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

Provisional Application Serial No. 62/259,192, 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 invention relates generally to methods and apparatus for selectively distributing cord in a continuous flow of molten glass from a forming body by controlling an orientation of the forming body.

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 in the form of a surface corrugation with a variable spatial period in a range from about 1 millimeter to about 10 millimeters and a depth typically on the order of 10 nanometers. Cord is undesirable in glass sheets used in the manufacture of display devices, as it affects the quality of liquid crystal display panels, both visually and functionally. Visually, cord appears 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, which may affect operation of the device.

[0004] A fusion-formed glass sheet has two sides, 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 is also equal on both sides of the sheet. 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. SUMMARY

[0005] A method is described comprising supplying molten glass to a forming body including a trough extending along at least a portion of a length L of the forming body, the trough including opposing first and second side walls, the molten glass filling the trough and overflowing the sidewalls, wherein molten glass overflowing the first side wall forms a first flow of molten glass and molten glass overflowing the second side wall forms a second flow of molten glass, the first and second flows of molten glass joining along a bottom edge of the forming body to form a ribbon of glass including a first layer and a second layer,

respectively. The method further comprises rotating the forming body about a longitudinal axis thereof through an angle a greater than zero degrees relative to a vertical plane parallel to the bottom edge and in a direction toward the second side wall such that a mass flow rate of the molten glass overflowing the second side wall is greater than a mass flow rate of the molten glass overflowing the first side wall such that a thickness of the second layer is greater than a thickness of the first layer. The angle a may be equal to or less than about 1 degree. In some embodiments, a may be in a range from about 0.3 degrees to about 0.4 degrees

[0006] The method may further comprise rotating the forming body about a lateral axis orthogonal to the longitudinal axis through an angle β relative to a horizontal plane. For example in some embodiments, a height of the bottom edge at an inlet end of the forming body is greater than a height of an end of the forming body opposite the inlet end, relative to a horizontal plane.

[0007] The method may further comprise varying a viscosity of the molten glass overflowing the first and/or second side wall such that the viscosity of the molten glass overflowing the first side wall is different than a viscosity of molten glass overflowing the second side wall.

[0008] In some embodiments, an angle of a floor of the trough in a direction orthogonal to the longitudinal axis, relative to a horizontal plane, is in a range from greater than 0 degrees and equal to or less than about 1 degree.

[0009] In another embodiment, a method of forming a glass ribbon is described comprising supplying molten glass to a forming body including a longitudinal axis, and a trough extending along at least a portion of a length L of the forming body, the trough including opposing first and second side walls, the molten glass filling the trough and overflowing the first and second sidewalls. The molten glass then flows down exterior surfaces of the side walls and down exterior forming surfaces of the forming body. The method may further comprise rotating the forming body about a lateral axis perpendicular to the longitudinal axis through an angle β relative to a horizontal plane, and wherein molten glass overflowing the first side wall forms a first flow of molten glass and molten glass overflowing the second side wall forms a second flow of molten glass, the first and second flows of molten glass joining along a bottom edge of the forming body to form a ribbon of glass. A height of the bottom edge at an inlet end of the forming body can be greater than a height of the bottom edge at an end of the forming body opposite the inlet end, relative to a horizontal plane positioned below the forming body.

[0010] The ribbon of glass comprises a first layer and a second layer, respectively, and the method may further comprise rotating the forming body about the longitudinal axis thereof through an angle a greater than zero degrees relative to a vertical plane parallel with the bottom edge and in a direction toward the second side wall such that a mass flow rate of the molten glass overflowing the second side wall is greater than a mass flow rate of the molten glass overflowing the first side wall, and wherein a thickness of the second layer is greater than a thickness of the first layer. For example, in some embodiments, a is equal to or less than about 1 degree, for example in a range from about 0.3 degrees to about 0.4 degrees.

[0011] The method may further comprise varying a viscosity of the molten glass overflowing the first and/or second side wall such that the viscosity of the molten glass overflowing the first side wall is different than a viscosity of molten glass overflowing the second side wall.

