Note: Descriptions are shown in the official language in which they were submitted.
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PULLING ROLL FOR THE PRODUCTION OF SHEET GLASS
FIELD OF THE INVENTION
[0001 ] This invention relates to the manufacture of sheet glass. More
particularly, the
invention relates to improved pulling rolls for use in controlling the
thickness of glass
produced by, for example, the overflow downdraw fusion process. The invention
is of
particular value in the production of high quality sheet glass, which exhibits
low levels of
surface defects. Such glass can be used as substrates for flat panel displays,
e.g., liquid crystal
displays.
BACKGROUND OF THE INVENTION
[0002] Pulling rolls are used in the manufacture of sheet glass to apply
tension to the ribbon
of glass from which the sheets are formed and thus control the nominal sheet
thickness. For
example, in the overflow downdraw fusion process (see Dockerty, U.S. Pat. Nos.
3,338,696
and 3,682,609), pulling rolls are placed downstream of the tip or root of the
fusion pipe and
are used to adjust the rate at which the formed ribbon of glass leaves the
pipe and thus
determine the nominal thickness of the finished sheet. The rolls are
customarily used in pairs;
the formed ribbon of glass is fed between the rolls.
[0003] A frequently used configuration for such rolls comprises a profiled
refractory cover
over a steel shaft. A profiled cover is a cover that has portions with
different radii with
respect to the axis of the shaft. Pulling rolls are commonly designed to
contact the glass
ribbon at its outer edges, specifically, in regions just inboard of the
thickened beads that exist
at the very edges of the ribbon. The portion of the roll coming in contact
with the glass
ribbon is known as a pulling flat. The use of a roll with pulling flats
minimizes the contact
surface area between the pulling rolls and the glass, thereby minimizing
damage to the glass
sheet and the generation of wear debris from the rolls.
[0004] A successful pulling roll needs to meet a number of conflicting
criteria. First, the roll
needs to be able to withstand the high temperatures associated with newly
formed glass for
substantial periods of time. The longer a roll can last in such an environment
the better, since
roll replacement reduces the amount of finished glass a given machine can
produce and thus
increases the ultimate cost of the glass. The roll must also be able to
withstand any internal
stresses generated by unequal heating, or by different coefficients of
expansion that are
exhibited by different portions of the roll being constructed of different
materials.
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[0005] Second, the roll must be able to produce enough pulling force to
control the glass'
thickness. In order not to damage the central portion of the ribbon that
becomes the usable
finished glass, the roll can only contact the ribbon over a limited area at
its edges. Thus, the
required pulling forces must be generated using only this area. However, the
forces applied to
the glass cannot be too large since this can create surface damage that can
propagate into the
usable central portion of the ribbon. Accordingly, the roll must achieve a
balance between
applying too little and too much force to the edge regions of the glass.
[0006] Third, the pulling roll must not give off excessive amounts of
particles that can adhere
to the glass and form surface defects (known as "onclusions"). For glass that
is to be used in
such demanding applications as substrates for flat panel displays, onclusions
must be kept to
very low levels since each onclusion will typically represent a defective
region of the finished
product (e.g., one or more defective pixels). Because of the hot environment
in which pulling
rolls operate, finding materials which can apply sufficient pulling forces to
a glass ribbon and
yet not give off particles when hot is a difficult challenge. Work done
previously in the field
and the laboratory has established that when either a silica fiber or alumina
silica fiber formed
as a compressed cover on a shaft, impregnated with colloidal silica and fired
to form a rigid
body, is used in the application as the pulling roll cover media, clusters of
colloidal silica
particles and small amounts of fiber were the most prevalent form of dust and
debris picked
up by the convective currents, and transported up the draw to contaminate the
surface of the
sheet.
[0007] The present invention addresses these shortcomings in the art and
provides improved
pulling rolls which achieve higher levels of performance than existing pulling
rolls.
SUMMARY OF THE INVENTION
[0008] The roll of the present invention makes use of two or more materials in
a roll
configuration that provides a pulling flat containing a material optimized for
glass contact,
and remaining portions optimized for resistance to thermal and physical
stress. One
embodiment of the roll of the invention is made up of various components
disposed co-axially
about a shaft. These components are, in exterior view from one end of the
shaft to another, a
first end cap, a first end module, a first pulling flat, a center module, a
second pulling flat, a
second end module, and a second end cap. The modules and pulling flats may be
generally
cylindrical in form, with bores to accommodate the shaft. The modules and
pulling flats are
held in place on the shaft by such fastening means as end caps provided with
compression
fittings or with springs. The modules and pulling flats may be held in fixed
positions relative
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to each other by pins, screws, or other fasteners. In a variation on this
embodiment, the
pulling flats may take the form of cylinders with bores to accommodate, in
addition to the
shaft, the profile of either an end module or the center module, a reduced
profile of either an
end module or the center module, or reduced profiles or full profiles of an
end module and a
center module at opposite ends of the pulling flat bore. In this variation
either or both of the
center module and end modules serve as a support system for the pulling flat.
