[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S.

Provisional Application Serial No. 62/259, 189, filed on November 24, 2015, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

Field

[0002] The present disclosure relates generally to apparatus for forming a glass article, and in particular an asymmetrically configured forming body. Methods of forming the glass article with the asymmetric forming body are also disclosed.

Technical Background

[0003] Cord, a glass attribute, is an attenuated or thread-like glassy surface defect possessing optical and other properties different from those of the surrounding glass. This defect can manifest as a surface corrugation with a variable spatial period in a range from about 1 millimeter to about 10 millimeters and a peak-to-valley depth typically on the order of nanometers, for example about 10 nanometers. A large amount of cord is undesirable in glass sheets used in the manufacture of display devices, as cord can affect the quality of liquid crystal display panels produced therefrom, both visually and functionally. Visually, cord can appear as multiple dark lines along a draw direction of a glass ribbon (or glass sheet cut therefrom) as a result of the lensing effect produced by the curvature of the corrugated surface. Functionally, cord can produce a variation in the cell gap of a liquid crystal display (LCD) device that may affect operation of the device.

[0004] A fusion-formed glass sheet has two sides, typically referred to as the "A" side and the "B" side. In a conventional fusion down draw process the mass flow of glass over both sides of the forming body is maintained equal, and therefore cord distribution on both sides of the sheet is also equal. During the manufacture of display panels, the "A" side of the glass sheet is positioned to face the cell gap, while the "B" side of the glass sheet faces the backlight. Thus, only surface corrugation on the "A" side plays a role in finished LCD device performance. In the absence of a complete eradication of cord in the process of making the glass sheets, it would be beneficial to be able to control the side of the glass ribbon (and sheet) at which the cord is manifest and therefore mitigate the effect of the cord on display device appearance and function.

SUMMARY

[0005] In one embodiment, an apparatus for making glass is disclosed comprising a forming body including a trough extending along a length direction of the forming body, the trough defined by a first side wall comprising a first height HI at a predtermined position along a length of the trough, wherein HI is defined as a vertical distance between a floor of the trough where the floor of the trough intersects with the first side wall, and a top surface of the first side wall. The apparatus further comprises a second side wall comprising a second height H2 different than HI at the predetermined position along the length of the trough, wherein H2 is defined as a vertical distance between the floor of the trough where the floor of the trough intersects with the second side wall, and a top surface of the second side wall. The forming body may include a pair of converging forming surfaces that converge at or below a bottom edge of the forming body.

[0006] In some embodiments, HI is greater than H2.

[0007] In some embodiments, an elevation El of the top surface of the first side wall relative to the bottom edge at the predetermined position is substantially equal to an elevation E2 of the top surface of the second side wall relative to the bottom edge at the predetermined position.

[0008] In certain examples the floor of the trough may be arranged at an angle a relative to a horizontal plane tangent to the bottom edge.

[0009] In certain examples, HI relative to H2 may be constant over a length of the trough.

[0010] In another embodiment, a method of forming a glass substrate is described comprising flowing molten glass into a forming body including a trough extending along a length direction of the forming body, the trough defined by a first side wall comprising a first height HI at a predetermined position along the length of the trough, where HI is defined as a vertical distance between a floor of the trough where the first side wall intersects the floor of the trough, and a top surface of the first wall. The trough is further defined by a second side wall comprising a second height H2 less than HI at the predetermined position, where H2 is defined as a vertical distance between the floor of the trough where the second side wall intersects the floor of the trough, and a top surface of the second side wall at the

predetermined position. The molten glass flowing over the first and second side walls and down converging forming surfaces of the forming body forms first and second flows of molten glass, respectively, wherein the second flow of molten glass is greater than the first flow of molten glass.

[0011] The method may further comprise drawing the molten glass from a bottom edge of the forming body to form a ribbon of glass, the ribbon of glass comprising a first layer of glass including a first thickness at a centerline of the ribbon of glass and a second layer of glass including a second thickness at the center line of the ribbon of glass different than the first thickness.

[0012] A chemical composition of the first layer of glass may be substantially the same as a chemical composition of the second layer of glass. That is, with the exception of localized inhomogeneities, the molten glass supplying both flows of molten glass flowing over the side walls originates from the same source and is therefore nominally the same glass.