[0012] In still another embodiment, a method is disclosed comprising supplying molten glass to a forming body including a trough extending along at least a portion of a length L of the forming body, the trough including opposing first and second side walls and a floor connecting the first and second side walls, the molten glass filling the trough and overflowing the first and second sidewalls, and wherein an angle of the floor of the trough in a direction orthogonal to the longitudinal axis relative to a horizontal plane is in a range from greater than 0 degrees and equal to or less than about 1 degree such that a mass flow rate of molten glass overflowing the second side wall is greater than a mass flow rate of molten glass overflowing the first side wall. The molten glass overflowing the first side wall forms a first flow of molten glass and molten glass overflowing the second side wall forms a second flow of molten glass, the first and second flows of molten glass joining along a bottom edge of the forming body to form a ribbon of glass including a first layer and a second layer, respectively, and a thickness of the second layer is greater than a thickness of the first layer. [0013] In some embodiments, a viscosity of the molten glass overflowing the first side wall is different than a viscosity of the molten glass overflowing the second side wall. For example, in some embodiments, the viscosity of the molten glass overflowing the first side wall is greater than a viscosity of the molten glass overflowing the second side wall.

[0014] The method may further comprise heating and/or cooling the molten glass overflowing at least one of the first and second sidewalls.

[0015] In some embodiments, a height of the bottom edge at an inlet end of the forming body may be greater than a height of an end of the forming body opposite the inlet end relative to a horizontal plane

[0016] The method may further comprise rotating the forming body about a longitudinal axis thereof through an angle a greater than zero degrees relative to a vertical plane parallel to the bottom edge and in a direction toward the second side wall such that a mass flow rate of the molten glass overflowing the second side wall is greater than a mass flow rate of the molten glass overflowing the first side wall. The vertical plane may, for example, extend through the forming body trough. In some embodiments, the angle a may be greater than 0 degrees and equal to or less than about 1 degree.

[0017] 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.

[0018] 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

[0019] FIG. 1 is a schematic view of an example fusion down draw glass making process;

[0020] FIG. 2A is a cross sectional view of an exemplary forming body in an upright, non- rotated position; [0021] FIG. 2B is an edge view of a glass sheet cut from a ribbon drawn from the forming body of FIG. 2A depicting relative thicknesses of the layers of glass comprising the sheet;

[0022] FIG. 3 A is a cross sectional view of the forming body of FIG. 2A after rotation about a longitudinal axis thereof;

[0023] FIG. 3B is an edge view of a glass sheet cut from a ribbon drawn from the forming body of FIG. 3 A depicting relative thicknesses of the layers of glass comprising the sheet;

[0024] FIG. 4 is a graph depicting the flow rate of molten glass over side walls of the forming body of FIG. 3 A in pounds per inch length of the forming body per hour as a function of the roll angle a; and

[0025] FIG. 5 is a side view of an exemplary forming body rotated about a lateral axis through a pitch angle β.

DETAILED DESCRIPTION

[0026] Reference will now be made in detail to the 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] Shown in FIG. 1 is an exemplary glass manufacturing apparatus 10. In some examples, 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.

[0032] 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. In some examples glass melting vessel 14 may be constructed from refractory ceramic bricks.

[0033] In some examples, 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 the 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.

[0034] The glass manufacturing apparatus 10 (e.g., fusion down draw apparatus 10) can optionally include an upstream glass manufacturing apparatus 16 that is 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.

[0035] 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.

[0036] Glass manufacturing apparatus 10 can also optionally include a downstream glass manufacturing apparatus 30 positioned downstream relative to glass melting furnace 12. 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. [0037] 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.

[0038] 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.

[0039] 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 the 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.

[0040] 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.

[0041] 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, at least a portion of exit conduit 44 may be 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, 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 54a, 54b 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 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.

[0042] As described supra, cord can originate as regions of chemical and/or thermal inhomogeneity in a body of molten glass. If the molten glass is drawn (stretched), the regions of inhomogeneity are stretched as well. Accordingly, cord can manifest as strings (filaments) of inhomogeneity extending in the draw direction of the drawn glass. When the glass cools, these filaments of inhomogeneity may also manifest as slight thickness changes in the glass sheet, which can both visually and functionally affect the performance and perceived quality of the display device. The resultant lensing effect 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. Additionally, even small thickness changes on the surface of a glass sheet may affect deposition processes used to deposit electronic components on the glass sheet.

