WO2011066064A2

WO2011066064A2 - Method and apparatus for making a glass sheet with controlled thickness

Info

Publication number: WO2011066064A2
Application number: PCT/US2010/054964
Authority: WO
Inventors: Michael Y. Nishimoto
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 2009-11-24
Filing date: 2010-11-01
Publication date: 2011-06-03
Anticipated expiration: 2012-05-24
Also published as: KR20120102720A; KR101846035B1; WO2011066064A3; CN102725238A; JP5685264B2; JP2013512171A; TW201125828A; TWI547448B; CN102725238B

Abstract

A method of making a glass sheet includes providing a glass ribbon at a first temperature where at least a portion of the glass ribbon exhibits viscous behavior. A heat sink is provided adjacent to the at least a portion of the glass ribbon at a second temperature that is less than the first temperature. A plurality of heating elements is provided at a position where the heating elements are operable to shape a thermal profile of the heat sink. Heat is transferred from the at least a portion of the glass ribbon to the heat sink and at least a portion of the heat is absorbed into the heat sink.

Description

METHOD AND APPARATUS FOR MAKING A GLASS SHEET

WITH CONTROLLED THICKNESS

[0001] This application claims the priority of US Provisional Application 61/264017 filed on November 24, 2009.

TECHNICAL FIELD

[0002] The invention relates generally to methods and apparatus for forming a glass sheet. More specifically, the invention relates to a method and an apparatus for controlling the thickness of a glass sheet formed from molten glass.

BACKGROUND

[0003] U.S. Patent No. 3,682,609 (S. M. Dockerty) describes a system for controlling thickness of a sheet formed from molten glass. In the system of U.S. Patent No. 3,682,609, molten glass flows down opposite sides of a forming member and merges at a wedge root of the forming member to form a glass sheet. The glass sheet passes between a pair of opposing housings having front walls that face the glass sheet. The front walls are made of a material having high thermal conductivity, low expansion and low emissivity, such as silicon carbide. Fluid conduit tubes are arranged within the housings, with the nozzles of the fluid conduit tubes positioned in a spaced-apart relationship on the backside of the front walls. Each fluid conduit has an associated flow meter, which is provided with a control valve and is connected to a manifold. Each fluid conduit tube delivers cooling fluid or heated fluid to a backside area of the adjacent front wall. Typically, the delivered fluid is air. Heat exchange via thermal radiation occurs between the glass sheet and the front walls in order to control the thickness of the glass sheet. If a thickness trace of the glass sheet indicates that a particular area across the width of the glass sheet is thicker than desired, the thickness trace is corrected by cooling zones of the glass sheet adjacent to the thicker area, i.e., cooling the thinner areas. Fluid conduit tubes corresponding to the adjacent zones are activated to cool the adjacent zones (i.e., the thinner areas). The patent also suggests delivering heated fluid to the backside of the front walls as an alternative to delivering cooling fluid. In this case, the heated fluid would be delivered by the fluid conduit tubes corresponding to the thicker area. This would decrease viscosity in the thicker area and then thin the area. Heated fluid may be provided by electrical windings associated with the fluid conduit tubes. [0004] For the apparatus described above, the resolution of the apparatus's field of view is limited by the use of convective cooling of an intermediate wall that then diffuses the effect by thermal conduction. There is a need for higher resolution control of the glass ribbon during forming.

[0005] The present invention satisfied this and other needs.

SUMMARY

[0006] Several aspects of the present invention are disclosed herein. It is to be understood that these aspects may or may not overlap with one another. Thus, part of one aspect may fall within the scope of another aspect and vice versa. Unless indicated to the contrary in the context, the differing aspects shall be considered as overlapping with each other in scope.

[0007] Each aspect is illustrated by a number of embodiments, which, in turn, can include one or more specific embodiments. It is to be understood that the embodiments may or may not overlap with each other. Thus, part of one embodiment, or specific embodiments thereof, may or may not fall within the ambit of another embodiment, or specific embodiments thereof and vice versa. Unless indicated to the contrary in the context, the differing embodiments shall be considered as overlapping with each other in scope.

