1. Field of the Invention
The present invention relates to a method for manufacturing float glass.
2. Discussion of the Background
A molten metal bath used for the method of manufacturing float glass is divided generally into three regions along a direction of movement of a glass ribbon. A first region is called a fire polishing region which is adapted to receive molten glass on the surface of molten metal bath in which a glass ribbon having an equilibrium thickness is formed while the width of the glass ribbon is expanded and at the same time the surface of the ribbon is made flat. Generally, soda-lime glass is used for the molten glass and it is kept at a temperature of 1,110.degree. C.-950.degree. C. A second region is used for forming the glass ribbon in a predetermined thickness. The second region is so constructed especially that when glass having a thickness lower than an equilibrium thickness is to be formed, a pulling force is applied to the glass ribbon in its longitudinal direction while top rollers are engaged with both edges of the glass ribbon to suppress contraction of the glass ribbon in its width direction to thereby form the glass ribbon having a predetermined thickness. In the second region, the top rollers are placed to be engageable with the glass ribbon. Further, the second region is kept at a temperature sufficient to change the thickness of the glass ribbon when it is pulled by a pulling force. Namely, the glass ribbon is formed at a temperature range of about 950.degree. C.-800.degree. C. when soda-lime glass is used. A third region is so adapted that the glass ribbon formed in a predetermined thickness can be drawn from the molten metal bath and the glass ribbon is cooled to a temperature suitable to be transferred by means of rollers. The third region is kept at a temperature ranging from about 800.degree. C.-600.degree. C. when soda-lime glass is used.
A temperature distribution formed in the molten metal bath in its longitudinal direction has been attained by changing the depth of the metal bath as shown in Japanese Examined Patent Publication No. 18353/1966, or by arranging a barrier at the boundary of each region. However, in the method of obtaining a predetermined temperature distribution by changing the depth of the metal bath, it is necessary to use a molten metal bath having a depth of 40 mm in order to avoid reduction in processability. Accordingly, when a temperature distribution is formed in the metal bath in its longitudinal direction, there takes place a strong convection current in the molten metal bath, so that a gradient of temperature in the bath becomes flat. Therefore, it is necessary to obtain a predetermined temperature distribution by increasing the length of the molten metal bath. However, it increases an amount of released heat and a large-sized apparatus is required to manufacture a glass ribbon.
In the method of using a barrier in the molten metal bath, a large temperature difference is produced between the upstream side and the downstream side of the barrier and there takes place a strong convection in a spiral form along the barrier. The convection current changes a temperature distribution in the molten metal bath, whereby small stripe-like ridges and recesses, i.e. a so-called distortion results in the glass ribbon.
Further, in the later method, it is necessary to determine the upper end of the barrier to be 20 mm-30 mm lower than the bath surface of the molten metal in order to avoid the contact between the barrier and the glass ribbon. Therefore, an effect of interruption of heat from the molten metal can not be obtained. In the conventional method, the depth of the metal bath is so determined that the longest thin glass ribbon can be produced. Accordingly, when a glass ribbon having a relatively large thickness is manufactured, there is a useless glass ribbon forming region, which is results in increases heat loss.
On the other hand, use of a movable barrier system is proposed. However, a movable barrier can not be used for a vessel in which the cross-sectional area in its width direction is not uniform.