A manufacturing method of an optical element such as a glass lens, a so-called press molding method, has come to be widely employed in recent years. The press molding method directly conducts molding of an optical element by pressing glass heated at not lower than the deformation temperature (At) by use of a molding die including a heated pair of an upper mold and an lower mold. The method needs fewer manufacturing processes to be capable of manufacturing an optical element in a shorter time and at a lower cost compared to a conventional molding method to grind glass. Therefore, it has come to be widely employed as a manufacturing method of an optical element in recent years.
This press molding method can be roughly classified into a reheating method and a direct press method. A reheating method is a method, in which, after a gobpreform or a ground preform having a nearly final product form is prepared, these preforms are reheated at over the softening point followed by being pressed and molded by a pair of heated upper and lower molds to prepare a final product form. On the other hand, a direct press method is a method, in which molten glass drops are directly added drop-wise on a heated die from a glass melting furnace and are press-molded to prepare a final product form. In either of these press molding methods, it is necessary to heat a press die at a temperature neighboring or higher than a glass transition temperature (Tg) when glass is molded. Therefore, the higher is a glass transition temperature (Tg), the more easily caused is surface oxidation of a press molding die or variation of a metal composition to shorten a life of a molding die, resulting in an increased cost of manufacturing. Although deterioration of a molding die can be restrained by conducting molding under an inert gas atmosphere such as nitrogen, a manufacturing cost is increased because a molding system may become complicated to control the atmosphere and a running cost of an inert gas is also required. Therefore, glass utilized in a press molding method is preferably has a glass transition temperature (Tg) as low as possible.
Further, a nozzle comprising a material such as platinum is generally employed to drop molten glass drops. The weigh of glass added drop-wise is controlled by this nozzle temperature. In glass having a low liquid phase temperature (TL), since nozzle temperature can be set in a wide temperature range from high temperature to low temperature, it is possible to prepare an optical element of various size from large one to small one. On the other hand, in the case of glass having a high liquid phase temperature (TL), there is a problem that stable drop-wise addition is difficult since glass may be devitrified unless nozzle temperature is kept at not lower than a liquid phase temperature (TL) Further, in the case of glass having a high liquid phase temperature (TL), reactivity with a molding die will increase resulting in a problem of shortened life time of a molding die since temperature of glass added drop-wise itself becomes high.
Herein, in consideration of a working environment, it is desirable not to use a lead compound, an arsenic compound and a fluorine compound. Various technologies, which lower a glass transition temperature (Tg) and a liquid phase temperature (TL) without employing these compounds, have been studied and proposed, as disclosed in, for example, patent documents of US2002/042337 and JP-A 2005-8518 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection No.).
However some of optical glass shown in the above patent documents has a glass transition temperature (Tg) of not higher than 520° C., the liquid phase temperature (TL) is not satisfactorily low. Since a liquid phase temperature (TL) of the optical glass is not so low, the glass will be devitrified in a nozzle to induce nozzle clogging like hardening of the arteries. As a result, there is a problem that stable drop-wise addition and press molding of glass are difficult.
In view of such a problem, it is desired to provide optical glass of high productivity having predetermined optical constants without including compounds such as lead compound and arsenide.