[0008] Thus, according to a first aspect of the present invention, a method of making a glass sheet comprises: (A) providing a glass ribbon at a first temperature where at least a portion of the glass ribbon exhibits viscous behavior; (B) providing a heat sink adjacent to the at least a portion of the glass ribbon at a second temperature; (C) providing a plurality of heating elements at a position where the heating elements are operable to shape a thermal profile of the heat sink; and (D) transferring heat from the at least a portion of the glass ribbon to the heat sink and absorbing at least a portion of the heat into the heat sink.

[0009] In certain embodiments of the first aspect of the present invention, in step (B), the second temperature is lower than the first temperature, whereby at least part of the glass ribbon is cooled down by the heat sink.

[0010] In certain embodiments of the first aspect of the present invention, in step (B), the second temperature is higher than the first temperature, whereby at least part of the glass ribbon is preferentially heated by the heat sink. [0011] In certain embodiments of the first aspect of the present invention, in step (C), the heating elements are embedded in the heat sink.

[0012] In certain embodiments of the first aspect of the present invention, the method further comprises: (E) selectively adjusting an output of each of the heating elements to shape the thermal profile of the heat sink such that in step (D), heat is differentially absorbed into the heat sink.

[0013] In certain embodiments of the first aspect of the present invention, in step (E), the output of each of the heating elements is selectively adjusted such that heat is transferred from each of a plurality of areas on the at least a portion of the glass ribbon by an amount inversely proportional to a thickness of each of the areas.

[0014] In certain embodiments of the first aspect of the present invention, in step (E), the output of each of the heating elements is selectively adjusted such that more heat is transferred from thinner areas of the at least a portion of the glass ribbon than from thicker areas of the at least a portion of the glass ribbon.

[0015] In certain embodiments of the first aspect of the present invention, in step (E), the output of each of the heating elements is selectively adjusted such that heat is transferred from each of a plurality of areas on the at least a portion of the glass ribbon by an amount proportional to a temperature in each of the areas.

[0016] In certain embodiments of the first aspect of the present invention, in step (E), the output of each of the heating elements is selectively adjusted such that more heat is transferred from hotter areas of the at least a portion of the glass ribbon than from colder areas of the at least a portion of the glass ribbon.

[0017] In certain embodiments of the first aspect of the present invention, the method further comprises: (F) monitoring the thermal profile of the heat sink and using the result of the monitoring to selectively adjust the output of each of the heating elements in step (E).

[0018] In certain embodiments of the first aspect of the present invention, the method further comprises: (G) delivering cooling fluid to selected points on the heat sink to modify the shape of the thermal profile of the heat sink.

[0019] In certain embodiments of the first aspect of the present invention, the method further comprises: (H) moving the glass ribbon relative to the heat sink.

[0020] In certain embodiments of the first aspect of the present invention, step (H) is simultaneous with step (D). [0021] In certain embodiments of the first aspect of the present invention, step (A) comprises: (Al) providing separate streams of molten glass and forming the glass ribbon by merging the separate streams of molten glass at a wedge root of a forming member.

[0022] In certain embodiments of the first aspect of the present invention, in step (D), the at least a portion of the glass ribbon is in the vicinity of the wedge root.

[0023] In certain embodiments of the first aspect of the present invention, in step (D), the at least a portion of the glass ribbon is below the wedge root.

[0024] According to a second aspect to the present invention, provided is an apparatus for making a glass sheet, comprising: (i) a forming member for forming a glass ribbon, the forming member comprising a wedge-shaped part having a wedge root at which separate streams of molten glass merge to form the glass ribbon; (ii) a heat sink positioned in the vicinity of the wedge root such that the heat sink can absorb heat from the at least a portion of the glass ribbon; and (iii) a plurality of heating elements in contact with or adjacent to the heat sink and operable to shape a thermal profile of the heat sink. The heat sink has a surface having a thermal field of view covering at least part of a surface of the glass ribbon.

[0025] In certain embodiments of the second aspect of the present invention, the apparatus further comprises a plurality of tubes for delivering cooling fluid to selected points on the heat sink. Advantageously, the tubes are behind the heat sink and are not in the thermal field of view of the glass ribbon.

