CA1194697A - Pressure sizing and lateral stretch method for forming float glass - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A continuous ribbon of glass is reduced in thickness while sup-ported on molten metal by imposing super-atmospheric pressure over a molten glass layer and subsequently increasing the width of the ribbon by forces engaging the edges of the ribbon, whereby the optical power of any residual surface defects is reduced.

Description

PRESSUR~ SIZIN~ AND LATERAL STRETCH METHOD FOR FORMING FLOAT GLASS

~ackground of the [nvention This invention relates to the manu~acture of flat glass wherein the glass is for~ed into a flat sheet while supported on a pool of molten metal9 commonly referred to as the float process. More particularly9 this invention relates to a process for producing less than equilibrium thick-ness float glass while reducing ~he amount of distortion in the glass.
In a float forming process molten glass is delivered onto a pool of molten metal, usually tin or an alloy thereof, and thereafter formed into a continuous ribbon or sheet of glass. Under the competing forces of gravity and surface tension, the molten glass on the molten metal spreads outwardly to an equilibrium thickness of about 6.8 millimeters. In order to produce glass of thicknesses less than the equilibrium ~hickness the prior art has resorted to various arrangements for stretching the glass ribbon while still in a viscous state on the molten metal. These arrange-ments usually involve engaging marginal edge portions of the ribbon with mechanical devices, usually toothed rolls. The contact between the glass ribbon and these mechanical devices is believed to create disturbances in the ribbon as well as the molten metal pool which cause optical distortion to be imparted to the glass. Moreover, as disclosed in U.S. Patent 4,305,745, by R. J. Mouly, issued 15 December 1981, attenuating a glass ribbon in the longitudinal direction, as is the common practice, tends to increase the visibility of surface distortion. There it is proposed to carry out transverse attenuation subsequent to longitudinal attenuatlon so as to at least partially offset the harmful effects of the h~ r~, .~b `', longitudinal attenuation. It would be desirable to improve upon such a process by minimizing the amount of perturbation introduced to the float forming process.
The use of super-atmospheric chambers for thinning float glass has been suggested in the prior art, for example, in U.S. Patent Nos.
3,241,937 (Michalik et al.); 3,241,93~ (Michalik); 3,241,939 (Michalik);
3,248,197 (Michalik et al.); 3,345,149 (Michalik et al.); 3,615,315 (Michalik et al.); 3,749,563 (Stingelin); 3,~83,338 (Stingelin);
3,885,944 (Stingelin); 3,432,283 (Galey). An improved pressure sizing arrangement is disclosed in ~S. Patent 4,395,272 (corresponding to Canadian application 413,013 of^7 October 1982) by G. E. Kunkle et al.
Pressure sizing has the potentiali~y of producing below equilibriu~
thickness float glass with considerably less distortion-producing perturbation than mechanical stretching. It would be desirable to improve the practicality of pressure sizing by reducing the size of the pressurized chamber required and by minimizing the consumption of pressurized gas, which usually must be a non-oxidizing gas and may require pre-heating.

SUMMARY OF THE INVENTION
In the present invention, a layer of molten glass floating on a pool of molten metal is first partially thinned by super-atmospheric pressure imposed over the glass, and then reduction to the final thickness is completed by lateral stretching. Because the glass is only partially thinned by pressure, ~he pressure sizing requiremen~s are lessened, and therefore the pressure chamber may be economically compact and the atmo-sphere pressure and volume requirements reduced. The low level of per-turbations to the fluid glass and molten metal in the pressure sizing zone yields a ribbon having relatively low surface distortion. The distortion X