The support
system may be constructed of a compressible substance so as to absorb the
diametrical
expansion of the shaft while locking the pulling flat in place so that it
cannot move on the
shaft. The support system also serves as a stable platform to absorb the load
applied to the
pulling flat to pull the sheet of glass. The rolls of the invention may be
configured so that
they may be disassembled, fitted with replacement pulling flats, and
reassembled.
[0009] The pulling flat of the invention contains a TBS (thermally bonded
shape). The TBS
is constructed from a material that does not contain free colloidal silica or
fibrous particles in
a size range or shape that leads to fluidization of these particles in the
convective currents
produced in the sheet glass manufacturing process. The TBS may be a compressed
pulling
flat material such as a ceramic fiber felt formed from silicoalumina fiber
that is produced
through the blowing technique and washed to remove the shot. The washed
silicoalumina
fibers may then be dispersed into water slurry to which up to 4.5% by weight
of a sodium-free
solution of borate has been added. The resulting fiber suspension is then
pressed until a
compaction having the desired block density is obtained. This compaction is
then fired at
temperature at or about 1350 C to produce a ceramic body. The density of the
block can be
optimized to meet specific operating conditions. The TBS material has
sufficient high-
temperature compressive strength to pull the glass. Any wear debris produced
by the TBS
material is not easily fluidized because of its combination of specific
gravity, particle size and
morphology. The TBS material is a sintered compressed product having a
chemical bond
between the colloidal component and the fiber, which eliminates the dust seen
in rigidized
materials in which the colloidal component is not bonded to the fiber but
rather remains loose
within the fiber matrix.
[0010] Materials that may be used as the TBS material of the invention include
TBS-Nl,
TBS-N2 and 1260TBS, available from Isolite Insulating Products Co., and having
the
following composition and characteristics:
[0011] TBS-Nl : Maximum temperature = 1200 C, density = 620 kg/m3, A1203
content = 41.7
wt%, Si0z content: 53.8%, flexural strength = 11.9 Mpa, thermal expansion =
3.7 x 10-6 / K,
thermal conductivity = 0.23 W/mK at 350 C.
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[0012] TBS-N2: Maximum temperature = 1200 C, density = 950 kg/m3, A1203
content = 41.7
wt%, Si02 content: 53.8%, flexural strength = 24.8 Mpa, thermal expansion =
3.7 x 10-6 / K,
thermal conductivity = 0. 3 7 W/mK at 3 5 0 C.
[0013] 1260TBS: Maximum temperature = 1260 C, density = 550 kg/m3, A1203
content =
40.5 wt%, Si02 content: 55.5%, flexural strength = 9.8 Mpa, thermal expansion
= 3.7 x 10-6 /
K, thermal conductivity = 0.14 W/mK at 3 5 0 C.
[0014] The center module and the end modules are typically composed of fiber
materials
(such as the aluminosilicate fiber material commercially available as ZYALITE
and the
silicate fiber material available as ZYACIL) rigidized with colloidal-sized
ceramic particles
such as amorphous silica particles. End modules constructed of such rigidized
fiber materials
contain very hard surfaces that are resistant to wear and impact damage while
simultaneously
being resilient enough under the surface to be able to absorb the strain
imparted by the
thermal expansion of the shaft. These properties are imparted by making use of
a coating
material with a suitable particle size on a pre-form of the fiber material, by
the method of
application of the coating material to the pre-form and by the choice of an
appropriate method
of heat treatment. Application of a spray coating produces a roll that may be
compressed and
expanded. In one embodiment of the invention, the spray coating is applied to
the center
module and the end modules, but not to the pulling flats. Use of such
rigidized fiber materials
results in modules and rolls that are relatively lightweight. Suitable
materials for these
modules are described in U.S. Patents No. 5,205,398, 5,355,996 and 5,378,219,
incorporated
herein by reference. In one embodiment of the invention, the center module
contains an
aluminosilicate fiber material and the end modules contain a silica fiber
material.
[0015] In the embodiment of the invention in which the flat is underlain by a
portion of the
center module or an end module, the underlying portion provides for mechanical
and physical
support of the flat during the pulling of the sheet by a set of rolls. The
underlying portion
provides a stable platform to absorb the load exerted on the flat in the
pulling process. The
underlying portion also provides a compressive layer over the shaft and
absorbs the diametric
expansion of the shaft while locking the flat in place so that it cannot move
in the shaft. In
addition, as a result of their surface hardness, underlying portions of the
center and end
modules can prevent uneven wear of the flat due to slight variations on the
density of the
material from which the flat is constructed.
[0016] The hard center module provides adequate support to the pulling flats,
even at reduced
profile, thereby allowing the main diameter of the center module to be
reduced. This
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increases the effective center gap between pairs of rollers which in turn
allows wider sheets of
glass to be pulled without touching the rollers at the center, thereby
eliminating "micro-touch"
defects near the center of the glass sheet. The hardness of the center module
and the end
modules results in extended service lives with respect to the pulling flats.
[0017] Additional features and advantages of the invention are set forth in
the detailed
description which follows, and in part will be readily apparent to those
skilled in the art from
that description or recognized by practicing the invention as described
herein.
