WO2010024937A2

WO2010024937A2 - Isopipes having improved dimensional stability

Info

Publication number: WO2010024937A2
Application number: PCT/US2009/004923
Authority: WO
Inventors: Andrea W. Bookbinder; Dana C. Bookbinder; Paul M. Schermerhorn
Original assignee: Corning Incorporated
Priority date: 2008-08-29
Filing date: 2009-08-28
Publication date: 2010-03-04
Also published as: TW201024235A; JP5613670B2; KR101618390B1; TWI385129B; CN102149646B; JP2012501289A; CN102149646A; KR20110058832A; WO2010024937A3

Abstract

An isopipe (13) for forming a glass ribbon (19) by a fusion process is provided which comprises a trough (11) and first and second weirs (1,2). Each weir (1,2) has an aperture (35) that extends along at least part of the weir's length. The apertures (35) can be used to cool the interior of the weirs (1,2) and thus reduce their creep rate. Alternatively, the apertures (35) can include structural members (41,42) to stabilize the weirs. In certain embodiments, the structural members (41,42) do not completely fill the apertures (35) thus allowing for weir stabilization through a combination of mechanical support and internal temperature reduction.

Description

IsopiPES HAVING IMPROVED DIMENSIONAL STABILITY

RELATED APPLICATION

[0001] This application claims the benefit of priority of U.S. Provisional Application Serial No. 61/092,931 filed on August 29, 2008, entitled, "Isopipes Having Improved Dimensional Stability".

FIELD

[0002] This invention relates to isopipes used in the production of sheet glass by the fusion process and, in particular, to apparatus and methods for controlling the dimensional changes which isopipes exhibit during use.

DEFINITIONS

[0003] For ease of reference, the description of the present invention employs the word "isopipe," which historically has been used to describe forming apparatus employed in the fusion process. It is to be understood that the use of this terminology is not intended to, and should not be interpreted as, limiting the invention to forming apparatus produced by isostatic pressing.

[0004] Rather, as set forth in the claims (see also the Summary), the invention is applicable to all types of forming apparatus used in the fusion process, irrespective of the material(s) from which the forming apparatus is produced and/or the manner in which those material(s) are processed.

[0005] Also, for ease of description, the isopipe (forming apparatus) is treated as consisting of: 1) a first portion which comprises the apparatus' first and second weirs and 2) a second portion which comprises the apparatus' wedge-shaped portion. The second portion also has a first outer surface which is a continuation of the outer surface of the first weir and a second outer surface which is a continuation of the outer surface of the second weir. Since this terminology is used only for ease of description, the exact location of the transition between the first and second portions is not critical and can be considered to lie at any location below the bottom of the weirs and above the bottom of the apparatus' wedge-shaped portion. BACKGROUND

[0006] Manufacturers of flat panel displays, such as, liquid crystal displays (LCDs), use glass substrates to produce multiple displays simultaneously, e.g., six or more displays at one time. The width of a substrate limits the number of displays that can be produced on a single substrate, and thus wider substrates correspond to increased economies of scale. Also, display manufacturers need wider substrates to satisfy a growing demand for larger size displays.

[0007] In addition, such manufacturers are seeking glass substrates that can be used with polycrystalline silicon devices that are processed at higher temperatures, hi particular, a need exists for high strain point glass compositions that do not undergo compaction during display manufacture. Such glasses generally require higher forming temperatures, and thus a need exists for glass forming processes that can withstand such higher temperatures. [0008] The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. See, for example, Varshneya, Aran K., "Flat Glass," Fundamentals of Inorganic Glasses, Academic Press, Inc., Boston, 1994, Chapter 20, Section 4.2., 534-540. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of liquid crystal displays (LCDs). [0009] The fusion process, specifically, the overflow downdraw fusion process, is the subject of commonly assigned U.S. Patents Nos. 3,338,696 and 3,682,609, to Stuart M. Dockerty. A schematic drawing of the process of these patents is shown In FIG. 1. As illustrated therein, the system includes a supply pipe 9 which provides molten glass to a collection trough 11 formed in isopipe 13.