[0026] In certain embodiments of the second aspect of the present invention, the heat sink is placed at a location below the wedge root. In certain embodiments the plate has a flat surface facing the glass ribbon.

[0027] In certain embodiments of the second aspect of the present invention, the heat sink comprises a plate comprising a ceramic material having a thermal conductivity of at least one third of silicon carbide at the operating temperature of the heat sink.

[0028] In certain embodiments of the second aspect of the present invention, the apparatus comprises a plate comprising silicon carbide and/or silicon nitride.

[0029] In certain embodiments of the second aspect of the present invention, the heating elements are embedded in the heat sink.

[0030] In certain embodiments of the second aspect of the present invention, the heating elements are behind the heat sink and are not in the thermal field of view of the glass ribbon. [0031] In certain embodiments of the second aspect of the present invention, the heating elements are resistive heating elements.

[0032] In certain embodiments of the second aspect of the present invention, the apparatus further comprises a plurality of temperature sensors coupled to the heat sink to monitor the thermal profile of the heat sink.

[0033] In certain embodiments of the second aspect of the present invention, the temperature sensors are thermocouples.

[0034] In certain embodiments of the second aspect of the present invention, the apparatus further comprise a controller for selectively adjusting an output of each of the heating elements based on an output of each of the temperature sensors.

[0035] In certain embodiments of the second aspect of the present invention, the apparatus further comprises a sensor for collecting a thickness distribution information of the ribbon before the ribbon enters the thermal field of view of the heat sink, and the thickness distribution information is fed to the controller for selectively adjusting output of each of the heating elements and/or the cooling tubes.

[0036] These and other aspects of the present invention will be described in more detail below.

BRIEF DESCRIPTION OF DRAWINGS

[0037] The following is a description of the figures in the accompanying drawings. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

[0038] FIG. 1 is a schematic illustration of an apparatus for making a glass sheet with controlled thickness in one embodiment of the present invention.

[0039] FIG. 2 is a schematic illustration of a cross-section of a heat sink for differentially absorbing heat from a portion of a glass ribbon in one embodiment of the present invention.

[0040] FIG. 3 is a schematic illustration of a cross-section of a sheathed heating element for use with the heat sink of FIG. 2 in one embodiment of the present invention.

[0041] FIG. 4 is a block diagram of an apparatus for controlling the thermal profile of the heat sink of FIG. 2 in one embodiment of the present invention.

[0042] FIG. 5 schematically illustrates how the heat sink of FIG. 2 can be used to differentially absorb heat from a portion of a glass ribbon. DETAILED DESCRIPTION

[0043] The present invention will now be described in detail, with reference to the accompanying drawings. In this detailed description, numerous specific details may be set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art when the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps may not be described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals may be used to identify common or similar elements.

[0044] "Heat sink" as used herein means a device for regulating the temperature of an apparatus or a system, by absorbing and/or irradiating heat from and to the

surrounding.

[0045] FIG. 1 illustrates an apparatus 100 for forming a glass ribbon 113 having a width W and thickness T. The apparatus 100 includes a downdraw forming member 101 comprising a wedge-shaped part having converging sides 103, 105 terminating in a wedge root 107. The glass ribbon 113 starts as two streams 109, 111 of molten glass flowing down the converging sides 103, 105 of the forming member 101 and merging at the wedge root 107 to form a glass sheet. The molten glass streams 109, 111 are formed by delivering molten glass into a channel within the forming member 101 and allowing the molten glass to overflow the channel in a known manner, such as described in U.S. Patent Nos. 1,829,641 and 3,338,696. The glass ribbon 113 is drawn away in sheet-form from the wedge root 107, as indicated by arrow 108. As the glass ribbon 113 is drawn away from the wedge root 107, the glass ribbon 113 cools down and the glass transitions from the viscous regime to the elastic regime. The cooling pattern of the glass ribbon 113 in the viscous regime affects the thickness profile of the glass ribbon 113 in the elastic regime. Therefore, it is important to control cooling of the glass in the viscous regime in order to achieve a desired thickness profile in the elastic regime.