[0013] The method may further comprise forming a viscosity difference between a viscosity of the molten glass flowing over the top of the second side wall and a viscosity of the molten glass flowing over the top of the first side wall. For example, the viscosity difference may be formed by heating the molten glass flowing over the second side wall. Alternatively, or additionally, the viscosity difference may be formed by cooling the molten glass flowing over the first side wall.

[0014] Additional features and advantages of the embodiments disclosed herein will be 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, including the detailed description which follows, the claims, as well as the appended drawings.

[0015] It is to be understood that both the foregoing general description and the following detailed description present embodiments intended to provide an overview or framework for understanding the nature and character of the claimed invention. The accompanying drawings are included to provide further understanding, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure, and together with the description serve to explain the principles and operations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a schematic view of an example;

[0017] FIG. 2A is a lateral cross sectional view of a forming body suitable for use in the fusion down draw glass making apparatus of FIG. 1 ; [0018] FIG. 2B is a cross sectional edge view of a glass sheet produced by the forming body of FIG. 2A;

[0019] FIG. 3 A is a lateral cross sectional view of another embodiment of a forming body suitable for use in the fusion down draw glass making apparatus of FIG. 1;

[0020] FIG. 3B is a cross sectional edge view of a glass sheet produced by the forming body of FIG. 3A; and

[0021] FIG. 4 is a lateral cross sectional view of still another embodiment of a forming body suitable for use in the fusion down draw glass making apparatus of FIG. 1.

DETAILED DESCRIPTION

[0022] Reference will now be made in detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. However, this disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0023] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, for example by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0024] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0025] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order, nor that with any apparatus specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification.

[0026] As used herein, the singular forms "a, " "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a" component includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0027] Shown in FIG. 1 is an exemplary glass manufacturing apparatus 10. In some embodiments, the glass manufacturing apparatus 10 can comprise a glass melting furnace 12 that can include a melting vessel 14. In addition to melting vessel 14, glass melting furnace 12 can optionally include one or more additional components such as heating elements (e.g., combustion burners or electrodes) that heat raw materials and convert the raw materials into molten glass. In further examples, glass melting furnace 12 may include thermal management devices (e.g., insulation components) that reduce heat lost from a vicinity of the melting vessel. In still further examples, glass melting furnace 12 may include electronic devices and/or electromechanical devices that facilitate melting of the raw materials into a glass melt. Still further, glass melting furnace 12 may include support structures (e.g., support chassis, support member, etc.) or other components.

[0028] Glass melting vessel 14 is typically comprised of refractory material, such as a refractory ceramic material, for example a refractory ceramic material comprising alumina or zirconia, although in further embodiments other refractory materials may be used. In some examples glass melting vessel 14 may be constructed from refractory ceramic bricks.

[0029] In some embodiments, the glass melting furnace may be incorporated as a component of a glass manufacturing apparatus to fabricate a glass substrate, for example a glass ribbon of a continuous length. In some examples, the glass melting furnace of the disclosure may be incorporated as a component of a glass manufacturing apparatus comprising a slot draw apparatus, a float bath apparatus, a down draw apparatus such as a fusion process, an up draw apparatus, a press rolling apparatus, a tube drawing apparatus or any other glass manufacturing apparatus that would benefit from aspects disclosed herein. By way of example, FIG. 1 schematically illustrates glass melting furnace 12 as a component of a fusion down draw glass manufacturing apparatus 10 for fusion drawing a glass ribbon for subsequent processing into individual glass sheets.

[0030] The glass manufacturing apparatus 10 (e.g., fusion down draw apparatus 10) can optionally include an upstream glass manufacturing apparatus 16 positioned upstream relative to glass melting vessel 14. In some examples, a portion of, or the entire upstream glass manufacturing apparatus 16, may be incorporated as part of the glass melting furnace 12.

[0031] As shown in the illustrated example, the upstream glass manufacturing apparatus 16 can include a storage bin 18, a raw material delivery device 20 and a motor 22 connected to the raw material delivery device. Storage bin 18 may be configured to store a quantity of raw materials 24 that can be fed into melting vessel 14 of glass melting furnace 12, as indicated by arrow 26. Raw materials 24 typically comprise one or more glass forming metal oxides and one or more modifying agents. In some examples, raw material delivery device 20 can be powered by motor 22 such that raw material delivery device 20 delivers a predetermined amount of raw materials 24 from the storage bin 18 to melting vessel 14. In further examples, motor 22 can power raw material delivery device 20 to introduce raw materials 24 at a controlled rate based on a level of molten glass sensed downstream from melting vessel 14. Raw materials 24 within melting vessel 14 can thereafter be heated to form molten glass 28.