[0010] Once steady state operation has been achieved, molten glass passes from the supply pipe to the trough and then overflows the top of the trough on both sides, thus forming two ribbons of glass that flow downward and then inward along the outer surfaces of the isopipe. The two ribbons meet at the bottom or root 15 of the isopipe, where they fuse together into a single ribbon. The single ribbon is then fed to drawing equipment (represented schematically by arrows 17), which controls the thickness of the ribbon and thus the ultimate sheets by the rate at which the ribbon is drawn away from the root. The drawing equipment is located well downstream of the root so that the single ribbon has cooled and become rigid before coming into contact with the equipment.

[0011] As can be seen in FIG. 1, the outer surfaces of the final glass ribbon do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces only see the ambient atmosphere. The inner surfaces of the two half ribbons which form the final ribbon do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final ribbon. In this way, the superior properties of the outer surfaces of the final glass sheets cut from the ribbon are achieved. [0012] As is evident from the foregoing, isopipe 13 is critical to the success of the fusion process. In particular, the dimensional stability of the isopipe is of great importance since changes in isopipe geometry affect the overall success of the process. Significantly, the conditions under which the isopipe is used make it susceptible to dimensional changes. The isopipe typically operates at elevated temperatures on the order of 1000°C and above. Moreover, the isopipe operates at these elevated temperatures while supporting its own weight as well as the weight of the molten glass overflowing its sides and in trough 11 , and at least some tensional force that is transferred back to the isopipe through the fused glass as it is being drawn. Depending on the width of the glass sheets that are to be produced, the isopipe can have an unsupported length of 2.0 meters or more.

[0013] To withstand these demanding conditions, isopipes 13 have been manufactured from isostatically pressed blocks of refractory material (hence the name "iso-pipe"). In particular, isostatically pressed zircon refractories have been used to form isopipes for the fusion process.

[0014] Even with such high performance materials, in practice, isopipes exhibit dimensional changes which limit their useful life. For example, isopipes exhibit sag such that the middle of the unsupported length of the pipe drops below its outer supported ends. These dimensional changes occur both along the root of the isopipe and along the weirs at the top of the pipe.

[0015] In view of the foregoing, it can be seen that a need exists for apparatus and methods which will allow the fusion process to be used effectively and economically to produce glass sheets which have larger widths and/or are composed of glasses having higher strain points. -A-

In particular, a need exists to improve the dimensional stability of isopipes and thereby extend their service life and thus minimize process down time and isopipe replacement costs.

SUMMARY

[0016] In accordance with a first aspect, the invention provides an apparatus (e.g., isopipe 13) for forming a glass ribbon (19) by a fusion process including: a first portion (21) including a trough (11) and first and second weirs (1,2), each weir having an inner surface (25), a top surface (27), and an outer surface (29); and a second portion (23) having a first outer surface (31) which is a continuation of the outer surface (29) of the first weir (1) and a second outer surface (32) which is a continuation of the outer surface (29) of the second weir (2), the first and second outer surfaces (31,32) being oriented with respect to one another so that at least a part (33) of the second portion (23) has a wedge-shaped cross section; wherein each weir (1,2) includes an aperture (35) that: (i) extends along at least part of the weir's length, and (ii) at least part of the aperture (35) is located between the weir's inner and outer surfaces (25,29).

[0017] In accordance with a second aspect, the invention provides a method of making a glass ribbon (19) using a fusion process including:

(A) providing molten glass to a forming apparatus (e.g., isopipe 13) that includes first and second weirs (1,2), each weir having:

(i) an inner surface (25), (ii) a top surface (27), (iii) an outer surface (29); and (iv) an aperture (35) that:

(a) extends along at least part of the weir's length, and

(b) includes at least a part that is located between the weir's inner and outer surfaces (25,29); and

(B) passing a fluid (e.g., a gas or gas mixture or a liquid or liquid mixture) through the apertures (35) to cool the weirs (1,2). [0018] In certain embodiments, the apertures (35) in the weirs (1,2) can include structural members (41 ,42) which completely or partially fill the apertures. The structural members can be solid or hollow. In other embodiments, the body of the forming apparatus can also include one or more apertures (43), which can include structural members (45). [0019] The reference numbers used in the above summaries of the aspects of the invention are only for the convenience of the reader and are not intended to and should not be interpreted as limiting the scope of the invention. More generally, 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.