~3~ 7 1 quality of the glass is not deteriorated, and may be improved, by the sub-sequent mechanical attenuation since it is limited to stretching in sub-stantially the lateral direction only (transverse to the direction of glass travel~. Major sources of transmitted light distortion in float glass are thickness variations and corrugations that extend in the longitudinal direc- -tion. It is believed that lateral stretching diminishes the observability of these defects by reducing their spatial frequency across the width of the glass ribbon.
Any of the above-cited arrangements for pressure sizing glass may be employed in conjunction with the present invention, but the preferred embodiment is that disclosed in the aforementioned U.S. Patent ~ bn-~ en~
54~ier~ of Kunkle et al. The features of that arrangement in combination with the present invention result in a particularly compact and economical system for producing thin, high quality float glass. In that arrangement, molten glass is metered into the pressure sizing chamber as a relatively wide layer covering the full width of the pressure chamber, thereby minimizing the amount of sizing to be perEormed in the pressure chamber. The glass in the pressure chamber is in contact with the side walls, whereby the amount of escaping pressurized gas is reduced and attainment of desired pressures is expedited.
A preferred mode of carrying out the pressure sizing method of the present invention entails delivery of molten glass to the pressure si%ing chamber at temperatures higher than those customarily employed in float processes, i.e.~ at least 2100F. (1150C.) and preferably at least 2300F. (126QC.). At the low glass viscosities accompanying such high temperatures ~he super-atmospheric pressure in the pressure chamber has a rapid effect on the glass thickness so that thickness reduction 1 can be achieved in a short period of time and, accordingly, the length of the pressure cha~ber may be relative~y short. The low viscosity also permits any perturbations introduced by delivering the molten glass onto the molten metal to flow out rapidly. These temperatures are higher than those at which conventional edge gripping attenuating devices are effective.
In a conventional glassmaking operation, a chamber known as a refiner or conditioner is interposed between the melting furnace and the forming chamber, the function of at least a substantial portion being to permit the glass to cool Erom a melting temperature to a temperature suita-ble for forming. But when the glass is formed at higher than conventional temperatures as is permitted by the present invention, the cooling function of the refiner/conditioner is reduced and, thus, it may be reduced in size, thereby effecting further economies.
Another aspect of sizing the glass at relatively high temperatures is that the sized glass may leave the pressure chamber at temperatures com-parable to those at which glass enters conventional float forming processes, e.g., 1900F. (1040C.) to 2100F. (1150C.). Such temperatures and the accompanying low glass viscosities following pressure sizing are compatible with the transverse attenuation that follows.

The Drawings FIG. 1 is a schematic plan view wi~h the top cut away of an embodiment of the float glass forming operation of the present invention.
FIG. 2 is a longitudinal cross section of the float glass orming operation of FIG. 1 taken along line 2-2 in FIG. 1.

1 Detailed Description A detailed description of the invention will be made with refer-ence to a specific preferred embodiment as shown in FIGS. 1 and 2. It should be understood that the invention may take various other specific forms.
In FIGS. 1 and 2 a refiner or conditioner lO contains a body of molten glass 11. A threshold member 12 separates the conditioner or refiner 10 from the forming chamber designated generally as 13. The threshold may include a conduit 1~ for the passage of cooling medium. As is the conven-tional practice, a cut-off tweel 15 may be provided for shutting off the ~low of molten glass from the conditioner into the forming chamber. In the forming chamber a bath or pool of molten metal 16 is contained within a refractory basin 17. The molten metal is tin or an alloy thereof such as tin/copper alloys. Coolers 18 aid containment of the molten metal at the hot end of the forming chamber. Oxidation of the molten metal is retarded by providing a non-oxidizing atmosphere (e.g., nitrogen or forming gas) within the forming chamber. Maîntenance of the non-oxidizing atmosphere within the forming chamber is assisted by a gas tight casing 19 around the forming chamber.

In the preferred embodiment, as shown in FIG. 2, molten glass from the conditioner 10 is metered into the forming chamber 13 by a meter-ing tweel 20 which may be provided with a conduit 21 in its lower portion for circulating coolant in order to e~tend its life. The tweel 20 overlies a deep portion 22 of the molten metal in the basin 17, and the distance between the lower edge of ~he tweel and ~he underlying molten metal may be adjusted by vertical movement of the tweel so as to establish a predeter~
~ined flow rate of molten glass into the forming chamber~ The molten glass 1 is delivered to the ~ull width of the first zone of the forming chamber, which is a pressure chamber 25 in which the glass G is maintained in contact with the side walls 26 and 27. Maintaining glass contact with the side walls may be assisted by employing wettable materials for the side walls (most ceramic refractory materials) and by avoiding use of non-wettable materials, such as graphite. Fluidity of the glass along the sides may be assisted by edge heating means such as the bar type electrical resistance heaters 27 shown in the drawings. Coolers may be provided in the pressure forming chamber to begin cooling the glass, and preferably the cooling is directed toward center portions of the glass ribbon. In the arrangement shown, the coolers are comprised of conduits 28 for carrying water or other heat transfer medium provided with sleeves 29 of insulating material at each end.
The downstream end of the pressure sizing chamber 25 is closed by a vertically adjustable exit seal 35. The bottom edge of the exit seal 35 is spaced a small distance (e.g., a few millimeters) above the top surface of the glafis ribbon to minimize leakage of the pressurized atmosphere from the pressure sizing chamberO In order to extend the life of the exit seal and to cool the glass leaving the pressure chamber, the exit seal 35 may be provided with a conduit 36 for passage of a cooling medium. Except for the gap under the exit sealS the pressure sizing chamber 25 is essentially gas tight, thereby permitting imposition of pressures greater than atmospherie.
Pressurized gas may be introduced to the pressure sizing chamber through a conduit 37. As in conventional float forrning operations, the atmosphere in the pressure chamber 25 as well as the remainder of the forming chamber is preferably a non-oxidizing atmosphere such as nitrogen or forming gas.

