Glass manufacturing – Processes – With wearing away of surface material
Reexamination Certificate
1999-09-08
2002-09-03
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
With wearing away of surface material
C065S064000, C065S066000, C065S102000, C065S106000
Reexamination Certificate
active
06442975
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing by press forming a thin-plate glass with a thickness of about 3 mm or less which is used as a substrate of a magnetic recording medium, a magneto-optic recording medium, an optical recording medium and another information recording medium or as a plate glass for a camera filter, mask blanks or the like, also relates to a method of manufacturing a glass substrate for an information recording medium and further relates to a magnetic recording medium.
2. Description of Related Arts
A glass substrate has been increasingly used as a substrate for a magnetic recording medium or another information recording medium lately. Further, as a method of manufacturing the glass substrate, instead of cutting out of a plate glass, molten glass is directly pressed by using a forming die, that is, a direct pressing method is used.
A prior-art direct pressing method is disclosed in Japanese Patent Application Laid-open No. Hei 5-105458. In this method, a forming die with a die release agent layer formed on a forming face is used. A glass material is pressed for a sufficient period of time until the glass material is at a softening point or a lower temperature and is thermally in balance with upper and lower dies, to form a disc-like glass product having a final configuration with a small warp.
However, in the method described above in which the press forming is performed for a sufficient time until the glass material has a softening point or a lower temperature and brought in thermal balance with the upper and lower dies, the press forming takes much time. Thus, mass productivity is disadvantageously deteriorated.
Also, in the method disclosed in Hei 5-105458, it is difficult to control temperatures completely. The warp can be minimized only to some degree. The glass product which is obtained in the manufacture method has been heretofore able to be used as the magnetic recording medium substrate. Recently, however, a demand for densification of the magnetic recording medium has been increased. A substrate with a smaller warp than a conventional substrate is requested for. Especially, the magnetic recording medium substrate which can be used on an MR head is requested to have a high flatness. Therefore, the prior-art method may not be desirable.
Also in the method disclosed in Hei 5-105458, when manufacturing a glass substrate for the MR-head magnetic recording medium, the glass substrate needs to be finally ground and polished to conform to predetermined specifications. The grinding is performed by using a grinding plate while a pressure is applied to both side faces of a thin-plate glass. The thin-plate glass with a warp is deflected when ground. Thus, problem is that when pressure is released from both sides of the ground thin-plate glass, the thin-plate glass is again warped. To solve the problem, when the thin-plate glass is ground, the pressure to be applied thereto needs to be constantly adjusted finely so that the thin-plate glass fails to be deflected. This lengthens the grinding time. Also in this respect, the mass productivity of the thin-plate glass with a good flatness cannot be enhanced.
Another direct pressing method is disclosed in Japanese Patent Application Laid-open No. Hei 7-133121. In this method, surface temperatures of pressing faces of upper and lower dies are set to a transition point of a glass to be press-formed or its vicinity. Additionally, while inner-surface temperatures of cylindrical dies are set higher than the pressing-face surface temperatures described above, molten glass is pressed between the upper and lower dies to form a disc-like glass product close to a final product.
Also in this method, however, the obtained glass substrate needs to be ground/polished to conform to the predetermined specifications in the same manner as the method described above. For the same reason as described above, the mass productivity is deteriorated.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of manufacturing a thin-plate glass having a good flatness with a high mass productivity. Further object is to provide a method of manufacturing a glass substrate for an information recording medium and to provide a magnetic recording medium.
To attain these and other objects, the present invention provides a method of manufacturing a thin-plate glass in which press forming is performed between a lower die onto which molten glass is supplied and an upper die which is opposed to the lower die. The press forming is performed between the lower and upper dies which are kept at a predetermined temperature. The press forming is finished when the inside of the thin-plate glass has a temperature higher than a glass transition point. Subsequently, a warp of the press-formed thin-plate glass is modified. The warp modifying process is finished when the inside of the thin-plate glass has a temperature higher than the glass transition point. To shorten the time for pressing, the press forming and the warp modification pressing are preferably finished when the temperature is higher than a glass softening point. Also, the glass temperature during the warp modifying process is preferably lower than the press forming temperature. Thus, since the pressing is finished at the temperature which is higher than the glass transition or softening points, the press-formed glass basically maintains a configuration given by forming faces of the dies even after released from the dies, and can be slightly deformed by a external force. Each pressing needs a period of time preferably within two seconds, more preferably within 1.8 seconds.
Here, the inside of the glass described above means a glass main portion covered with a surface layer which remarkably radiates heat. The temperature of the inside of the glass influences the configuration maintaining property and deformability.
Also, the thin-plate glass of the invention means a thin-plate glass substrate represented by a glass substrate for a magnetic disc. Typically, the glass has a thickness of 2 to 4 mm and a diameter and a length both of 15 cm or less.
The lower die is designed to successively go through glass gob supply process, press forming process, warp modifying process, formed product taking process and the like. For example, plural lower dies are preferably arranged on the circumference of a turntable, and the turntable is preferably rotated in such a manner that the lower dies go through the processes. The lower dies may be designed to move linearly. Also, a single lower die may be supplied to each of the processes.
On the other hand, the upper die is disposed opposite to the lower die which is positioned in the press forming process. Therefore, the number of the upper dies needs to be at least the same as the number of the lower dies for use in one press forming, but more number of the upper dies may be used. Also, a single upper die may be used on the condition that by removing the heat transferred from the molten glass to the upper die after the press forming, temperature can be controlled in a short time to allow the temperature of the upper die to reach an appropriate temperature at the time of press forming.
Subsequently, the temperature of the forming faces of the upper and lower dies (hereinafter, often referred to as the forming die) needs to be adjusted to a predetermined temperature when the press forming is started.
Here, the predetermined temperature of the forming die means the temperature adequate to form a glass material to a thin plate. The temperature is appropriately determined by glass species, thickness, glass plate size and the like.
Further, in order to adjust the temperature of the upper and lower die forming faces to the above predetermined temperature when starting the press forming, the upper and lower dies are heated or cooled as required.
As means for heating the dies used is a heating method in which plural Nichrome heaters are arranged around the lower die (upper die), an indu
Murakami Akira
Nagakura Katsuya
Otagiri Takeo
Saito Jun
Saito Tetsuya
Frishauf Holtz Goodman & Chick P.C.
Hoya Corporation
Vincent Sean
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