[0032] Glass manufacturing apparatus 10 can also optionally include a downstream glass manufacturing apparatus 30 positioned downstream from glass melting furnace 12 relative to a flow direction of molten glass. In some examples, a portion of downstream glass manufacturing apparatus 30 may be incorporated as part of glass melting furnace 12. In some instances, first connecting conduit 32 discussed below, or other portions of the downstream glass manufacturing apparatus 30, may be incorporated as part of glass melting furnace 12. Elements of the downstream glass manufacturing apparatus, including first connecting conduit 32, may be formed from a precious metal. Suitable precious metals include platinum group metals selected from the group of metals consisting of platinum, iridium, rhodium, osmium, ruthenium and palladium, or alloys thereof. For example, downstream components of the glass manufacturing apparatus may be formed from a platinum-rhodium alloy including from about 70 to about 90% by weight platinum and about 10% to about 30% by weight rhodium. However, other suitable metals can include molybdenum, palladium, rhenium, tantalum, titanium, tungsten and alloys thereof.

[0033] Downstream glass manufacturing apparatus 30 can include a first conditioning (i.e., processing) vessel, such as fining vessel 34, located downstream from melting vessel 14 and coupled to melting vessel 14 by way of the above-referenced first connecting conduit 32. In some examples, molten glass 28 may be gravity fed from melting vessel 14 to fining vessel 34 by way of first connecting conduit 32. For instance, gravity may drive molten glass 28 through an interior pathway of first connecting conduit 32 from melting vessel 14 to fining vessel 34. It should be understood, however, that other conditioning vessels may be positioned downstream of melting vessel 14, for example between melting vessel 14 and fining vessel 34. In some embodiments, a conditioning vessel may be employed between the melting vessel and the fining vessel wherein molten glass from a primary melting vessel is further heated to continue the melting process, or cooled to a temperature lower than the temperature of the molten glass in the melting vessel before entering the fining vessel.

[0034] Bubbles may be removed from molten glass 28 within fining vessel 34 by various techniques. For example, raw materials 24 may include multivalent compounds (i.e. fining agents) such as tin oxide that, when heated, undergo a chemical reduction reaction and release oxygen. Other suitable fining agents include without limitation arsenic, antimony, iron and cerium. Fining vessel 34 is heated to a temperature greater than the melting vessel temperature, thereby heating the molten glass and the fining agent. Oxygen bubbles produced by the temperature-induced chemical reduction of the fining agent(s) rise through the molten glass within the fining vessel, wherein gases in the molten glass produced in the melting furnace can diffuse or coalesce into the oxygen bubbles produced by the fining agent. The enlarged gas bubbles can then rise to a free surface of the molten glass in the fining vessel and thereafter be vented out of the fining vessel. The oxygen bubbles can further induce mechanical mixing of the molten glass in the fining vessel.

[0035] Downstream glass manufacturing apparatus 30 can further include another conditioning vessel such as a mixing vessel 36 for mixing the molten glass. Mixing vessel 36 may be located downstream from the fining vessel 34. Mixing vessel 36 can be used to provide a homogenous glass melt composition, thereby reducing chemical or thermal inhomogeneity that may otherwise exist within the fined molten glass exiting the fining vessel. As shown, fining vessel 34 may be coupled to mixing vessel 36 by way of a second connecting conduit 38. In some examples, molten glass 28 may be gravity fed from fining vessel 34 to mixing vessel 36 by way of second connecting conduit 38. For instance, gravity may drive molten glass 28 through an interior pathway of second connecting conduit 38 from fining vessel 34 to mixing vessel 36. It should be noted that while mixing vessel 36 is shown downstream of fining vessel 34, mixing vessel 36 may be positioned upstream from fining vessel 34. In some embodiments, downstream glass manufacturing apparatus 30 may include multiple mixing vessels, for example a mixing vessel upstream from fining vessel 34 and a mixing vessel downstream from fining vessel 34. These multiple mixing vessels may be of the same design, or they may be of different designs. [0036] Downstream glass manufacturing apparatus 30 can further include another conditioning vessel, such as delivery vessel 40 that may be located downstream from mixing vessel 36. Delivery vessel 40 may condition molten glass 28 to be fed into a downstream forming device. For instance, delivery vessel 40 can act as an accumulator and/or flow controller to adjust and/or provide a consistent flow of molten glass 28 to forming body 42 by way of exit conduit 44. As shown, mixing vessel 36 may be coupled to delivery vessel 40 by way of third connecting conduit 46. In some examples, molten glass 28 may be gravity fed from mixing vessel 36 to delivery vessel 40 by way of third connecting conduit 46. For instance, gravity may drive molten glass 28 through an interior pathway of third connecting conduit 46 from mixing vessel 36 to delivery vessel 40.