[0046] The apparatus 100 includes a cooling device 115 made of a heat sink 201, a plurality of heating elements 207 for heating the heat sink 201, a plurality of temperature sensors 209 for monitoring the temperature distribution within the heat sink 20 land a plurality of tubes 120 for delivering cooling fluid jets to the heat sink 201. In operation, the heat sink 201 is positioned adjacent to a portion 121 of the glass ribbon 1 13. The heat sink 201 is maintained at a lower temperature than the glass ribbon portion 121 so that heat is transferred from the glass ribbon portion 121 to the heat sink 201 and absorbed into the heat sink 201. The heating elements 207 are used to shape the thermal profile of the heat sink 201. How to shape the thermal profile of the heat sink 201 would depend on the temperature profile (or thickness profile) of the glass ribbon portion 121.

[0047] Consider the hypothetical example shown in FIG. 5, where the glass ribbon portion 121 has areas 501, 503, 505 and 506 with temperatures T501, T503, T505, T506, respectively. T501, T503, T505 and T506 may vary along three dimensions, but for simplicity, T501, T503, T505 and T506 will be considered to be single-valued. Now, suppose that T501 > T503 > T505 > T506, i.e., the temperature distribution within the glass ribbon portion 121 is not uniform and that the problem is to make the temperature distribution within the glass ribbon portion 121 uniform. In this case, the thermal profile of the heat sink 201 can be shaped such that the heat sink 201 differentially absorbs heat from the glass ribbon portion 121 until T501 ~ T503 ~ T505 ~ T506. Suppose then that the heat sink 201 has areas 507, 509, 511 and 513 with temperatures T507, T509, T511 and T513, respectively. Each of the heat sink areas 507, 509, 511, 513 has one or more associated heating elements 207 and one or more associated temperature sensors 209. Further, suppose that heat sink is sufficiently close to the glass ribbon 121 and therefore area 507 absorbs heat from glass ribbon area 501, heat sink area 509 absorbs heat from glass ribbon area 503, heat sink area 511 absorbs heat from glass ribbon area 505 and heat sink area 507 absorbs heat from glass ribbon area 506, as indicated by arrows 515, 517, 519 and 520, respectively. To achieve T501 ~ T503 ~ T505 ~ T506 in the glass ribbon portion 121, T501, T503, T505 and T506 would have to be reduced by some amount a, b, c and d, respectively, where a > b > c > d. The output of the heating elements 207 in the heat sink areas 507, 509, 511, 513 can be adjusted such that T507, T509, T511, T513 are at the appropriate settings to absorb the desired amount of heat from the glass ribbon areas 501, 503, 505 and 506, respectively. Typically, the following relationship will be true: T507 < T509 < T511 < T513. Adjusting the temperature distribution across the heat sink 201 so that the heat sink 201 can differentially absorb heat from the glass ribbon portion 121 will be referred to as shaping the thermal profile of the heat sink 201.

[0048] Another way of thinking of the glass ribbon portion 121 is that the glass ribbon portion 121 has hot areas and cold areas. To even out the temperature profile of the glass ribbon portion 121, more heat would have to be transferred out of the hot areas than from the cold areas. The heat sink 201 can be used to control this transfer of heat. By providing relatively cold and hot areas on the heat sink 201, the heat sink 201 can differentially absorb heat from the glass ribbon portion 121 such that the temperature distribution within the glass ribbon portion 121 becomes more even. Alternatively, the glass ribbon portion 121 may be thought of as having thick and thin areas. To even out the thickness profile of the glass ribbon portion 121 , more heat would be transferred from the thin areas and less heat from the thick areas. The heat sink 201 can again be designed to differentially absorb heat from the glass ribbon portion 121 such that the thickness across the glass ribbon portion 121 becomes more uniform.