[0020] 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. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. It is to be understood that the various features of the invention disclosed in this specification and in the drawings can be used in any and all combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a perspective, schematic drawing illustrating a representative construction for an apparatus for use in an overflow downdraw fusion process for making flat glass sheets.

[0022] Figures 2-8 are schematic, cross-sectional drawings illustrating representative embodiments of isopipes having weirs in which apertures are formed for receiving a cooling fluid and/or a structural member.

[0023] The reference numbers used in the drawings are set forth below in Table 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] As discussed above, a preferred method for producing LCD substrates is to use the fusion process in which molten glass is formed into a ribbon by being passed over a large ceramic structure known as an isopipe. Over time, LCD substrates have increased in size with the current size (Gen 10) being about 2850 mm x 3050 mm. Each increase in size (width) has meant a corresponding increase in isopipe length.

[0025] This migration to larger isopipes has placed significant challenges on the ability of isopipes to have multi-year useful lives. For current glass compositions, the weirs of the isopipe typically operate at a temperature of approximately 1220°C and above while the root operates between 1180°C and 114O⁰C. This high temperature condition causes the isopipe's refractory material, e.g., zircon, to undergo creep. The larger the Gen size, and hence the larger the isopipe, the more creep will be generated.

[0026] Stress analysis shows that as a first approximation, the absolute deflection (D) of an isopipe depends on the intrinsic rate of creep (έ = dε/dt) of the material from which the isopipe is made (units of 1/hr), as well as the time (t) the isopipe is in service and the isopipe's length (L) and height (H), wherein (k) is a constant:

D ~ k ^• έ ^• L⁴/H^{2 •} t.

[0027] As can be seen from this equation, doubling the length of the isopipe increases the deflection 16 fold for the same isopipe material, height, and time in use. [0028] This increase in deflection can potentially be addressed by increasing the isopipe height. However, isopipe height is already near the fundamental limits for the isopress equipment currently available in the industry. Another alternative is to improve the compressive forces which are applied to the sides of the isopipe to combat creep (see commonly assigned US Patent Publication No. 2003/0192349), but this approach can place significant constraints on isopipe design that may result in glass flows below desired targets. Lowering the overall operating temperature is another possibility but it would require the development of new glass compositions capable of being processed at the lower temperatures. Finally, the creep rate of the ceramic material used to produce the isopipe can be reduced through the development of improved materials (see commonly assigned PCT Patent Publication No. WO 2002/044102). However, even in this case, future substrate sizes and isopipe designs may continue to push the ceramic materials into areas that are not capable of achieving sufficiently long usable lives.

[0029] As noted above, during operation, the hottest part of the isopipe is normally the weirs. As illustrated in, for example, WO 2002/044102 referred to above, the creep rate of zircon refractories (as well as other high temperature refractories) increases with temperature.

Thus, compared to the body of the isopipe, the weirs will exhibit the highest creep rates.

[0030] In addition, during use, the weirs are subjected to both downward forces due to gravity as well as outward forces (bulging forces) due to the molten glass contained in the isopipe's trough. Moreover, not only are there two forces, but the weirs have a thickness that is much less than that of the body of the isopipe, thus making the weirs especially subject to dimensional instability over time.

[0031] One possible approach to combat this susceptibility to dimensional changes is to use thicker weirs. However, this approach can place significant constraints on isopipe design that may result in glass flows below desired targets.

[0032] Various proposals have been made to reduce the sag of isopipes through the use of support rods and holes in the body of the isopipe. See U.S. Patent No. 3,437,470; Japanese

Patent Publication No. 11-246230; Japanese Patent Publication No. 2006-298736; Japanese

Patent Publication No. 2006-321708; and Japanese Patent Publication No. 2007-197303.

Significantly, none of these references has recognized the enhanced susceptibility of the weirs to dimensional changes as a result of: 1) the higher operating temperatures to which the weirs are normally exposed, 2) the fact that the weirs are subjected to both vertical and horizontal deforming forces, and 3) the weirs' relative thinness compared to the remainder of the isopipe.

Similarly, none of these references have proposed a solution to this problem.

[0033] The present invention specifically deals with the weir instability problem by including at least one aperture in each weir. As an initial benefit, the aperture reduces the weight of the weir and thus the associated sag-producing load on the weir.

