Note: Descriptions are shown in the official language in which they were submitted.
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 `',
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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
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~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
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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.
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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
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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.
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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:
P2-P
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
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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
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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.
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