[0049] The heat sink 201 is able to differentially absorb heat from the glass ribbon portion 121 because it has a shaped thermal profile. The thermal profile of the glass ribbon can be actively shaped through control of the heating elements 207. For example, certain heating elements 207 can be controlled to make certain areas of the heat sink 201 relatively hot, while certain heating elements 207 can be controlled to make certain areas of the heat sink 201 relatively cold. The tubes (120 in FIG. 1) can also be used to deliver cooling fluid jets to points on the heat sink 201 in order to influence the shape of the thermal profile of the heat sink 201. However, the resolution of the shaping possible by the cooling fluid jets alone would not be as fine as what would be possible with the aid of heating elements 207.

[0050] The heat sink 201 is a mass of material having a high heat capacity and a low thermal expansion. The surface of the heat sink 201 in opposing relation to the glass ribbon portion 121 is continuous. This allows the heat sink 201 to create an unobstructed heat dump for the glass ribbon portion 121. In certain embodiments, the heat sink 201 is in the form of a plate, which may be flat as shown, or may have other shapes, as will be further described below. In certain embodiments, the material of the heat sink 201 is a ceramic material, examples of which include, but are not limited to, silicon nitride and silicon carbide. Silicon carbide is a good thermal spreader. Silicon nitride has good high temperature strength, creep resistance and oxidation resistance. Silicon nitride also has good thermal shock resistance compared to most ceramic materials including silicon carbide. The thermal conductivity of silicon nitride is less than half of that of silicon carbide. Therefore, silicon nitride can potentially provide a finer temperature profile than silicon carbide. Other types of heat sink materials not based on ceramics, e.g., those based on alloys or nano materials, may be used for the heat sink 201. [0051] FIG. 2 shows a cross-section of the heat sink 201. The heating elements 207 are shown embedded in the heat sink 201. The heating elements 207 may be embedded in the heat sink 201, for example, by forming holes in the heat sink 201 and inserting the heating elements 207 in the holes. In an alternate embodiment, the heating elements 207 may be adjacent and very close to a surface of the heat sink 201, rather than being embedded in the heat sink 201. This alternate embodiment permits easy replacement of a faulty heating element. In certain embodiments, the heating elements 207 are resistive heating elements made of high-temperature material. The high-temperature material may be an inert material, i.e., one that is resistant to oxidation. Examples of suitable high- temperature materials include platinum, platinum alloys and precious metal alloys. In certain embodiments, each heating element 207 is a conductive wire made of a high- temperature material. The heating elements 207 can be linear heating elements or nonlinear heating elements. If the heating elements 207 are linear heating elements, fine control of the temperature profile across the heat sink 201 can be achieved through fine spacing between adjacent heating elements 207.

[0052] The heating elements 207 may be embedded in the heat sink 201 with or without sheathing depending on the material of the heat sink 201. For example, if the material of the heat sink 201 is an electrical insulator such as silicon nitride, sheathing will not be needed for the heating element. On the other hand, if the material of the heat sink 201 is an electrical conductor such as silicon carbide, sheathing will be needed for the heating element. FIG. 3 shows an example of a sheathed heating element 207 including a high-temperature conductor (or wire) 300, surrounded by a high-temperature insulator 302, surrounded by a high-temperature sheath 304. For example, the high- temperature conductor 300 may be made of platinum, platinum alloy, precious metal alloy and the like. The high-temperature insulator 302 may be made of magnesium oxide, aluminum oxide, hafnium oxide, beryllium oxide and the like. The high temperature sheath 304 may be made of platinum alloy or other high-temperature metal or alloy.

Other materials suitable for use as a high-temperature conductor, high-temperature insulator and high-temperature sheath may be used.

[0053] The temperature sensors 209 are embedded at least partially within the heat sink 201 in FIG. 2. The temperature sensors 209 may be embedded in the heat sink 201, for example, by forming holes in the heat sink plate 201 and inserting the temperature sensors 209 at least partially in the holes. In an alternate embodiment, the temperature sensors 209 may be mounted on a surface of the heat sink 201. The temperature sensors 209 may be, for example, thermocouples or thermistors. Typically, it is desirable that the temperature sensors 209 are made of a material that is inert in an oxidizing atmosphere and that can withstand high temperature. Where the temperature sensors 209 are high- temperature thermocouples, the thermocouples may be made of platinum, platinum alloy, or precious metal alloy. As in the case of the heating elements 207, electrical isolation may or may not be needed between the temperature sensors 209, e.g., thermocouples and the heat sink 201 depending on the material of the heat sink 201. Where electrical isolation is needed, such as if the material of the heat sink 201 is silicon carbide, a similar approach to sheathing the heating elements 207 may be used for the temperature sensors 209.