[0034] hi addition, in certain embodiments, the aperture is used to reduce the internal temperature of the ceramic material making up the weir. For example, a fluid at a controlled temperature less than the nominal temperature of the weir can be passed through the aperture at a controlled flow rate. As shown below, even a relatively small change in the temperature of the material making up the weir can have a significant effect on the creep rate of the material.

[0035] The fluid can be an inert gas, such as nitrogen, a non-inert gas, such as air provided that if molybdenum is used it is not exposed to the non-inert gas, or a liquid, such as water. Gas or liquid mixtures can also be used if desired. For some applications, a liquid can be more effective than a gas because of its higher heat capacity.

[0036] The fluid can be passed through the aperture in either direction, although in some cases it may be desirable to pass the fluid through the aperture starting at the inlet end of the isopipe since the inlet end, where the molten glass enters the isopipe, is normally hotter than the isopipe's distal end. If the fluid picks up more heat from the inlet end, it can help smooth out thermal gradients along the weir and thus help control glass flow. Also, having the fluid enter the aperture on the inlet end can help reduce the temperature of the molten glass at the inlet which may be desirable for certain applications. The fluid can also make multiple passes through the aperture through the use of a heat exchanger structure. For example, the fluid can be introduced into a tube having a center bore connected to a surrounding annulus. The fluid can, for example, pass down the center bore and back through the annulus. In this way, heat can be more effectively transferred to the fluid. Alternatively, the fluid can make its first pass through the annulus and its second pass through the center bore. More complex heat exchanger structures can, of course, be used if desired.

[0037] In other embodiments, the aperture is used to house a structural member composed of a material having a lower creep rate than the material making up the weir. The structural member can be solid and can fully or partially fill the cross-section of the aperture. In the latter case, the unfilled part of the aperture can be used for cooling both the structural member and the weir material, e.g., by passing a fluid through the unfilled part of the aperture and thus over the exposed surfaces of the structural member and the inner wall of the aperture. [0038] In further embodiments, the structural member can be hollow and its outer envelope can fully or partially fill the cross-section of the aperture, hi either case, the hollow part of the structural member can be used for cooling, e.g., by passing a fluid through the inside of the structural member. If the outer envelope of the hollow structural member only partially fills the cross-section of the aperture, the unfilled part of the aperture can also be used for cooling.

[0039] The aperture will normally extend through the entire length of the weir, although an aperture with one or both ends closed can be used in the practice of the invention, e.g., when a cooling fluid is passed through a heat exchanger structure. The structural member can be contained within the weir or can extend beyond the weir and engage a support structure on one or, preferably, both of its ends.

[0040] In addition to an aperture in each weir (with or without a structural member), the isopipe can include one or more apertures in the body of the isopipe, i.e., at a level below the weirs. As with the apertures formed in the weirs, the apertures formed in the body can contain structural members, which can completely or partially fill the aperture. Also like the apertures in the weirs, the apertures in the body can be used for internal cooling of the isopipe to reduce the creep rate of the material making up the body of the isopipe. [0041] The apertures in the weirs and the body of the isopipe (when used) can be manufactured by core drilling into the isopipe or preferably into the blank from which the isopipe is formed, or can be formed in situ during the manufacture of the blank. [0042] The normal process for manufacturing ceramic blanks for an isopipe is a multiple step process. For example, batch materials of zircon or other ceramic materials along with a binder can be prepared by, for example, spray drying. The batch materials can then be placed in a flexible bag and vibrated to allow particle settling and to achieve initial compaction. The bag can then be hermetically sealed and placed in an cold isostatic press to more fully compact the structure. The compacted structure can then be fired to a dense ceramic at high temperature.

