Many types of flat glasses are being used in various fields such as a window glass, an automobile window screen, a mirror, and the like. A flat glass may be manufactured by various techniques, and among them, a typical technique is a production technique using a float method. For example, a thin glass plane or a glass film for a thin-film-transistor (TFT) display is manufactured primarily by a float method, and a glass manufactured by a float method is called a float glass.
A method for manufacturing a float glass includes a continuous circulation process, and is gaining attention as a typical method for manufacturing a flat glass in that the method is operable discontinuously and permanently, for example, for at least several years almost without interruption as possible.
FIG. 1 is a diagram schematically illustrating a partial construction of an apparatus for manufacturing a float glass according to a related art.
As shown in FIG. 1, a float glass is generally formed using a float bath 10 in which a metal melt M such as a tin melt or a tin alloy melt is stored and circulates. In this instance, a glass melt having a lower viscosity than the metal melt M and being lighter by approximately ⅔ than the metal melt M is continuously supplied into the float bath 10 through an inlet of the float bath 10 via a spout lip 11. Inside the float bath 10, the glass melt G moves to a downstream side of the float bath 10 while the glass melt G is floating and spreading on the metal melt M. In this process, the glass melt G reaches roughly an equilibrium thickness by the effects of its surface tension and the gravity, so a glass strip or ribbon solidified to some extent is formed.
Subsequently, the glass melt is pulled toward an annealing furnace by a lift out roller adjacent to an outlet of the float bath to pass through an annealing process. In this instance, a thickness of a resulting glass may change by adjusting and changing an amount of glass poured through the inlet, a pulling rate determined by a rotation rate of rollers, and a forming means such as top rollers installed in a float chamber.
FIG. 2 is a diagram illustrating a spreading shape of the glass melt poured through the inlet of the float bath according to the related art, when viewed from the top of the float bath. In FIG. 2, an arrow denotes a movement direction of the glass melt.
Referring to FIG. 2, the metal melt M is received by an inner wall 12 of the float bath, and the glass melt G is provided from the spout lip 11 onto the metal melt M. Also, the provided glass melt G gradually spreads in a widthwise direction while moving in a downstream direction of the float bath 10. That is, as seen in the drawing, the glass melt G gradually spreads in upper and lower directions (widthwise direction) of the drawing while moving in a left to right direction of the drawing. In this instance, ends of the glass melt G in the widthwise direction are indicated by ‘a’ in FIG. 2. Like this, the glass melt G moves in the downstream direction while spreading in the widthwise direction in a state that the glass melt G floats on the metal melt M, and in this instance, a spreading shape and a spreading speed of the glass melt G may be determined by a density of glass, an atmospheric gas, a metal melt, a viscosity of glass, an interfacial tension between glasses, and the like.
However, when the spreading speed of the glass melt poured onto the metal melt is low, to manufacture a wide float glass, the problem that the float bath 10 should have a sufficient length is posed. Accordingly, it is advantageous to use a glass melt spreading fast as possible.
Conventionally, to increase the spreading speed of the glass melt, a high temperature driving condition method has been widely used. The high temperature driving condition method is a method that increases an upstream temperature of the float bath 10 to increase temperature of a glass melt and consequently to spread the glass melt fast.
However, this method using high temperature requires high cooling performance of the float bath 10 as well as a high amount of power. Also, because the high temperature driving condition may shorten the life of refractories constituting the float bath 10, it is unfavorable in an aspect of management of an apparatus. Furthermore, when a glass melt supply condition or a driving condition changes at an upstream of the float bath 10 where the glass melt spreads, formation of a glass ribbon may become unstable, and high temperature driving method is problematic in that unstability may be worsened.