Note: Descriptions are shown in the official language in which they were submitted.
~IS3~3
Background of the Invention
Field of the Invention: This invention relates to apparatus
for the manufacture of a continuous sheet of flat glass by supporting
molten glass on a pool of molten metal while forming and cooling the
glass. More particularly, thls invention relates to an apparatus and
method for delivering molten glass onto a pool of molten metal for
forming.
Description of the Prior Art: Molten glass may be delivered
onto molten metal and formed into a continuous sheet or ribbon of glass
according to the teachings of Heal, ~.S. Patent No. 710,357 and of Hitchcock,
U.S. Patent ~o. 789,gll, and according to the patents of Pilkington, U.S.
Patent No. 39083,551 and U.S. Patent No. 3,220,816. In all of the prior
art, molten glass is delivered over some rigid element, usually a refractory
member, onto molten glass. In the practice disclosed by Pilkington, the
molten glass is delivered through a long, narrow canal and over a lip from
which the molten glass falls onto the molten metal and spreads outwardly
on the molten metal. In the methods of Heal and Hitchcock, the molten
glass is delivered over a refractory bridge or wall onto molten me~al con-
tained ad~acent to such a refractory wall downstream of the furnace in which
the molten glass is prepared.
According to the disclosures of both Heal and Hitchcock, molten
glass flows from a pool of molte~ glass over a generally flat surface and
then onto molten metal in a forming chamber. Molten glass flowing over a
flat surface like the surfaces disclosed in Heal and Hitchcock imposes a
significant drag upon the surface over which it flows. This causes sub-
stantial erosion of the underlying surface and generally causes the bottom
surface of the glass ribbon produced from such a strPam of glass to be
characterized by llnear defects associated with this drag. These defects
- 2 - ~
.-~
53~ 3
extend along the dlrection of draw and appear as regions of optical
distortion when visually observed. These lines become more pronounced
in appearance when the bottom surface of the glass is either silvered
or etched.
Recognizing the problem of bottom surface damage experienced
from practicing tlle methods of Heal or Hitchcock and using the apparatus
disclosed therein, Pilkington discloses a method whereby molten glass
after flowing over a flat delivery surface is caused to fall and flow
rearwardly and outwardly from the location where molten glass falls onto
the pool of molten metal. In this way, the glass which has been flowing
in contact with the flat delivery surface is caused to spread outwardly
such that it becomes concentrated in the outwardly disposed marginal
portions of a continuous ribbon or sheet of flat glass formed on the
molten metal. There the glass which had been in contact with the flat
refractory surface may be severed from the remainder of the glass sheet
and discarded. The present inv~ntion provides means for overcoming the
problems associated with the practices of Heal or Hitchcock and without
introducing additional problems which occur when practicing the method
disclosed by Pilkington.
Summary of the Invention ~-~
Thus the present invention in one aspect provides an
apparatus for the manufacture of flat glass comprising:
(a) a glass melter; -
(b) a glass conditioner connected to the glass melter; ,
(c) means for forming a continuous, dimensionally stable sheet -
of glass including a pool of molten metal upon which glass ;
floats during forming; and
(d) means connected to the glass conditioner and to the forming
means for delivering molten glass from the glass condi-
tioner onto the pool of molten metal of the forming means;
wherein the delivery means is an enclosed channel comprising:
.
- 3 - -
.
,
r
53~3 ~
(1) a threshold member over which molten glass may flow extending
transversely across the bottom of said channel having an
upwardly-facing, convex upper surface having a highest
portion located at least at the elevation of the upper
surface of the pool of molten metal, the threshold member
being located for separating the pool of molten metal
from molten glass in the glass conditioner;
(2) side members extending upwardly from both ends of the
transversely extending threshold me~ber; and
(3) a roof member extending between the side members and
ex~ending transversely across the top of the channel.
In another aspect the invention provides a method for making -
flat glass comprising the steps of:
(a) melting glass;
(b) establishing a pool of molten glass in a container;
(c) delivering molten glass from the container over a threshold
me~ber having an upwardly-facing, convex upper surface onto
a pool of molten metal having an upper surface maintalned
at an elevation substantially at or below a highest portion
:. :
of the upwardly facing convex surface of the threshold
member;
(d) cooling the delivered molten glass on the pool of molten metal -
to form a continuous, dimensionally stable sheet of glass; and
(e) withdrawing the continuous sheet of glass from the pool of
molten metal.