[0037] Downstream glass manufacturing apparatus 30 can further include forming apparatus 48 comprising the above-referenced forming body 42 and inlet conduit 50. Exit conduit 44 can be positioned to deliver molten glass 28 from delivery vessel 40 to inlet conduit 50 of forming apparatus 48. For example, exit conduit 44 may be open ended and nested within and spaced apart from an inner surface of inlet conduit 50, thereby providing a free surface of molten glass positioned between the outer surface of exit conduit 44 and the inner surface of inlet conduit 50. As best seen with the aid of FIGS. 1 and 2A, forming body 42 in a fusion down draw glass making apparatus can comprise a trough 52 positioned in an upper surface of the forming body and converging forming surfaces 54a and 54b that converge in a draw direction along a bottom edge 56 of the forming body. Molten glass delivered to the forming body trough via delivery vessel 40, exit conduit 44 and inlet conduit 50 overflows side walls of the trough and descends along the converging forming surfaces 54 as separate flows of molten glass. The separate flows of molten glass join below and along bottom edge 56 to produce a single ribbon of glass 58 that is drawn in a draw direction 60 from bottom edge 56 by applying tension to the glass ribbon, such as, for example, by gravity, edge rolls and pulling rolls (not shown), to control the dimensions of the glass ribbon as the glass cools and a viscosity of the glass increases. Accordingly, glass ribbon 58 goes through a visco-elastic transition and acquires mechanical properties that give the glass ribbon 58 stable dimensional characteristics. Glass ribbon 58 may, in some embodiments, be separated into individual glass sheets 62 by a glass separation apparatus (not shown) in an elastic region of the glass ribbon. A robot 64 may then transfer the individual glass sheets 62 to a conveyor system using gripping tool 65, whereupon the individual glass sheets may be further processed.

[0038] As described supra, cord can originate as regions of chemical or thermal inhomogeneity in a body of molten glass. If the molten glass is drawn (stretched), the regions of inhomogeneity are also stretched. Accordingly, the regions of inhomogeneity may manifest as strings or filaments (cord) of inhomogeneity extending in the draw direction of the drawn glass. When the glass cools, these filaments of chemical and/or thermal inhomogeneity can exhibit an index of refraction that may differ from the index of refraction of the surrounding glass. Additionally, cord may also manifest as filaments of thickness change in the glass sheet, which can both visually and functionally affect the performance and perceived quality of the display device. For example, the lensing effect produced by raised filaments on the glass sheet can be readily observable, particularly if the glass is formed into thin glass sheets that are used in the manufacture of visual display devices, such as, but not limited to, television and computer monitors. The more stretched the inhomogeneities, the more visible the cord. Moreover, even small thickness changes on the surface of a glass sheet may affect deposition processes used to deposit electronic components on the glass sheet.

[0039] Inhomogeneities are stretched not only by being drawn from the forming body, but simply by passage through the various connecting and conditioning conduits and vessels. Flowing the molten glass through a mixing process, such as through mixing vessel 36 as described supra, can significantly reduce the volume of inhomogeneity in the molten glass supplied to the forming body, but may not completely eliminate inhomogeneities. Thus, efforts to completely eliminate cord from the finished product have not been successful.