[0043] A process of this type can be modified to produce apertures in the blank by placing one or more rods composed of graphite or other combustible materials, for example, a solid or foamed natural or synthetic polymer, into the isopress bag to act as mandrels. The batch is then poured around the rod(s) and vibrated to allow the batch particles to arrange in a more densely packed structure. The bag is then hermetically sealed and isopressed. The compacted batch and rod(s) can then be placed in a furnace to first burn out the rod(s) followed by sintering at high temperature. In an alternative process, the batch binder can be burnt out first, followed by pre-sintering of the structure. After cooling to room temperature, the rod or rods are removed from the blank followed by high temperature sintering. [0044] The apertures can have a variety sizes ranging from, for example, a few millimeters to several inches for a Gen 10 isopipe. In general, smaller apertures are used when the purpose of the aperture is to cool the interior of the isopipe by passing a fluid through the aperture. As discussed above, this fluid flow provides a means for extracting heat from the interior of the isopipe thereby lowering the internal temperature and reducing material creep. Small changes in the temperature can significantly reduce the level of creep in an isopipe. For example, as shown in Table 2, for zircon, a reduction in temperature from 1250⁰C to 1180°C results in approximately a 50% reduction in creep rate. The amount of fluid flow needed will depend on the heat capacity of the fluid, the fluid's temperature, the desired internal temperature reduction, and the specific geometry of the isopipe. Flow rates for specific applications can be readily determined by skilled workers based on the present disclosure.

[0045] As discussed above, in certain embodiments, the apertures can include structural members. These structural members preferably are composed of a material which exhibits less creep than the material used for the isopipe. For example, for a zircon isopipe, the structural members can be composed of such materials as Al₂O₃, SiN, SiC, molybdenum, or fiber-reinforced structures. In the case of molybdenum rods, the rod is preferably platinum clad or blanketed in an inert atmosphere, e.g., N₂, to reduce oxidation. Materials of these types are capable of demonstrating very low creep even at 1250°C and thus can provide additional support to isopipes composed of zircon or other refractories during operation. [0046] Among the benefits provided by the invention is the ability to continue to use proven materials, such as zircon, in the manufacture of LCD substrates. Such materials are known to be compatible with the glass compositions which have been qualified by display manufacturers. The invention also widens the design window for isopipes. For example, isopipes having reduced heights can be produced without impacting sag and thus operating life. This reduces costs both in terms of the isopipe itself and the overall size of the fusion machine. A reduced height can also help reduce the chances of forming secondary crystals such as those described in commonly-assigned PCT Patent Publication No. 03/055813. [0047] Without intending to limit it in any manner, the invention is further illustrated by the exemplary embodiments of FIGS. 2 through 8.

[0048] FIG. 2 shows an embodiment which employs apertures 35 in weirs 1 and 2, and aperture 43 in the body of the isopipe. Apertures 35 in weirs 1 and 2 are completely filled with structural members 41 and 42, respectively, while aperture 43 is completely filled with structural member 45. [0049] FIG. 3 shows a variation of the embodiment of Figure 2 in which apertures 35 in weirs 1 and 2 are connected to one another by further aperture 37, which like apertures 35 is filled with a structural member. This embodiment also uses an aperture 43 which has a circular configuration, rather than a rectangular configuration as in FIG. 2. [0050] FIG. 4 shows an embodiment suitable for use in cooling portions of the interior of the isopipe. This embodiment employs five relatively small apertures, two of which are located in the weirs, i.e., apertures 35, and three of which are located in the body of the isopipe, i.e., apertures 43.

[0051] FIG. 5 shows a variation of the embodiment of FIG. 4 which again employs apertures suitable for cooling. This embodiment employs one relatively large aperture 43 in the body of the isopipe and elliptically-shaped apertures 35 in weirs 1 and 2. [0052] FIG. 6 shows a variation of FIG. 5 where structural members 41 and 42 have been introduced into apertures 35 and structural member 45 has been introduced into aperture 43. A cooling fluid can be passed through the unfilled parts of the apertures or the structural members alone can be used to reduce the sag of the isopipe.

[0053] FIGS. 7 and 8 illustrate the use of hollow structural members which, as illustrated, can be elliptical in shape. The use of hollow structural members has the advantage of reducing the weight of the structural members. Also, a cooling fluid can be passed through the central portion of the structural member, if desired. It should be noted that irrespective of whether the outer envelope of the structural member fills the aperture, the hollow portion of the structural member constitutes an unfilled portion of the aperture. [0054] A variety of modifications that do not depart from the scope and spirit of the invention will be evident to persons of ordinary skill in the art from the foregoing disclosure. For example, although the invention has been illustrated in terms of isopipes whose weirs have vertical sides, it is also applicable to isopipes having weirs with sloping sides, e.g., an isopipe having the cross-sectional shape of a V or of a Y with no corner on the outside surface of the pipe at the upper end of the wedge-shaped portion of the pipe. Similarly, although unitary isopipes are shown in FIG. 2 through 8, isopipes composed of two or more separate components (which can be composed of the same or different materials) can be used in the practice of the invention. The following claims are intended to cover the specific embodiments set forth herein as well as modifications, variations, and equivalents of these and other types.