Glassmaking materials are melted in a glass melter. From this
melter molten glass flows into a glass refiner or conditioner connected
to the glass melter. In the c~nditioner, the molt~n glass is gradually
cooled to a suitable temperature for forming and the molten glass is then
delivered from the conditioner onto a pool of molten metal in a glass
forming chamber. In the glass forming chamber, the molten glass which
- 3a - `
.. . .
~o53~3
has been delivered onto the surface of the molten metal in the form of
a wide, relatively shallow stream ls cooled to form a continuous,
dlmensionally stable sheet of glass. It may be stretched or attenuated
by the application of longitudinal forces, lateral forces or both to
form ~ sheet of deslred thickness. This continuous sheet of glass is
then withdrawn from the forming chamber for further processlng.
The molten glass is delivered from the conditioner to the glass
forming chamber through a delivery means, generally comprising an
enclosed channel. The molten glass is withdrawn from the refiner and
caused to flow along a substantially horizontal path, the stream of
glass having greater width than depth, through an opening defined by a
bottom or threshold member, side members and a roof member. At least
one of these opening defining members should be movable so that the
size of the opening may be varied. Preferably, a top member is movable
and is characterized as a metering member which extends downwaxdly
from the roof member. The molten glass is delivered onto the molten
metal (preferably tin) in the forming chamber while maintaining the
bottom surface of the glass at about the same elevation as the top
surface of the threshold member. The glass flow is maintained sub-
- 20 stantially undisturbed in this manner.
The threshold member extends across the path of glass flow
in the bottom of the delivery means. The upwardly facing surface of
this threshold member over which the molten glass flows is convex.
The channel through which the molten glass flows onto ths molten
metal also includes side members or jambs which extend upwardly from the
ends of the threshold member. The marginal edges of the stream of molten
glass flowing over the threshold member engage the side members. Joining
the side members and extending over the threhold is a roof. In general,
the metering member or tweel extends downwardly from the roof over the
threshold member so as to engage the upper surface of the molten glass
~ . .
, . .
: : . :, -
~1~53~3
flowing over the threshold member. The tweel is used to control the
size of the opening through which molten glass flows. In general, the
meterlng member i8 aligned with the threshold member with its lowermost
portion substantially in line with the highest portion of the upwardly-
facing convex upper surface of the threshold. In some instances, it may
be desirable to locate the metering member slightly downstream of the
uppermost portion of the threshold member. The directions "upstream"
and "downstream" are defined by the direction of glass f~ow through the
process; that is, glass flows from an upstream portion of the glassmaking
apparatus toward a downstream portion of the glassmaking apparatus.
The threshold member preferably comprises a refractory such as
fused silica or fused cast alumina. However, the ~hreshold may be con-
structed of a material such as an inert metal, particularly platinum,
which does not react with the glass. The preferred materials for the
threshold and other delivery members are substantially pure (greater
than 90 percent by weight) alumina or silica, Such materials fused in
the substantial absence of other materials that could form undesired eutectic
phases provide glass contact surfaces that do not cause glass marking like
that caused by conventional refractories. Fused silica is particularly
preferred because of its dimensional stability over a wide temperature
range.
Preferably, the downstream portion of the upper convex surface
of the threshold is partially submerged in the pool of molten metal in the
forming chamber. The upper convex surface of the threshold may be shaped as
a continuous curve as would be generated by rotating a straight line about
one or more axes. For example, the convex surface may be a section of a
cylinder or may be parabolic or hyperbolic in shape. It has been found,
however, convenient both for construction and operation to provide a
.. ~ - ~.
~05~ 3
threshold member having an upper surface comprising a plurality of sub-
stantially flat or planar surfaces with their regions of intersections
sllghtly rounded to blend the relatively flat surfaces into a continuous
convex surface. A particularly useful threshold shape is one having a
substantially horizontal portion at its highest region. A particularly
useful threshold shape is one having three substantially planar portions
comprising its upper surface: one, horizontal, as an uppermost portion;
a second, about 15 to 35 degrees from the horizontal, as an upstream
portion; and a third, 5 to 20 degrees from the horizontal, as a downstream
portion. Such a threshold, when used in combination with a metering member
or tweel, substantially aligned with the intersection between the middle
horizontal portion and the downstream portion, has yielded particularly
beneficial results.
The threshold member may be provided with means for heating or
cooling the threshold as described in U.S. Patent 3,843,346. For `
example, cooling pipes may extend through the threshold member, or resis- ;
tance heating elements may extend through it.