[0054] The heating elements 207 are designed to generate heat. For example, if the heating elements 207 are resistive heating elements, electrical power can be delivered to the heating elements 207 to cause the heating elements 207 to generate heat. The heat generated by the heating elements 207 is dissipated to the heat sink 201. FIG. 4 shows that the temperature sensors 209 and heating elements 207 are coupled to a controller 400. The controller 400 has three functions: temperature reading, power instruction and power output. The controller 400 receives output signals from the temperature sensors 209. The output signals are used to create a current thermal profile for the heat sink 201. The current thermal profile for the heat sink 201 is compared to the desired thermal profile for the heat sink 201. Then, the controller 400 regulates the output power to the heating elements 207 accordingly. Through signal-output control feedback loop, the controller 400 adjusts the current thermal profile to match the desired thermal profile. The desired thermal profile of the heat sink 201 will be dictated by the temperature or thickness profile of the glass ribbon portion (121 in FIG. 1), as explained above.

[0055] The heat sink 201 is shown as a flat rectangular plate in FIG. 2. In alternate embodiments, the heat sink 201 or a surface of the heat sink 201 that will be in opposing relation to the glass ribbon portion (121 in FIG. 1) may have a non-flat shape, e.g., curved shape, in order to maximize the radiation view factor between the heat sink 201 and the glass ribbon portion (121 in FIG. 1). Radiation view factor is the fraction of thermal energy leaving the surface of the glass ribbon portion (121 in FIG. 1) and reaching the surface of the heat sink 201 determined entirely from geometric considerations of the heat sink 201 and the glass ribbon portion (121 in FIG. 1). The thickness of the heat sink 201 will depend on the conductivity of the material of the heat sink. In general, the lower the thermal conductivity of the material of the heat sink, the thinner will thickness of the heat sink be required. For example, a heat sink made of silicon nitride can be half as thick as a heat sink made of silicon carbide and deliver equivalent heat flux (q) as the silicon carbide, where q = K ΔΤ/ X, K is thermal conductivity, ΔΤ is temperature difference and X is the thickness of the substrate. For example, if 1 inch (2.54 cm) thickness of silicon carbide is needed to deliver a particular q, then 0.5 inch (1.27 cm) of silicon nitride would be needed to deliver the same q.

[0056] Returning to FIG. 1, a method of making a glass sheet involves forming the glass ribbon 113, as described above. While forming the glass ribbon 113, the heat sink 201 is positioned adjacent to a portion 121 of the glass ribbon 113 such that heat is transferred from the glass ribbon portion 121 to the heat sink 201 by radiation. The heat sink 201 essentially acts as a heat dump for the glass ribbon portion 121. The glass ribbon portion 121 typically is at a temperature where the glass exhibits viscous behavior, while the heat sink 201 is at a temperature lower than the temperature of the glass ribbon portion 121. The location of the glass ribbon portion 121 will typically be in the vicinity of the wedge root 107 (above or below the wedge root 107), where the glass is still likely to be in the viscous regime. The width of the heat sink 201 determines the width of the glass ribbon portion 121 for which the heat sink 201 will act as a heat dump for the glass ribbon portion 121. Typically, the width of the heat sink 201 is similar to the width of the glass ribbon 113, but may in other examples be shorter or longer than the width of the glass ribbon 113.

[0057] The heat sink 201 differentially absorbs heat from the glass ribbon portion 121. The differential absorption is determined by the thermal profile of the heat sink 201, which can be controlled by the heating elements 207 and optionally by cooling fluid jets from the tubes 120, as explained above. In certain embodiments, the thermal profile of the heat sink 201 is such that heat is transferred from different areas of the glass ribbon 121 to the heat sink 201 in an amount inversely proportional to the thickness of the glass in those areas. In certain embodiments, the thermal profile of the heat sink 201 is such that more heat is transferred to the heat sink from thinner areas of the glass ribbon portion 121 than is transferred from thicker areas of the glass ribbon portion 121 to the heat sink 201. The end result may be that the heat sink 201 differentially absorbs heat from the glass ribbon portion 121 so that the temperature profile or thickness profile of the glass ribbon portion 121 is more uniform.