1 Molten glass spreads on molten metal until it attains an equi-librium thickness in accordance with the following relationship:

2Pt~Sl~S2-S3) Pgg(pt-pg) where hl = equilibrium glass thickness Pt = density of molten metal pg = density of molten glass Sl = atmosphere - glass surface tension (dynes/cm) S2 = glass-metal surface tension S3 = atmosphere-metal surface tension g = acceleration of gravity For conventional sodallime/silica flat glass on molten tin, the equilibrium thickness is about 0.27 inches (6.8 millimeters). Increasing the pressure on the glass has the apparent effect of increasing the density of the glass. Therefore, in accordance with the equation above, an increase in the apparent density of the glass results in a smaller equilibrium glass thickness. The reduced glass thickness may be calculated as follows:

h2 - hl -P~ g where hl = equilibrium glass thickness h2 = reduced glass thickness Pl = atmospheric pressure P2 ~ pressure in pressure si~ing c~amber pg = density of glass g = acceleration of gravity 1 It may be noted that the atmospheric pressure Pl in the equation above is actually the pressure on the exposed molten metal within the forming cham-ber outside the pressure sizing zone and may be slightly above the natural atmospheric pressure outside the Eorming chamber. Within the pressure siz-ing chamber no portion of the molten metal is exposed to the pressurized atmosphere. Small pressure differences yield significant reductions in glass thickness as may be seen in the following table of calculated examples:

P2-Pl Glass Thickness (mm) (mm water c_lumn) 1.8 6.3

2.5 5.8

3.8 5.3 5.1 4.8 6.4 4.3 7.6 3.8 8.9 3.3 10.2 2.~
11.4 2.3 12.7 1.8 1~.0 1.3 15.2 0.8 16.5 In preferred embodiments of the invention, the economy and com-pactness of the pressure sizing chamber are urther enhanced by delivering the molten glass into the pressure sizing chamber at temperatures consider-ably higher than those conventionally employed for float forming. In con-ventional float processes, the molten glass is delivered onto the molten metal typically at about 2000F. (1090C.), but in the preferred embodi-ments of the present invention the delivery temperature is in excess of 2100~F. (1150~C.) and most preferably above 2300F. (1260 C.). Even higher temperatures could be employed to advantage, but temperatures may be limited by the durability of conventional refractory materials. Higher temperatures do not affect the final glass thickness, but the reduced 1 viscosities which accompany high temperatures permit the glass to attain the final thi~kness in a shorter period ~f time. Therefore, less residence time is required in the pressure sizing chamber, and the pressure sizing chamber ~ay be of reduced length. These temperatures refPr to conventional soda/lime/silica ~lat glass and will differ for other glass compositions.
The use of unusually high temperatures for the pressure sizing step is made possible by the fact that pressure sizing does not require mechanical engagement of the glass ribbon.
As the ribbon of glass ~. is drawn out of the pressure chamber 25 it enters an attenuating zone ~1 in which a pressure lower than that of the pressure chamber is maintained. The glass separates from the sidewalls as it enters zone ~1. In the reduced pressure environment, the ribbon has a tendency to shrink in width and increase in thickness as long as the tem-perature of the glass remains sufficiently high for the glass to be in a plastic state. Therefore, it is necessary to maintain the ribbon width by forces applied to the edges, such as by edge roll means 40, until the glass has cooled to a substantially stable condition. In the present invention, the ribbon width is not only maintained but enlarged by the rolls 40~ The rolls are angled outwardly to impart a lateral component of force to the ribbon. Preferably, no substantial longitudinal acceleration is imparted to the ribbon at that point to avoid longitudinal stretching.
Predominant sources of optical distortion in flat glass are longitudinally extending surface irregularities. Scanning transversely across the ribbon with optical measuring devices reveals that the opti-cal power of this distortion is strongly dependent on the spatial fre-quencies of the surface irregularities in accordance with the following relationship:

. g P = khf2 where P iB optical power, k iB a constant, h is the height or amplitude of the surface defect, and f is the spatial frequency of the distortion pat-tern. Widening the ribbon has been ~ound to decrease the frequency of longi-tudinal surface defect~ present in the ribbon, which in turn has a beneficial second power effect on the optical power of the distortion. The frequency alteration is proportional to the change in ribbon width as follows:

f2 = fl X Wl/W2 where fl is the optical distortion frequency entering the transverse attenu-ating zone, f2 is the optical distortion frequency leaving the transverse attenuation zone, Wl is the ribbon width entering the transverse attenuation zone, and W2 is the ribbon width leaving the transverse attenuation zone.
Because of the second power relationship, small changes in ribbon width can provide significant benefits to the optical quality of the glass. Accord-ingly, improvements may be obtained by widening the ribbon to a final width at least 1.05 times its width leaving the pressure chamber and preferably at least 1.1 times its width. When the glass passes from the pressure siz-ing chamber, it should be at a temperature suitable for engagement by the edge retaining device~, typically below about 1900F. ~1040 C.). Thus, the glass may be permitted to cool considerably as it passes through the pres~
sure sizing chamber, and a5 it passes into the attenuating zone 41 it may be further cooled. The cooling may be aided by coolers 42 within zone 41.
Subsequent to the lateral attenuation, the glass is permitted to cool, with or without the aid of coolers, to a temperature at which the rib-bon iæ dimensionally stable and can be lifted from the molten metal pool (e.g., 1100F., 60QC.). At the exit end of the forming chamber, conventional means ~ 10 --1 such as lift-out rolls 50 may be provided for lifting the dimensionally stable ribbon of glass G from the molten metal over a lip 51 at an exit opening 52.
It is contemplated that one variation may entail a pressure sizing chamber in which the side walls taper away from one another so that the glass may increase in width slightly as it is reduced in thickness.
Other modifications as are known to those of skill in the art may be resor~ed to without departing from the scope of the invention as defined by the claims which follow.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of producing float glass of less than equilibrium thickness comprising: continuously delivering a stream of molten glass onto a molten metal pool to form a layer thereon; imposing a pressure greater than atmospheric on the molten glass layer in a pressure zone so as to reduce the thickness of the glass layer as it passes through the pres-sure zone to a thickness less than the equilibrium thickness; withdrawing the glass as a thinned ribbon from the pressure zone to an attenuating zone where the pressure is lower than in the pressure zone, and in the attenuat-ing zone supporting the glass ribbon on molten metal while increasing the width of the ribbon by forces applied to edge portions of the ribbon; cool-ing the widened ribbon to a dimensionally stable condition; and withdrawing the dimensionally stable glass ribbon from the cooling chamber.

2. The method of claim l wherein the molten glass enters the pressure chamber at a temperature of at least 2100°F. (1150°C.).

3. The method of claim l wherein the molten glass enters the pressure zone at a temperature of at least 2300°F. (1260 C.).

4. The method of claim 1 wherein the width of the ribbon is increased in the attenuation zone by a factor of at least 1.05.

5. The method of claim 4 wherein the factor is at least 1.1.

6. The method of claim 4 or 5 wherein no substantial longitudinal attenuation is imparted to the ribbon after leaving the pressure zone.

7. The method of claim 1 wherein the layer of molten glass in the pressure zone is maintained in contact with side walls defining a pressurized enclosure.

8. The method of claim 1 wherein the stream of molten glass is delivered onto the molten metal pool at a width substantially equal to the width of the ribbon being withdrawn from the pressure zone.

9. The method of claim 8 wherein the molten glass stream being delivered into the pressure zone is supported by molten metal before entering the pressure chamber.

10. The method of claim 8 wherein the stream of molten metal is delivered into the pressure zone through an opening defined by a horizontally elongated refractory member on the upper side and a pool of molten metal on the underside.