One of the advantages of using a threshold having a convex up-
wardly-facing shape in the making of flat glass is that the flowing molten
glass, as it engages the upstream portion of the convex threshold, has its `
velocity diminished because of the converging cross section available
for glass flow. While the glass closest the threshold flows slowly, the ~
glass farther from the threshold continues to flow at a relatively greater ~`
velocity. Thus, glass which has not been in contact with or proximate to
the threshold in the main part of the flowing molten glass proceeds over ~"
the threshold in a much greater amount than that glass which is and has
been flowing close to the threshold. The applicants have found that the
rate of damage to a refractory element in contact with the flowing glass
.
- 6 - , ;, ~
.
. , ~, ' '.
~o53~3
is grea~ly influenced by the rate at which the glass flows adjacent the
material, and have discovered that by employing a convex threshold the
rate of erosion and dissolution of refractory threshold is drammatically
diminished. They have also found that the bottom surface of a ribbon of
glass produced from molten glass that has flowed over such a threshold is
of excellent quality.
The upstream sloped face of a threshold having a multifaced,
upper surface preferably extends along a slope sufficient to place the
intersection of the upstream sloped face and the upstream vertical face
of the threshold at an elevation above the bottom of the conditioner so
that when flows are established in a pool of molten glass in the conditioner
the neutral flow plane will be at about the same elevation as this inter-
section. Such a flow may be established by appropriate thermal control
in the conditioner according to the teachings of U.S. Patent 3,843,346.
Such a threshold is subject to less wear than a threshold having this
intersection at a higher elevation. This is because the glass flow adjacent
the upstream sloped face is virtually zero in this arrangement. Meanwhile,
upward flow adjacent the upstream vertical face of the threshold is avoided
by so locating the threshold with respect to the neutral flow plane. This
avoids the danger of glass flow adjacent the threshold bringing subsurface
glass from near the bottom of the conditioner into the stream of molten
glass being delivered for forming. Since such subsurface glass is often
the source of stones, devitrified glass and Ither defect-causing impurities,
this preferred embodiment of the invention provides a means for insuring
the production of glass free from the defects caused by such impurities.
In a preferred embodiment, the continuous sheet of glass being
drawn is drawn at a rate such that its width is substantially equal to the
spacing between the side members of the delivery means. In this way, the
- 7 -
3~153~6~3
process is much more stable than that taught by Pilkington, particularly
when the glass being producsd has a thickness less than that at~ained at
equilibrlum with the molten metal forming bath. This is attributed to
the Eact that the ribbon or body of glass is anchored along a line rather
than only at a point during attenuation.
This invention will be further understood with reference to the
drawings accompanying this application.
Brief Description of the Drawings
.
FIG. 1 is a longitudinal sectional view of a delivery section
connecting a glass conditioner to a glass forming chamber showing the
convex threshold of the applicants.
FIG. 2 is a horizontal sectional view taken along sectional
line 2-2 of FIG. 1.
FIG. 3 i9 an enlarged longitudinal sectional view of a preferred
embodiment of the present invention.
Description of the Preferred Embodiments
Referring now to FIGS. 1 and 2, there is shown a glassmaking
facility including a glassmaking furnace and a glass forming chamber.
A glass melter 11, such as a conventional regenerative gas- or oil-fired
furnace, is provided for melting raw materials to form molten glass.
Connected to the melter 11 is a refiner or glass conditioner 13.
Molten glass is refined in the refiner or conditionex 13 in preparation
for forming and is contained therein prior to delivery to a forming
chamber.
- 8 -
,. . . . . . . . . .. . .. . .
~31)Q3 ~:
Connected to the refiner 13 is a forming chamber lS. Con-
nected to the forming chamber 15 is a glass take-out apparatus 17.
The refiner 13 and forming chamber 15 are connected through
a glass delivery means 21. ~ `
The glass delivery means 21 comprises a bottom threshold member
23, slde members 25, a roof 27, a metering member 29 and a shut-off mem-
ber 31. The metering member 29 is movable so that the opening defined
by it, along with the side members or jambs 25 and the threshold 23, may
be varied. Thus, the rate of glass flow through the opening can be con-
trolled.
Molten glass 33 in the refiner 13 is cooled to a temperaturesuch that its viscosity is within the range of from about 103 to about 104
poises. The cooling establishes a flow toward the threshold in an upper ` `
portion of the pool and a flow a~ay from the threshold in a lower portion `
of the pool. The molten glass is withdrawn from the refiner and caused
to flow through the opening defined by the threshold, the side members
and the metering member onto a pool of molten metal 35 in the forming
chamber 15. The body of glass formed on the surface of the pool of
molten metal 35 is cooled and attenuated to form a dimensionally stable,
continuous sheet or ribbon 37 of flat glass which is removed from the
pool of molten metal and the forming chamber by the take-out appara~us
17 generally comprising supporting and conveying rolls.
