JPH10324531A - Method and apparatus for forming sheet glass - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the waviness of a sheet glass and to obtain the sheet glass high in surface quality by discharging molten glass from the orifice while indirectly heating an orifice of platinum or platinum alloy formed at the bottom of a glass melting tank, thereby forming a sheet. SOLUTION: An orifice 11 of platinum or platinum alloy and a lip 11' projected downward are formed at the bottom of a feeder 10 at the bottom of a glass melting tank. The orifice 11 is in contact with a heating element 13 of platinum or platinum alloy through an insulator 12 formed with an alumina ceramic, and they are covered with a heat insulating material. When a current is applied to the heating element 13 to generate Joule heat, the orifice 11 is heated wholly and uniformly through the insulator 12, the molten glass G in the feeder 10 flows out from the lip 11' as a glass ribbon 15 and is formed into a sheet glass 16. The waviness of the sheet glass caused by the nonuniform generation of heat due to the crystal boundary when the orifice 11 is directly heated is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sheet glass forming method and apparatus suitable for forming a sheet glass requiring a high surface quality, such as a glass substrate for a liquid crystal display.

[0002]

2. Description of the Related Art Conventionally, various methods have been known as a method of forming a sheet glass. Particularly, when forming a sheet glass requiring a high surface quality, such as a glass substrate for a liquid crystal display, a slit-down method is used. Method or fusion method is suitable.

[0003] The slit down method is a method in which molten glass flows out of an elongated orifice formed at the bottom of a glass melting tank (feeder), forms a sheet glass, and then lowers it while cooling it. The molten glass is caused to flow out of the orifice formed at the bottom of the glass melting tank, and the molten glass is supplied to the upper part of the liquid control body provided thereunder, and is diverted along both side surfaces of the liquid control body. And then pull it down while cooling.

As a constituent material of an orifice used in a molding apparatus for carrying out these methods, platinum or a platinum alloy is used because it is less deteriorated even when exposed to a high temperature for a long time. An electric current is applied to the platinum alloy and the metal is heated by causing the metal itself to generate Joule heat.

[0005]

However, when a glass substrate for a liquid crystal display is produced over a long period of time using a conventional molding apparatus, fine flat undulations are locally generated in the glass sheet, and the surface quality is impaired. It was easy to have the problem of being

[0006] When a glass sheet having such undulation is used as a glass substrate for a liquid crystal display, it causes a defect in display quality. Therefore, the undulation is usually reduced by polishing the surface of the glass sheet in a later step. doing.

However, such a polishing step is a major factor in increasing the production cost of a glass substrate for a liquid crystal display, and the polishing step is performed by producing a plate glass having no undulation or small undulation in a molding step. Omission or shortening is desired.

The present invention has been made in view of the above circumstances, and when the molten glass flows out of an orifice made of platinum or a platinum alloy by a slit-down method or a fusion method to form the glass sheet, the undulation of the glass sheet is reduced. It is an object of the present invention to provide a method and an apparatus that can be made smaller.

[0009]

Means for Solving the Problems The present inventors have conducted various experiments in order to achieve the above object, and as a result, have found that the cause of undulation in the plate glass is uneven heating of the orifice. The inventors have found that such uneven heating can be eliminated by indirectly heating the platinum or platinum alloy constituting the orifice, and have proposed the present invention.

That is, the sheet glass forming method of the present invention comprises:
The present invention is characterized in that molten glass flows out and is formed into a plate shape while indirectly heating the orifice without flowing an electric current to platinum or a platinum alloy constituting the orifice formed at the bottom of the glass melting tank.

Further, the sheet glass forming apparatus of the present invention includes an orifice formed at the bottom of a glass melting tank and made of platinum or a platinum alloy for discharging molten glass, and a heating element in contact with the orifice via an insulator. The orifice is indirectly heated by heating the heating element.

[0012]

Generally, an orifice made of platinum or a platinum alloy used in a plate glass forming apparatus is manufactured by casting and forming. Before use, the crystal grains of platinum or a platinum alloy are very fine. When used for a long time and exposed to high temperatures, the crystal grains become very coarse.