[0040] Glass sheet used in the manufacture of display panels includes two major surfaces, typically identified as the "A" side and the "B" side. If contact with the glass sheet (or ribbon) during manufacture of the glass sheet is necessary, the "B" side of the glass sheet is the contacted side. For example, during the process of separating glass sheet 62 from glass ribbon 58, the glass ribbon, and subsequently the glass sheet, may be contacted by robot 64. For example, robot 64 may contact the glass ribbon, such as with a gripping tool 65 (e.g., comprising suction cups) before, during or after scoring of the glass ribbon with a scoring tool. The robot may then apply, via the gripping tool, a bending moment to the glass ribbon across the score line. The side of the glass ribbon (and the subsequent glass sheet) contacted by the robot and gripping tool is designated as the "B" side. To wit, the "A" and "B" side designation is a function of the draw equipment arrangement itself, and so the draw equipment, for example forming body 42, may itself include "A" side and "B" side designations. The "A" side and the "B" side designations of the glass sheet may be carried through and communicated to the display panel manufacturer so that the display panel manufacturer can distinguish between the potentially contacted side and the potentially more pristine "A" side and therefore selectively utilize and orient the glass sheet in subsequent processes. Thus, for example, in a liquid crystal display panel, the electronics (e.g., thin film transistors) that control the orientation of the liquid crystal material within a display panel can be deposited on the pristine "A" side of the glass sheet (hereinafter the backplane glass sheet, or more simply, the backplane). If cord is present on the "A" side of the backplane, the threads of raised cord can result in a narrowing of the cell gap between the backplane and the opposing color filter glass sheet, and may further produce a lensing effect that results in visual distortion of the image as seen by an observer of the display panel. On the other hand, if the cord is present on the "B" side of the backplane, facing the backlight, the diffuse nature of the light coming from the backlight eliminates or substantially reduces the visual effect of the cord. Accordingly, cord on the "B" side of the glass sheet may in certain circumstances not be considered a rejectable defect.

[0041] The propensity for cord to appear on either the "A" side or the "B" side of a fusion- drawn glass substrate can be controlled by varying the separate flows of molten glass descending over converging forming surfaces 54a,b of forming body 42. That is, the side with the greater flow will experience more cord. The interface where the separate flows join is referred to as the "fusion line", which is readily observable if a fusion-drawn glass sheet is viewed from the edge thereof. Thus, while the interface is generally planar, the term "fusion line" arises from the appearance of the interface when viewed from an edge of the glass substrate. In a conventional, properly operating fusion down draw apparatus, the flow rate of molten glass over the forming body side walls is balanced and the thicknesses of the two flows, and hence the opposing layers of the drawn glass ribbon, are equal, and the fusion line extends through the center of the glass ribbon edge (i.e., is generally equidistant between the two major surfaces of the glass sheet cut from the ribbon). Consequently, the amount of cord on one side of the glass ribbon compared to the opposite side of the glass ribbon is substantially equal. However, in accordance with the present disclosure, the manifestation of cord in the glass ribbon, and thus the resultant individual glass sheets separated therefrom, can be controlled by varying the flow of molten glass on one side of the forming body relative to the flow of molten glass on the other side of the forming body. Thus, the flow of molten glass over the forming body can be shifted such that the greater flow exists on the side of the forming body producing the "B" side of the glass ribbon and subsequent glass sheets. As a result, a position of the fusion line may be asymmetric, wherein one layer of glass is thicker than the opposing layer of glass, and cord present in the glass sheet will manifest more predominately in the layer formed by the greater flow, i.e., the thicker layer. [0042] As shown in FIG. 2A and 2B, in an exemplary fusion down draw process, molten glass is delivered to forming body 42 via inlet 50 connected to trough 52 extending along an upper surface of the forming body. Trough 52 is defined, inter alia, by a bottom floor 68 and two side walls 70, 72. The molten glass entering trough 52 overflows the tops 74, 76 of side walls 70, 72 and flows down and over converging forming surfaces 54a,b that join along bottom edge 56 of forming body 42. In a typical forming body suitable for use in a fusion down draw process, the side walls 70, 72 extend a predetermined height above the floor of the trough. That is, for any particular cross sectional location along the length L of the forming body (i.e., in a plane perpendicular to a longitudinal axis of the forming body), the height of a side wall is determined as the vertical distance between the floor at that particular longitudinal location and the top surface of the side wall (i.e., the distance between the top surface of the wall and the point where the wall intersects the floor). The floor 68 may be flat, such that the intersection of the floor and a first plane perpendicular to a longitudinal axis of the forming body forms a straight horizontal line. However, floor 68 may have a curved profile in the lateral cross sectional direction. In still other examples, the floor may have a more complex shape comprised of planar surfaces and curved surfaces. However, typically the floor will be symmetric relative to a central vertical plane extending through the trough and in which bottom edge 56 lies. In the exemplary forming body 42 of FIG. 2A, the height HI of side wall 70 is equal to the height H2 of the opposing side wall 72 and the flows of molten glass over the side walls of the forming body are symmetric. Additionally, the elevation of the top surface 74 of first side wall 70 relative to bottom edge 56 is equal to the elevation of the top surface 76 of second side wall 72 at any position along a length of the trough, also relative to bottom edge 56. As a result, the flow of molten glass over the top surfaces of the side walls at a predetermined position along a length of the trough is equal. Accordingly, glass sheet 62 produced by the forming body, shown as viewed from an edge thereof in FIG. 2B, comprises a first glass layer 78 including a first thickness Tl and a second glass layer 80 with a thickness T2 separated from the first layer by fusion line (interface) 82, and T2 is equal to Tl . Moreover, the symmetric flow results in an equally symmetric disposition of cord in the first and second layers 78 and 80.