The present apparatus and method are useful in making flat glass
of any composition~ such as, for example, soda-lime-silica glasses, alumin~
silica glasses, borosilicate glasses and the like. In the making of soda- ~`~
lime-silica glasses, it has been found particularly beneficial to employ a
threshold, side members and a metering member comprising fused cast silica.
Fused silica is prepared in a manner similar to that described in U.S.
Patent No. 3,151,964 to North, and is crushed and ground to form fused
.
. .,-.
~53~6~3
silica powder. The ground fused silica powder is screened and classified.
A casting slip is prepared from the fused silica powder fines, and this
811p i9 cast in~o suitably shaped plaster molds to make a threshold
member, slde members and a metering member. The resulting green members
are slowly dried and then fired in a kiln to produce a thre~hold, side
members and a metering member, each having extremely smooth surfaces for
glass contact.
The physical characteristics of this preferred fused cast silica
threshold may be appreciated from a microscopic examination of slivers
cut from a threshold or other fused cast silica members, each extending
inwardly from its glass contact surface. A distinguishable skin or sur-
face layer is observed. It is about 50 microns thick and comprises fine
(10 microns diame~er) grains of silica bonded together. Beneath the skin
is a distinguishable, dense layer about 5 to 10 millimeters thick com-
prising large (100 to 350 microns diameter~ gsains bonded by a matrix of
very fine (5 microns and smaller diameter) grains. The interior of the
fused cast silica body comprises a porous rnatrix of coarser (5 to 20 microns
diameter~ grains surrounding large (100 to 350 microns diameter) grains.
~oids having diameters up to one millimeter are found in the porous matrix. ','A,.-' ,.
During the production of soda-lime-silica glass, molten glass is
delivered over such a fused cast silica threshold at viscosities within
the range of from 103 to 104 poises. After months of operation, the threshold
shows little sign of wear, and glass produced during the period of operation
is free of observable surface marking.
The metering member is a slab shape having a longitudinal cross
section, substantially as shown in FIG. 3. This member is si~ilar in shape
to a conventional tweel as used in prior processes whereln glass flows over
a lip and down onto casting rolls or onto a molten metal forming bath. The
-- 10 --
.. . ..
. . ~ ~ . . , ;, .
r
3~)3
metering member preferably has a tapered top portion to secure it in a
supporting structure that may be raised and lowered to control the posi-
tion o the metering member with respect to the threshold.
The side members or Jambs are preferably shaped to partially
rest on the threshold and partially rest outside the ends of the threshold
so that they may be mechanically urged downwardly and inwardly against the
threshold. As shown in FIG. 2, the side members are preferably concave-
shaped in plan cross section in order to receive the metering member and
prevent it from moving downstream due to the hydrostatic and flow forces
of molten glass against it.
The threshold is an elongated block shape with its long dimension
being that lying transverse to the flow of molten glass. Its width and
depth (or height) are preferably as small as possible consistent with pro-
viding a threshold of sufficient mechanical strength to permit handling
and installation. The threshold is preferably provided with access holes
extending through it along its transverse length. These holes are for
receiving heaters or coolers to control the threshold temperature during
use .
The threshold has a convex upwardly-facing top surface, As
shown in FIG. 3, the top of the preferred threshold is defined by an
upstream face 41, a top, horizontal face 43 and a downstream face 45.
The metering member preferably extends downwardly toward the threshold
in the vicinity of the top face-downstream face intersection.
In a preferred embodiment, the top face is about horizontal and
the upstream and downstream faces are angled downward therefrom. The
upstream face is angled from the horizontal about 20 degrees, and the
downstream face is angled from the horizontal about 10 degrees. The
' " ' ~ ' '
.
~53~3
downstream face extends a sufficient distance so that molten metal of
the pool of molten metal in the forming chamber extends over a portion
of the downstream face. As molten glass is delivered over the threshold
and onto the molten metal, glass flowing ad~acent the metering member
or the threshold flows sufficiently slowly that erosion of these members
is minimal.
Although this invention has been described with reference to
particularly preferred embodiments, those skilled in the art of glass-
making will recognize that variations may be made in the practice of this
invention without departing from the concepts disclosed here. Accordingly,
this disclosure is intended to be illustrative rather than limiting, and
the applicants have defined their invention in the claims accompanying
this disclosure.
~, . ~. .
- 12 ~