This invention relates to the manufacture of flat glass by forming it while supporting it on molten metal. More particularly, this invention relates to an improved method for controlling the convective cooling of the molten glass, particularly, immediately following delivery onto the molten metal for forming.
There are several well-known processes for making flat glass by forming it from a body of glass supported on molten metal. Molten glass may be delivered onto a pool of molten metal, such as molten tin, and cooled and advanced along the surface of molten metal to form a continuous sheet or ribbon of glass according to the teachings of Heal U.S. Pat. No. 710,357; of Hitchcock U.S. Pat. No. 789,911; or of Pilkington U.S. Pat. Nos. 2,911,159, 3,083,551 and 3,220,816.
From the earliest of these disclosures it has been appreciated that the temperature of the glass may be controlled while the glass is supported on the molten metal. According to Heal, "By varying the temperature in the heating chambers the flow of the sheet may be accelerated or retarded in proportion to the varying degree of fluidity of the glass." The disclosure of Hitchcock primarily concerns the division of a glass supporting molten metal bath into segregated pools so that controlled cooling of the glass may be facilitated. Hitchcock also discloses the use of burners to heat an arch roof extending over the molten glass being supported by and formed on the molten metal.
According to recent patents, workers in the art have coordinated temperature control with the application of tractive forces to glass being formed in order to produce flat glass of varying thickness. For example, U.S. Pat. No. 3,352,657 to Charnock discloses a method for making flat glass in which the glass upon delivery onto the molten metal in the forming chamber is immediately cooled at a rapid rate and thereafter remains at a substantially constant temperature for a prolonged period while applying longitudinal tractive forces to the glass at two locations and permitting the edges of the glass to be laterally unconstrained. According to this patent, flat glass of less than equilibrium thickness may be made in the manner described. Equilibrium thickness glass is flat glass having the thickness that it will attain when permitted to rest as molten glass on a pool of molten metal until it ceases to spread outwardly on the molten metal. According to the patent of Charnock, the hot glass that is supported on molten metal for forming has its temperature controlled by undefined temperature regulators located above and below the glass at locations well downstream of the inlet end of the forming chamber.
In the patent of Charnock and in U.S. Pat. No. 3,589,886 to Montgomery, there are shown radiation gates extending across float forming chambers. These gates are used to shield edge rolls or edge stretching devices located in a particular region from radiation of heat from other regions in a forming chamber. They shield such devices from heat radiated from roof portions of the forming chamber extending both upstream and downstream from the protected region. These gates permit the free movement of gases along the upper surface of glass along its entire length in the chamber.
In U.S. Pat. Nos. 3,248,197 and 3,615,315 to Michalik and Misson, it has been shown that the headspace above molten glass being formed while supported on a pool of molten metal in a forming chamber may be subdivided at its ends by providing barriers that are connected to a roof overlying the bottom portion of the chamber. This is shown with particular reference to methods for providing zones of differing atmosphere pressure within a forming chamber in order to effectuate control of the thickness of glass being produced; hence, such devices are used on conjunction with the supply of gas to the headspace under substantially superatmospheric pressures and directed at the glass being formed.
Flat glass produced by the techniques of Pilkington (including the variation of Charnock) or by techniques which are improvements of the basic Heal or Hitchcock processes, as well as techniques such as described in the patents of Michalik and Misson, have generally good optical quality. Such glasses are generally optically flatter than flat glasses produced by so-called sheet processes, such as the Pittsburgh process, the Colburn process or the Fourcault process. That is, these glasses are not characterized by optical distortion as intense as that which characterizes sheet glasses. Nevertheless, flat glass produced by supporting it on molten metal during forming is characterized by some optical distortion. This is more evident in thin glasses than in thick glasses, particularly equilibrium glass, and it is more evident when glass is observed at a small angle by reflected light rather than at an angle at about 90.degree. by transmitted light. The optical distortion is variously called "batter", "broken line distortion" and "reflective distortion".
The optical distortion in flat glass may be qualitatively evaluated by observing a shadow projection of the glass using a point-light source aimed perpendicularly at a target screen and positioning the glass at an angle with respect to the target screen so that an image of the glass is projected onto the screen. The optical distortion may also be observed in Schlerin photographs of the glass. The optical distortion pattern is regular, repetitive and isotropic in glass produced by methods like that of Pilkington wherein the glass falls onto the molten metal and spreads outwardly and then is drawn inwardly again. In glass produced in the manner of Hitchcock following horizontal delivery onto motlen metal while maintaining its width substantially unchanged, the distortion pattern is regular, repetitive and anisotropic. In both types of glass the distortion may be sufficiently intense (that is, the variation of lightness to darkness and the projected shadow is great) to be easily observed by even an untrained eye when the glass is coated with a reflected coating and observed by reflected light.
The optical distortion apparent in flat glass is recognized as an effect that may be caused by variations in the surfaces of the glass. If the surfaces are not perfectly flat, but rather are characterized by a plurality of elevations and depressions, a beam of light passing through the glass will be bent or distorted. This optical distortion in the glass may be detected during production, using a device such as that disclosed and claimed by Simko in U.S. Pat. No. 3,799,679 or determining the surface variation of a sheet of glass from its optical characteristics in the manner described in U.S. Pat. No. 3,792,930 to Obenreder. The present invention provides a method for making glass of improved optical quality having distortion of diminished intensity.