[0043] In accordance with the present disclosure, a forming body 42 is described comprising asymmetric side walls such that the flow of molten glass over the side walls is also asymmetric.

[0044] In one embodiment, shown in FIG. 3A, forming body 42 is illustrated comprising a symmetric floor 68, that is, symmetric relative to vertical plane 82 extending through the trough and containing bottom edge 56 along the length of the bottom edge, seen from an edge of the plane in the figure. For example, if floor 68 is a flat floor (without transverse curvature, i.e., curvature in a direction orthogonal to vertical plane 82), floor 68 is nominally a flat, horizontal surface, although in some embodiments the corners of trough 52 may be beveled or curved to reduce stress in the forming body. In other embodiments, floor 68 may comprise one or more surface features to modify the flow of molten glass in trough 52. For example, U. S. Patent Publication 2005/0268658, published on December 8, 2005, describes a variety of surface features that may be included on floor 68 for distributing the flow of molten glass.

[0045] As illustrated, in FIG. 3A, first side wall 70 comprises a height HI measured from floor 68 proximate the inside-the-trough base of the first side wall (at the intersection of the first side wall and the floor) to the top surface 74 of first side wall 70 greater than the height H2 of the top surface 76 of opposing second side wall 72 similarly measured (i.e., from the inside-the-trough base of the second side wall to the top surface 76 of second side wall 72). While the height of each side wall may vary along a length of the trough, a ratio of HI to H2 (HI :H2) is constant along the length of trough 52. The flow of molten glass over top surface 76 of second side wall 72 will be greater than the flow of molten glass over top surface 74 of first side wall 70, and therefore include more cord than the flow of molten glass over the first side wall. The asymmetric flow of molten glass is manifest in the glass ribbon (and subsequently in glass sheets cut from the glass ribbon) by an asymmetric fusion line 82. That is, referring to glass sheet 62, a thickness Tl of first glass layer 74 is less than a thickness T2 of second glass layer 76, as illustrated in FIG. 3B. Thus, cord is shifted to the second glass layer 76 and away from first glass layer 74.

[0046] Additionally, the flow asymmetry described supra can be augmented and enhanced by selectively adjusting the viscosity of the molten glass flowing over first and second side walls 70, 72, respectively. For example, a decrease in viscosity of the glass flowing over the second side wall 72 can function to further increase the flow of molten glass over the second side wall. Conversely, an increase in viscosity of the molten glass flowing over the second side wall 72 can decrease the flow of molten glass over the second side wall. The viscosity of the molten glass can be selectively varied, for example, by adjusting thermal elements 90 positioned above the forming body, and in particular, over one or both of tops 74, 76 of first and second side walls 70, 72, respectively. For example, thermal elements 90 can be electrical heating elements. In one example, heating elements over second side wall 72 can be supplied more electrical power than heating elements over first side wall 70, thereby heating the molten glass flowing over the second side wall and reducing the viscosity thereof. It should be apparent from the foregoing that similar effects can be achieved by adjusting the viscosity of the opposing (first) side wall 70, or by cooling one side wall relative to the opposing side wall. For example, thermal elements 90 can be cooling elements, such as cooling coils in fluid communication with a source of cooling fluid, or thermal elements 90 may include both heating and cooling elements. In some embodiments, the thermal elements may be distributed heating and/or cooling elements. That is, in some embodiments, a plurality of heating and/or cooling elements may be positioned above each side wall (e.g., above the tops of the side walls), and separately controllable, so that the viscosity of the molten glass overflowing at least one (for example either the first, second or both of the first and second side walls) can be varied along a length of the side wall.