[0058] As the glass ribbon 113 moves away from the wedge root 107 of the forming member 101, the glass ribbon portion 121 with modified temperature or thickness profile will move with the glass ribbon 113. A new glass ribbon portion will replace the old glass ribbon portion 121. Heat can be differentially absorbed from the new glass ribbon portion by the heat sink 201 as explained above for the old glass ribbon portion 121. This process can be repeated for every new glass ribbon portion positioned adjacent to the heat sink 201 due to the glass ribbon 113 continuously moving away from the wedge root 107. In certain embodiments, a sensor or a plurality of sensors are installed to monitor the thickness of the glass ribbon above the heat sink before the ribbon enters in the thermal field of view of the heat sink, and the thickness distribution information across the width of the glass ribbon is fed to the control system of the heating elements and/or the cooling fluid tube, to preferentially adjust the temperature distribution of the heat sink, thereby effectively adjusting the temperature and/or thickness of the glass ribbon when it passes through the thermal field of view of the heat sink.

[0059] The heat sink 201 with the shaped thermal profile can be used alone to control the thickness of the glass ribbon portion 121. Alternatively, the heat sink 201 with the shaped thermal profile can be used together with cooling fluid jets from the tubes 120 to control the thickness of the glass ribbon portion 121. As previously explained, the cooling fluid jets would have an effect on the shape of the thermal profile of the heat sink 201, although such effect may be of a global nature, while the heating elements 207 would be relied upon for fine control of the shape of the thermal profile. The tubes 120 may be similar to the fluid conduit tubes described in U.S. Patent No. 3,682,609 and may be connected to a manifold (not shown) via a flow meter (not shown) and control valve (not shown). The fluid delivered by the tubes 120 may be air. The heat sink 201 will be used in place of the intermediate wall in U.S. Patent No. 3,682,609. It should be noted that shaping of the thermal profile of the heat sink 201 to achieve thickness control of the glass ribbon portion 121 would require some knowledge of the temperature distribution or thickness profile of the glass ribbon portion 121. This may involve active measurement on the glass ribbon portion 121 or may be based on historical data obtained using a particular set of process setup and parameters. [0060] The heat sink assembly 201 may be a single unit having a width sufficient to cover the width of the glass ribbon portion 121, where the width of the glass ribbon portion 121 may be the same as or differ from the width W of the glass ribbon 113.

Alternately, the heat sink 201 may have a modular construction, where a plurality of modules can be arranged next to each other to form a heat sink 201 of desired width. Alternatively, a plurality of modules can be arranged separately in only portions of the glass ribbon 113 requiring thickness control.

[0061] In certain embodiments, it is also possible that the temperature of the heat sink is controlled in such a way that at least part of the heat sink surface facing the glass ribbon has a higher temperature than the corresponding area of the glass ribbon within the thermal field of view of the heat sink. In these embodiments, heat is transferred from the heat sink to the glass ribbon, effectively raising the temperature and glass viscosity of the exposed area, thereby reducing the thickness thereof while the glass ribbon is being drawn.

[0062] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

What is claimed is:

1. A method of making a glass sheet, comprising:

(A) providing a glass ribbon at a first temperature where at least a portion of the glass ribbon exhibits viscous behavior;

(B) providing a heat sink adjacent to the at least a portion of the glass ribbon at a second temperature;

(C) providing a plurality of heating elements at a position where the heating

elements are operable to shape a thermal profile of the heat sink; and

(D) transferring heat from the at least a portion of the glass ribbon to the heat sink and absorbing at least a portion of the heat into the heat sink.

2. A method of claim 1 , wherein in step (B), the second temperature is lower than the first temperature.

3. A method of claim 1 , wherein in step (B), the second temperature is higher than the first temperature.

4. The method of any of the preceding claims, wherein in step (C), the heating

elements are embedded in the heat sink.

