1. Field of the Invention
This invention relates to a method and apparatus for the manufacture of a continuous sheet of flat glass by supporting molten glass on a pool of molten metal and advancing it along the surface of the pool of molten metal while cooling it to form a continuous sheet of flat glass. More particularly, this invention relates to a method for selectively adjusting and maintaining the condition of the glass at different locations across its width to stabilize the path of movement of the glass as it advances along the surface of the pool of molten metal.
2. Description of the Prior Art
Flat glass may be produced in many different ways. Several methods have been disclosed in the past which involve floating or supporting glass on the surface of a pool of molten metal as it is advanced along that surface and cooled to form a continuous sheet of flat glass. For example, molten glass may be delivered onto a pool of molten metal and formed into a continuous sheet or ribbon or glass according to the teachings of Heal, U.S. Pat. No. 710,357 or of Hitchcock, U.S. Pat. No. 789,911 or according to the methods disclosed in the patents of Pilkington, U.S. Pat. Nos. 3,083,551 and 3,220,816 or according to the teachings of Edge and Kunkle found in U.S. Pat. No. 3,843,346. These patents describe processes in which molten glass delivery techniques vary, yet they share the common disclosure that a continuous sheet of flat glass may be formed by advancing a layer of glass along the surface of a pool of molten metal while cooling the glass until it assumes a final width and thickness as a dimensionally stable, continuous sheet of glass.
In the method of Heal, molten glass is delivered over a refractory bridge and then flows slightly downwardly onto the surface of a pool of molten metal confined between two side walls of a forming chamber. This layer of glass advances along the surface of the pool of molten metal between the side walls and remains in contact with them as it is advanced and cooled to form a dimensionally stable, continuous sheet of glass. In the method of Hitchcock, molten glass is delivered through a slot in a refractory wall and flows horizontally onto the surface of a pool of molten metal in a forming chamber. The glass advances along the surface of the pool of molten metal as a layer of constant width and is cooled and advanced at a sufficient speed to form a dimensionally stable, continuous sheet of glass of desired thickness. In the method described by Pilkington, molten glass is delivered through a long, narrow canal and over a refractory lip and then falls freely downwardly onto the surface of a pool of molten metal. It then spreads laterally, outwardly and rearwardly in an unhindered fashion. An advancing layer of glass is drawn along the surface of the pool of molten metal from this laterally, outwardly moving body of molten glass. This layer of glass is advanced as its width diminishes and as it is cooled to form a dimensionally stable, continuous sheet of glass of desired thickness and width. In the method disclosed by Edge and Kunkle, molten glass in a pool of molten glass is conditioned to establish a region of forward flow near its surface, and this forwardly flowing glass is delivered over a threshold member substantially horizontally onto the surface of a pool of molten metal maintained at or near the elevation of the threshold over which the glass is delivered. This flowing molten glass is advanced along the surface of the pool of molten metal as it is initially cooled either having its marginal edges free of contact with side members or having them in contact with selected side members for a short distance. The glass is thereafter advanced along the surface of the pool of molten metal while being further cooled to form a dimensionally stable, continuous sheet of glass.
The present invention provides a method and apparatus for steering and stabilizing an advancing layer or ribbon of glass supported on molten metal in a forming chamber to avoid the persistent tendency of advancing glass to drift from side to side when the glass is delivered and formed in the manner disclosed by Pilkington. Meanwhile, the present method and apparatus provide for a further improvement in the flattening of the velocity for the flow of molten glass immediately following its delivery for forming. This assists in the making of glass having excellent optical quality extending throughout the width of the glass.
In the practice of this invention a continuous sheet of flat glass is produced by a method including the following steps: A layer of molten glass is delivered onto the surface of a pool of molten metal maintained within a forming chamber. The molten metal is preferably tin, an alloy of tin or some other metal having a specific gravity greater than the glass and having a melting point lower than the glass to be formed while being substantially nonreactive to the glass at its melting temperature. The layer of molten glass is advanced along the surface of the pool of molten metal and is cooled during such advance to form a dimensionally stable, continuous sheet of glass.
Forces are applied to the glass while it is advanced along the surface of the pool of molten metal. Forces are applied to the glass which are aligned substantially along its path to cause it to be advanced. These forces may be sufficient to cause the glass to be attenuated to a thickness less than an equilibrium thickness during its advance. These forces are characterized as longitudinal tractive forces and may be applied to the glass at any location along the length of the glass sheet. They are preferably applied to the glass at locations well along its path of advance, preferably beyond the supporting pool of molten metal, and are transferred to the hot or more fluid glass primarily due to the surface tension of the glass. Other forces may be applied to the glass in a manner such that they are aligned substantially across or transverse to the path of glass advance. These forces are characterized as transverse forces. The resultant forces caused by the application of the described forces in combination with the reactive surface tension and gravity forces acting on the glass cause the glass to be formed into a continuous, flat sheet of desired thickness.
After the glass has been cooled sufficiently to become dimensionally stable (that is, if it has reached its final width and thickness) it is advanced farther along the surface of molten metal and then is lifted from the surface of the pool of molten metal and conveyed from the forming chamber. It may be lifted slightly and conveyed along a substantially horizontal path from the forming chamber, or it may be lifted and conveyed upwardly from the pool of molten metal in the manner described by Gerald E. Kunkle in his copending, commonly assigned patent application, Ser. No. 483,508, filed June 27, 1974, which is incorporated by reference herein. The glass is cooled sufficiently prior to lifting it from the surface of the pool of molten metal so that its width and thickness remain unchanged during lifting and conveyance from the forming chamber.
As the glass is being formed during its advance along the surface of the pool of molten metal and as it is conveyed from the pool of molten metal, it is controllably cooled. The cooling is coordinated with the rate of glass advance to form a continuous sheet of glass of desired width and thickness. If the layer of glass is permitted initially to spread laterally outwardly in an unhindered manner, the cooling and rate of glass advance are advantageously coordinated to simultaneously attenuate the thickness and the width of the glass in the manner disclosed by Charnock in U.S. Pat. No. 3,352,657 and by Dickinson et al. in U.S. Pat. No. 3,695,859. Nevertheless, a preferred practice involves maintenance of the width of the glass equal to or less than the width of the initially delivered layer of glass for this in and of itself helps to stabilize the path of advance of the glass.