass molded product by the precision press molding, it is necessary to perform press molding to a glass preform material under a high temperature, as described above. Therefore, the molding die used in this case is also exposed to the high temperature, and a high pressure is added thereto. Thereby, in respect to the optical glass that constructs the glass preform material, it is desired to make its glass transformation temperature (Tg) as low as possible from the viewpoint of preventing the wear of the molding surface of the molding die caused by oxidation of the surface in accordance with high temperature environment and the damage of the molding surface of the molding die in accordance with the high press pressure when press molding is performed.
As a method for manufacturing a glass preform material for performing precision press molding, there is a method for manufacturing a glass preform material having a shape close to the shape of the lens, which is a final product, or having a spherical shape by obtaining a glass block cut from a glass block material, and by grinding and polishing the glass block. However, since the cutting process of the glass block material, grinding and polishing processes are required, there is a problem that those processes take time. Further, there is a method for obtaining a glass preform material by dropping or flowing down a molten glass from the tip of an efflux pipe connected to a glass melting device, forming the molten glass in a die or the like, and cooling it. In this method, cutting from a glass block material, grinding and polishing processes are not required, and it is possible to obtain the glass preform material directly from the molten glass. Therefore, as a method for manufacturing a glass preform material, the latter method is the method that has the highest mass-productivity and the manufacturing cost is the lowest. The shape of the glass preform material obtained by the latter method is a biconvex lens-like or a spherical shape. In many cases, the biconvex lens-like shape is the shape close to the shape of an aspherical lens, which is a final product, or the like. Therefore, variations in shape when precision press molding is performed can be made small, and the biconvex lens-like glass preform material has an effect of remarkably improving the mass-productivity of the lens itself. Further, when it is a spherical shape, although variations in shape when precision press molding is performed become large in many cases, there is a merit that the glass preform material can be set easily in the center of a lower die of the molding die whose molding surface is usually a concave surface.
Incidentally, in the press molding technique of glasses in earlier technology, even though devitrification was generated on a surface of a glass preform material or a pressed glass molded product, the portion of devitrification on the surface was removed by grinding or polishing performed after press molding. Therefore, it was not a problem if devitrification was not generated in the inside of the glass. However, in the precision press molding, a glass molded product to which precision pressing is performed without performing grinding and polishing or hardly performing grinding and polishing after molding is used as an optical element, such as a lens or the like. Therefore, it cannot be used as a product even if devitrification is generated only on a surface of a glass preform material or a glass molded product. That is, in the glass preform material used for precision press molding and the optical glass used for precision press molding, it is required that devitrification is not generated at a temperature suitable for forming the glass preform material, and moreover, that devitrification is not generated when precision press molding is performed to the obtained glass preform material, in addition to the glass transformation temperature (Tg) being low, as mentioned above.
Devitrification of a glass is caused when the temperature range of the glass is in the range that the nucleation temperature range and the crystal growth temperature range, which is in the high temperature side rather than the nucleation temperature range, are overlapped. The longer the time that the glass is exposed to this temperature range is, the more the crystal is grown and the devitrification progresses. In the method for manufacturing a glass preform material by dropping or flowing down a molten glass, as mentioned above, when the viscosity of the molten glass for dropping or flowing down from the tip of an efflux pipe is too low, it becomes difficult to obtain a preform material having a smooth curved surface and a spherical shape or a shape close to a biconvex lens-like shape. Further, when the viscosity of the molten glass is too high, both dropping a glass having a weight for one piece of preform material from the tip of the efflux pipe, and separating a glass gob having a weight for one piece of preform from the molten glass flow flowed down from the tip of the efflux pipe by surface tension or the like, become difficult. Therefore, it is desired to adjust the viscosity (&eegr;) of the molten glass for being dropped or flowed down in a range of log&eegr;=approximately 1.5 to approximately 2.5.
Incidentally, for the molten glass for being dropped or flowed down from the tip of the efflux pipe, it is comparatively hard to generate devitrification since the molten glass does not remain in the temperature range that the nucleation temperature range and the crystal growth temperature range are overlapped for a long time generally. However, for the glass deposited to the peripheral portion of the tip of the efflux pipe, devitrification is easily generated since it is exposed to the temperature range lower than the upper limit of the temperature range that the nucleation temperature range and the crystal growth temperature range are overlapped, that is, the temperature range that devitrification is generated, for a long time. Then, the glass in which devitrification is generated is gradually involved in the molten glass which is dropped or flowed down. Therefore, after forming of the preform material is started, d
Bolden Elizabeth A.
Group Karl
Kabushiki Kaisha Ohara
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