[0018] It is to be understood that both the foregoing general description and
the following
detailed description are merely exemplary of the invention, and are intended
to provide an
overview or framework for understanding the nature and character of the
invention as
claimed.
[0019] The accompanying drawings are included to provide a further
understanding of the
invention, and are incorporated in and constitute a part of this
specification. The drawings
illustrate various embodiments of the invention, and together with the
description serve to
explain the principles and operation of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Fig 1 is a cross-section view of an apparatus according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021 ] The invention is best understood by reference to the accompanying
drawing. It is
emphasized that, according to common practice, the various dimensions of the
apparatus and
the associated component parts as shown in the drawings are not to scale and
in some cases
have been enlarged for clarity.
[0022] Referring now to the drawing, shown in FIG. 1, is a pulling roll 10,
containing a shaft
20 in the form of a cylinder, having two ends. Directions on the shaft towards
the ends are
designated "polar directions." Directions on the shaft towards the center of
the shaft are
designated "central directions." The shaft may be constructed of a metal such
as stainless
steel. Disposed coaxially around the shaft 20 is center module 22, having a
cylindrical outer
surface and two ends. Center module 22 has a bore to accommodate the shaft 20.
Pulling
flats 24 are disposed at either end of, and exterior to the cylindrical outer
surface of, center
module 22. Each pulling flat 24 has a cylindrical outer surface and two ends,
a polar and a
centrally directed end, in terms of their positions on the axis. Each pulling
flat 24 has a bore
to accommodate the profile of center module 22. Disposed coaxially around the
shaft 20, and
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in contact with either end of center module 22 and a polar end of a pulling
flat 24, is an end
module 26. Each end module 26 has a centrally directed end and a polar end.
Each end
module 26 has a bore to accommodate shaft 20. Disposed coaxially around the
shaft 20, and
in contact with a polar end of an end module 26, is an end cap 28. Each end
cap 28 has a bore
to accommodate shaft 20. A plurality of pins 32 fits into openings in a
surface of center
module 22, and aligned openings in the centrally directed surface of pulling
flat 24. A
plurality of pins 32 fits into openings in a polar surface of pulling flat 24,
and aligned
openings in the centrally directed surface of end module 26. End caps 28 may
be fastened to
the shaft 20 by any appropriate fastening means, such as by a spring or a
compression fitting.
Alternatively, the roll may be assembled without the use of pins 24; in this
embodiment,
mechanical linkage between the modules and pulling flats is provided by
compressive force
maintained by end caps 28 fastened to shaft 20, by other fasteners such as
screws, or by
providing the surfaces of the center module 22, pulling flats 24 and end
modules with mating
portions.
[0023] The pulling flats 24 of the invention may comprise a thermal bonded
shape produced
by washing silicoalumina fiber, dispersing the silicoalumina fibers in water
and sodium-free
borate solution to form a suspension, pressing the suspension to form a
compaction, and firing
the compaction to achieve a ceramic body. The thermal bonded shape obtained by
this
process is a fired ceramic containing a reduced-shot or shot-free
silicoalumina fiber
chemically crosslinked to sodium-free borate.
[0024] One example of a configuration for fastening an end cap 28 to shaft 20
is disclosed in
U.S. Patent Publication No. 2002/265180 Al, the subject matter of which is
incorporated
herein by reference.
[0025] Pulling roll 10 may be assembled by inserting the shaft 20 having two
ends into a
center module 22, inserting each end of shaft 20 into a respective pulling
flat 24 so that each
pulling flat 24 is provided with an exposed end and is engaged by the center
module, inserting
each end of shaft 20 into an end module 26 so that each pulling flat engages a
respective end
module 26, inserting each end of shaft 20 into a respective end cap 28, and
securing each end
cap 28 to shaft 20. The end caps 28 may be secured to shaft 20 with attachment
devices such
as spring or compression fittings.
[0026] If pins are used in the assembly, pulling roll 10 may be assembled by
inserting the
shaft 20 having two ends into a center module 22, inserting at least one pin
32 into each end
of center module 22, inserting each end of shaft 20 into a respective pulling
flat 24 so that
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each pulling flat 24 is provided with an exposed end and so that at least one
pin 32 in center
module 22 engages each of pulling flats 24, inserting at least one pin 32 into
the exposed end
of each pulling flat 24, inserting each end of shaft 20 into an end module 26
so that at least
one pin 32 inserted into an exposed end of a respective pulling flat 24
engages a respective
end module 26, inserting each end of shaft 20 into a respective end cap 28,
and securing each
end cap 28 to shaft 20. The end caps 28 may be secured to shaft 20 with
attachment devices
such as spring or compression fittings.
[0027] Numerous modifications and variations of the present invention are
possible. Within
the scope of the following claims, the invention may be practiced otherwise
than as
specifically described. While this invention has been described with respect
to certain
preferred embodiments, different variations, modifications, and additions to
the invention will
become evident to persons of ordinary skill in the art. All such
modifications, variations, and
additions are intended to be encompassed within the scope of this patent,
which is limited
only by the claims appended hereto.
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