This invention relates to a method of producing a glass gob to be subjected to press molding, a method of producing a glass molded product by pressing the glass gob, and an apparatus for producing the glass gob.
In order to produce an optical glass component, use has widely been made of a technique of press-molding a glass by the use of a die or a mold. Such technique includes a direct press method of directly pressing a molten glass gob and a reheating/pressing (RP) method of reheating and pressing a preform which has a shape approximate to that of a final or an end product.
Traditionally, the above-mentioned preform is prepared in the following manner. The molten glass is made to flow down from an outlet pipe and cut by a cutting tool such as a shear blade to obtain a molten glass gob having a predetermined weight. The molten glass gob is cooled into a glass gob to be directly used as the preform. Alternatively, the glass gob thus obtained is subjected to polishing before it is used as the preform. In the existing method of obtaining the glass gob by flowing down the molten glass from the outlet pipe, the weight of the glass gob is controlled by changing the diameter of a nozzle attached to an end of the outlet pipe. This means that the nozzle must be exchanged to another nozzle in order to produce the glass gob having a different weight. Furthermore, in order to reuse the nozzle once removed, a residual glass cooled and solidified in the nozzle must be eliminated.
In order to overcome the above-mentioned problem, the present applicant has already developed a method of blowing a gas to the molten glass before spontaneous falling of the molten glass from the nozzle, thereby forcibly drop down, as a molten glass drop, the molten glass having a particular weight as a dropping weight (see Japanese Unexamined Patent Publication No. H10-194752 (JP 10-194752 A, which will hereinafter be referred to as an earlier publication). In this method, the dropping weight of the molten glass drop is controlled by adjusting the timing of blowing the gas. By changing the timing, it is possible to continuously change the dropping weight from one to another. For example, if the gas is blown at a shorter time interval, the weight of the glass gob to be obtained is reduced. In the above-mentioned method, it is unnecessary to exchange the nozzle every time when a desirable dropping weight of the molten glass is changed from one to another. Upon press molding a video camera lens, a compact disk pickup lens, and so on, a small glass gob to be used as a preform is required. Such a small glass gob can be produced by this method.
The method disclosed in the earlier publication is excellent and useful as mentioned above. However, it is difficult to produce an extremely small or minute glass gob, which is further smaller in weight, with high weight accuracy. In the method disclosed in the earlier publication, the gas blown to the molten glass is guided to a nozzle portion by an air blow pipe. The air blow pipe is extended from an exterior of a heater through a lower opening of the heater to a nozzle end within the heater. Therefore, the gas blown to the molten glass has a temperature extremely lower than that of the molten glass.
The gas is intermittently blown to the molten glass at a predetermined timing corresponding to the dropping weight. Alternatively, in order to prevent the molten glass from wicking or wetting upwards along an outer peripheral surface of the nozzle (due to a capillary phenomenon or a flowing up movement caused by a gas flow), the gas is continuously blown relatively weakly at a relatively low pressure and, at a desired drop timing, the gas is blown relatively intensely at a relatively high pressure to forcibly drop the molten glass. In either event, the molten glass is dropped by relatively intensely blowing the gas at a controlled timing.
The present inventors have found out that, in order to obtain a glass gob having an extremely small weight with high weight accuracy by dropping the molten glass, it is important to keep a constant flow rate of the gas blown to the molten glass and to continuously flow the gas in a laminar flow condition. It has also been found out that, even upon occurrence of some fluctuation in flow rate of the gas, the influence of the fluctuation upon the weight accuracy of the glass gob is minimized under a particular condition. Furthermore, the present inventors have found out a method of setting the particular condition.
It is an object of this invention to provide a method of producing a glass gob with high weight accuracy.
It is a specific object of this invention to provide a method of producing an extremely small glass gob with high weight accuracy.
It is another object of this invention to provide a method of producing a glass molded product by the use of the above-mentioned glass gob.
It is still another object of this invention to provide an apparatus for producing the above-mentioned glass gob.
According to a first aspect of this invention, there is provided a method of producing a glass gob by successively dropping or dripping as molten glass drops a molten glass from a nozzle in a dropping direction, the method comprising the step of generating a gas flow continuously flowing in the dropping direction along an outer peripheral surface of the nozzle at a predetermined flow rate to apply a wind pressure or a blow/flow pressure upon the molten glass appearing from an end of the nozzle, thereby dropping the molten glass.
According to a second aspect of this invention, there is provided a method of producing a glass molded product, the method comprising the steps of preparing a glass gob by the use of the above-mentioned method in the first aspect, reheating the glass gob, and press-molding the glass gob to obtain the glass molded product.
According to a third aspect of this invention, there is provided an apparatus for producing a glass gob by dropping a molten glass as a molten glass drop, receiving the molten glass drop in a molding die, and molding the molten glass drop to produce the glass gob, the apparatus comprising a nozzle for flowing out the molten glass, a molding die for receiving the molten glass drop dropping down from the nozzle and molding the molten glass drop into the glass gob, a nozzle cover covering and surrounding an outer periphery of the nozzle to define a gas flow path between the nozzle cover and the nozzle and to define a nozzle cover opening between a nozzle cover end of the nozzle cover and a nozzle end of the nozzle, and gas supplying means for continuously supplying a gas of a predetermined flow rate or a predetermined quantity into the interior of the nozzle cover. The gas supplied into the interior of the nozzle cover passes through the gas flow path defined between the nozzle and the nozzle cover to be blown out through the nozzle cover opening towards the nozzle end so that a wind pressure of the gas blown out through the nozzle cover opening is applied to the molten glass appearing from the nozzle end protruding from the nozzle cover opening, thereby dropping the molten glass.