As the crystal grains of platinum or a platinum alloy become coarse in this manner, substances originally contained as impurities in the material are captured and accumulated in the crystal grain boundaries, or contact with the glass melt. From the surface, the substance contained in the glass diffuses into platinum or a platinum alloy, and is captured and accumulated at crystal grain boundaries. Such a phenomenon can be regarded as a process in which a very high-purity part (inside the crystal) and a low-purity part (crystal grain boundary) are formed in platinum or a platinum alloy.

In the conventional apparatus, an electric current is directly applied to the platinum or platinum alloy to heat the metal itself by generating Joule heat. The amount of heat obtained by Joule heat is obtained by (current) ² × (resistance). Therefore, under a constant current, the amount of heat generated in a grain boundary portion having a large amount of impurities and a high electric resistance is larger than that in a crystal having a small amount of impurities and a low electric resistance.

As a result, the platinum or platinum alloy constituting the orifice in which the crystal grains are coarsened due to Joule heat generated for a long period of time,
Since crystal parts having a small amount of heat generation are alternately present and intersect, it is difficult to uniformly generate heat. Such non-uniform heat generation is transferred to the molten glass passing through the orifice, and the glass also has non-uniform temperature distribution.

However, in the present invention, since the platinum or platinum alloy itself constituting the orifice is indirectly heated without generating Joule heat, the difference in the electrical resistance between the inside of the crystal of platinum or the platinum alloy and the grain boundary portion is reduced. The resulting difference in the amount of generated heat can be eliminated, and the entire orifice can be uniformly heated.

[0017]

The present invention will be described below in detail based on examples and comparative examples.

(Example) FIG. 1 is a longitudinal sectional view showing an apparatus for forming a sheet glass of the present invention by a slit down method.

As shown in FIG. 1, a slot-shaped orifice 1 made of a platinum-rhodium alloy is provided at the bottom of the feeder 10.
1 (length 1000 mm, width 10 mm), and a lip 11 ′ protruding downward is formed at the bottom of the orifice 11 over the entire circumference.

The orifice 11 is in contact with a heating element 13 made of a platinum-rhodium alloy via an insulator 12 made of alumina ceramics.
The heating element 13 is attached to the feeder 10 together with the heat insulating material 14.

When an electric current is applied to the platinum alloy constituting the heating element 13 of this device to generate Joule heat, the heat is transmitted to the orifice 11 via the insulator 12, and the entire orifice 11 is uniformly heated. The molten glass G in the feeder 10 flows out of the tip of the lip 11 ′ of the orifice 11 as a glass ribbon 15 and is formed into a sheet glass 16.

With such an apparatus, a thickness of 0.7 mm
Of glass substrates for liquid crystal displays (Nippon Electric Glass Co., Ltd .: OA-2)
After a lapse of time, the undulation of the formed sheet glass was measured using a stylus-type undulation meter and found to be 0.15 μm.

(Comparative Example) The orifice 11 and the heat insulating material 1 were provided without providing the insulator 12 and the heating element 13 in the embodiment.
4 at the bottom of feeder 10 and orifice 11
A similar sheet glass was produced by applying an electric current directly to the platinum alloy constituting the above to generate Joule heat in the metal itself.

When the undulation of the formed glass sheet was measured 100 hours after the start of production, it was as large as 0.3 μm, and the surface quality was inferior to the glass sheet of the example.

[0025]

As described above, according to the method of the present invention, since the orifice is heated indirectly, even if the sheet glass is produced for a long time, the entire orifice can be heated uniformly. The generation of undulation of the sheet glass is suppressed.

Further, the forming apparatus of the present invention can perform indirect heating of the orifice. Since the plate glass formed by this apparatus has a small undulation, it is particularly high, such as a glass substrate for a liquid crystal display. It is suitable as a sheet glass requiring surface quality.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a sheet glass forming apparatus of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Feeder 11 Orifice 12 Insulator 13 Heating element 16 Sheet glass

Claims (2)

[Claims] 1. An indirect heating of an orifice without flowing an electric current to platinum or a platinum alloy constituting an orifice formed at the bottom of a glass melting tank, and the molten glass is caused to flow out and formed into a plate shape. A sheet glass forming method characterized by the above-mentioned. 2. An orifice formed at the bottom of a glass melting tank and made of platinum or a platinum alloy for discharging molten glass, and a heating element in contact with the orifice via an insulator, and heats the heating element. A sheet glass forming apparatus characterized in that the orifice is indirectly heated by this.