[0047] In another embodiment, shown in FIG. 4A, the top surfaces 74, 76 of first and second side walls 70, 72 are at an equal elevation relative to bottom edge 56 (i.e., El is equal to E2), however floor 68 is depicted as being arranged at an angle a relative to horizontal plane 92. The effect is that even though the top surfaces 74, 76 of side walls 70, 72, respectively, are at an equal elevation relative to bottom edge 56, the heights HI , H2 of side walls 70, 72 relative to floor 68 are nonetheless asymmetric. That is, HI does not equal H2, and in accordance with the depiction of FIG. 4, H2 is greater than HI . Thus, the depth of molten glass within trough 52 is deeper and the flow thereof slower on the right side of the trough, adjacent second side wall 72, than the depth and flow of molten glass on the left side of the trough adjacent first side wall 70. As a result, the residence time of the molten glass on the right side of trough 52 is greater than the residence time on the left side of the trough. The longer residence time on the right side of the trough provides more time for the stretching of cord within the molten glass on the right side of the trough, and which molten glass predominately flows over the top surface of second side wall 68 compared to the molten glass adjacent first side wall 70. The relatively smaller amount of cord stretching for the molten glass overflowing the first side wall compared to molten glass overflowing the second side wall can reduce the effect of cord for the glass sheet originating from the left side of the forming body compared to the right side of the forming body, again with reference to the illustrated forming body, even if the flow of molten glass over the first and second side walls is substantially equal.

[0048] As previously described, the flow characteristics described in the foregoing embodiment can be augmented and enhanced by selectively changing the viscosity of the molten glass flowing through trough 52 and then flowing over the first and second side walls 70, 72. For example, a decrease in viscosity of the glass flowing along the right side of trough 52 can function to further decrease the residence time of molten glass through the right side of the trough and increase the flow rate of molten glass over the second side wall. Conversely, an increase in viscosity of the molten glass flowing through the right side of trough 52 can increase the residence time of molten glass flowing through the right side of the trough (an increase cord stretching), and decrease the flow of molten glass over the second side wall. The viscosity of the molten glass can be selectively varied, for example, by adjusting a temperature of thermal elements 90 positioned above the forming body side walls. For example, thermal elements 90 may be electrical heating elements, wherein one or more thermal elements 90 above one side wall, e.g., second side wall 72 can be supplied more electrical power than one or more thermal elements positioned over the opposing side wall, e.g., first side wall 70. As in the previous embodiment, it should be apparent from the foregoing description that similar effects can be obtained if thermal elements 90 are cooling elements rather than heating elements, for example cooling coils through which a cooling fluid supplied from a cooling fluid source is passed. Accordingly, the temperature of one or more cooling elements above one side wall can be lessened or increased relative to the amount of cooling provided by the cooling coils over the opposing side wall. Thermal elements 90 may include a combination of both heating elements and cooling elements. In the embodiment of FIG. 3 A or FIG. 4, for example, thermal elements 90 can be heating elements, wherein a temperature of the one or more heating elements over second side wall 72 can be increased, for example by increasing an electric current to the one or more heating elements, thereby heating the molten glass flowing over the second side wall to a temperature greater than the temperature of the molten glass flowing over first side wall 70. The increased temperature of the molten glass flowing over the second side wall reduces the viscosity of the molten glass, causing the flow of molten glass over the second side wall to increase relative to the flow of molten glass over the first side wall. In some embodiments, the thermal elements may be distributed heating and/or cooling elements. In some embodiments, a plurality of heating and/or cooling elements may be positioned above each side wall (e.g., above the tops of the side walls), and separately controllable, so that the viscosity of the molten glass overflowing at least one (for example either the first, second or both of the first and second side walls) can be varied along a length of the side wall. The propensity for cord to manifest at a surface of a subsequent glass sheet associate with the second side wall is greater than the propensity for cord to manifest at a surface of a subsequent glass sheet associate with the first side wall, and the side of the glass sheet associated with the second side wall can be designated the "B" side of the glass sheet.

[0049] It will be apparent to those skilled in the art that various modifications and variations can be made to embodiment of the present disclosure without departing from the spirit and scope of the disclosure. Thus it is intended that the present disclosure cover such modifications and variations provided they come within the scope of the appended claims and their equivalents.