5. The method of any of the preceding claims, further comprising:

(E) selectively adjusting an output of each of the heating elements to shape the thermal profile of the heat sink such that in step (D), heat is differentially absorbed into the heat sink.

6. The method of claim 5, wherein in step (E), the output of each of the heating

elements is selectively adjusted such that heat is transferred from each of a plurality of areas on the at least a portion of the glass ribbon by an amount inversely proportional to a thickness of each of the areas.

7. The method of claim 5 or claim 6, wherein in step (E), the output of each of the heating elements is selectively adjusted such that more heat is transferred from thinner areas of the at least a portion of the glass ribbon than from thicker areas of the at least a portion of the glass ribbon.

8. The method of any of claims 5 to 7, wherein in step (E), the output of each of the heating elements is selectively adjusted such that heat is transferred from each of a plurality of areas on the at least a portion of the glass ribbon by an amount proportional to a temperature in each of the areas. The method of any of claims 5 to 8, wherein step (E), the output of each of the heating elements is selectively adjusted such that more heat is transferred from hotter areas of the at least a portion of the glass ribbon than from colder areas of the at least a portion of the glass ribbon.

The method of any of claims 5 to 9, further comprising:

(F) monitoring the thermal profile of the heat sink and using the result of the monitoring to selectively adjust the output of each of the heating elements in step (E).

The method of any of the preceding claims, further comprising:

(G) delivering cooling fluid to selected points on the heat sink to modify the shape of the thermal profile of the heat sink.

The method of any of the preceding claims, further comprising:

(H) moving the glass ribbon relative to the heat sink.

The method of claim 12, wherein step (H) is simultaneous with step (D).

The method of any of the preceding claims, wherein step (A) comprises:

(Al) providing separate streams of molten glass and forming the glass ribbon by merging the separate streams of molten glass at a wedge root of a forming member.

The method of claim 14, wherein in step (D), the at least a portion of the glass ribbon is in the vicinity of the wedge root.

The method of claim 14 or claim 15, wherein in step (D), the at least a portion of the glass ribbon is below the wedge root.

An apparatus for making a glass sheet, comprising:

a forming member for forming a glass ribbon, the forming member comprising a wedge-shaped part having a wedge root at which separate streams of molten glass merge to form the glass ribbon;

a heat sink positioned in the vicinity of the wedge root such that the heat sink can absorb heat from the at least a portion of the glass ribbon; and

a plurality of heating elements in contact with or adjacent to the heat sink and operable to shape a thermal profile of the heat sink.

The apparatus of claim 14, further comprising a plurality of tubes for delivering cooling fluid to selected points on the heat sink.

19. The apparatus of claim 17 or claim 18, wherein the heat sink is placed at a location below the wedge root.

20. The apparatus of any of claims 17 to 19, wherein the heat sink is a plate

comprising a ceramic material having a thermal conductivity of at least one third of silicon carbide at the operating temperature of the heat sink.

21. The apparatus of any claims 17 to 20, wherein the heat sink is a plate comprising silicon carbide and/or silicon nitride.

22. The apparatus of any of claims 17 to 21, wherein the heating elements are

embedded in the heat sink.

23. The apparatus of any of claims 17 to 22, wherein the heating elements are not in the thermal field of view of the glass ribbon.

24. The apparatus of any of claims 17 to 23, wherein the heating elements are

resistive heating elements.

25. The apparatus of any of claims 17 to 24, further comprising a plurality of

temperature sensors coupled to the heat sink to monitor the thermal profile of the heat sink.

26. The apparatus of claim 25, wherein the temperature sensors are thermocouples.

27. The apparatus of claim 25 or claim 26, further comprising a controller for

selectively adjusting an output of each of the heating elements based on an output of each of the temperature sensors.

28. The apparatus of any of claims 17 to 27, further comprising a sensor for collecting a thickness distribution information of the ribbon before the ribbon enters the thermal field of view of the heat sink, and the thickness distribution information is fed to the controller for selectively adjusting output of each of the heating elements and/or the cooling tubes.