Table 1

Table 2

Claims

What is claimed is:

1. Apparatus for forming a glass ribbon by a fusion process comprising: a first portion comprising a trough and first and second weirs, each weir having an inner surface, a top surface, and an outer surface; and a second portion having a first outer surface which is a continuation of the outer surface of the first weir and a second outer surface which is a continuation of the outer surface of the second weir, the first and second outer surfaces being oriented with respect to one another so that at least a part of the second portion has a wedge-shaped cross section; wherein each weir comprises an aperture that:

(i) extends along at least part of the weir's length, and

(ii) at least part of the aperture is located between the weir's inner and outer surfaces.

2. The apparatus of Claim 1 wherein for each weir, the aperture extends along the weir's entire length.

3. The apparatus of Claim 1 wherein for each weir, the aperture has a circular cross-section.

4. The apparatus of Claim 1 wherein for each weir, the aperture has an elliptical cross-section.

5. The apparatus of Claim 1 wherein the apertures of the first and second weirs are connected by a further aperture which underlies the trough.

6. The apparatus of Claim 1 wherein the apparatus further comprises first and second structural members, the first structural member being located within the aperture of the first weir and the second structural member being located within the aperture of the second weir.

7. The apparatus of Claim 6 wherein the first and second structural members are composed of a material that exhibits less creep at the operating temperature of the apparatus than that exhibited by the material of which the weirs are composed.

8. The apparatus of Claim 6 wherein the structural members fill their respective apertures.

9. The apparatus of Claim 6 wherein the structural members partially fill their respective apertures.

10. The apparatus of Claim 6 wherein the structural members are hollow.

11. The apparatus of Claim 1 further comprising at least one additional aperture, no part of which is located between the weirs' inner and outer surfaces.

12. The apparatus of Claim 11 wherein the apparatus further comprises a structural member which is located within the additional aperture.

13. The apparatus of Claim 12 wherein the structural member which is located within the additional aperture is composed of a material that exhibits less creep at the operating temperature of the apparatus than that exhibited by the material of which the weirs are composed.

14. The apparatus of Claim 1 wherein the first and second portions are parts of a unitary body of material.

15. A method of making a glass ribbon using a fusion process comprising:

(A) providing molten glass to forming apparatus that comprises first and second weirs, each weir having:

(i) an inner surface, (ii) a top surface, (iii) an outer surface; and (iv) an aperture that:

(a) extends along at least part of the weir's length, and

(b) includes at least a part that is located between the weir's inner and outer surfaces; and

(B) passing a fluid through the apertures to cool the weirs.

16. The method of Cl aim 15 wherein :

(a) the forming apparatus further comprises first and second structural members, the first structural member being located within the aperture of the first weir and the second structural member being located within the aperture of the second weir;

(b) the structural members partially fill their respective apertures; and

(c) the fluid is passed through the unfilled portions of the apertures and cools both the weirs and the structural members.

17. The method of Claim 16 wherein the first and second structural members are composed of a material that exhibits less creep at the operating temperature of the forming apparatus than that exhibited by the material of which the weirs are composed.

18. The method of Claim 15 wherein:

(a) the forming apparatus comprises at least one additional aperture, no part of which is located between the weirs' inner and outer surfaces; and

(b) a fluid is passed through the additional aperture to cool the forming apparatus.

19. The method of Claim 18 wherein:

(a) the forming apparatus further comprises a structural member which is located within the additional aperture;

(b) the structural member partially fills the additional aperture; and

(c) the fluid is passed through the unfilled portion of the additional aperture and cools both the forming apparatus and the structural member.

20. The method of Claim 19 wherein the structural member which is located within the additional aperture is composed of a material that exhibits less creep at the operating temperature of the forming apparatus than that exhibited by the material of which the weirs are composed.