[0043] 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. However, it may not completely eliminate the inhomogeneities. Thus, efforts to completely eliminate cord from the finished product have not been successful.

[0044] Glass sheet used in the manufacture of display panels includes two major surfaces, typically identified as the "A" side and the "B" side. It should be apparent that while the fusion process is capable of producing a virtually pristine glass ribbon, it is necessary during downstream processing of the glass sheets to handle the subsequent glass sheets. If contact with the glass sheet (or ribbon) during manufacture of the glass sheet is necessary, contact is typically made on the "B" side of the glass sheet. For example, gripping tool 65 is configured to grip the glass sheet 62 on the "B" side of the glass sheet (or more appropriately, the side of the glass sheet contacted by the gripping tool is designated the "B" side), and all further contact is made against the "B" side. This designation between the "A" side and the "B" side of the glass sheet is carried through to the display panel manufacturer so that the panel manufacturer can distinguish between the potentially contacted side and the more pristine "A" side. Thus, for example, in an LCD display panel, the electronics (e.g., transistors) that control the orientation of the liquid crystal material are deposited on the "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 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.

[0045] Accordingly, a methods are disclosed that can be used to minimize, such as reduce, or eliminate cord from one surface of the glass ribbon (e.g., glass sheet), by forming the glass ribbon to have asymmetrically thick layers.

[0046] 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 flow of molten glass over opposing sides of forming body 42. That is, the separate flows of molten glass descending over converging forming surfaces 54a, 54b of forming body 42 and join together below bottom edge 56 of the forming body. 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 sides is balanced such that the thicknesses of the two flows, and hence the thicknesses of 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 equidistant between the two major surfaces of the glass sheet cut from the ribbon). 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. As a result, a position of the fusion line is asymmetric and one layer of glass is thicker than the opposing layer of glass. The inventors herein have discovered that the side of the forming body experiencing the greater flow of molten glass also manifests a greater amount of visible cord. 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.

[0047] As shown in FIGS. 1 and 2A, in an example fusion down draw process molten glass 28 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 to form glass ribbon 62. FIG. 2A illustrates the forming body in an upright vertical orientation with no angular roll. That is, the forming body depicted in FIG. 2A is symmetric about vertical plane 78 in which bottom edge 56 lies. As a result, glass ribbon 58 drawn from bottom edge 56 is symmetric in cross section about vertical plane 78. FIG. 2B is an edge view of a glass sheet 62 cut from glass ribbon 58 showing the relative thicknesses Tl and T2 as a result of the upright, non-rolled orientation of forming body 42. FIG. 2B shows the glass sheet comprising a first glass layer 80 and a second glass layer 82 joined to the first glass layer at an interface 84, which is the fusion line, and where Tl, representing the thickness of glass layer 80 is equal to T2, representing the thickness of glass layer 82.

[0048] In accordance with the embodiment shown in FIG. 3A, forming body 42 may be rotated about a long axis 86 of the forming body, effectively lowering the top of one side wall relative to the opposing side wall. For example, the forming body may be rotated through a roll angle a relative to vertical plane 78 greater than zero, for example in a range from about 0.1 degrees to about 1 degree, in a range from about 0.1 degrees to about 0.9 degrees, in a range from about 0.1 degrees to about 0.8 degrees, in a range from about 0.1 degrees to about 0.7 degrees, in a range from about 0.1 degrees to about 0.6 degrees, in a range from about 0.2 degrees to about 0.5 degrees, or in a range from about 0.3 degrees to about 0.4 degrees, and including all ranges and subranges therebetween. Long axis 86 may be placed anywhere along the vertical height of the forming body, as necessary. For example, long axis 86 may be positioned at the bottom edge 56 of the forming body, or moved upward relative to the bottom edge. Placement of the long axis (axis of rotation) at the bottom edge can be advantageous because, although the forming body may undergo a roll event, the location of the bottom edge of the forming body, from which the glass ribbon is drawn, does not shift. Thus, the glass ribbon is drawn from the same position as before the roll, with no impact on processing below the bottom edge. Roll angle a may be determined, for example, between vertical plane 78 and bottom floor 68. FIG. 3B illustrates the relative thicknesses of an exemplary glass sheet 62 cut from glass ribbon 58 drawn from the rotated forming body 42, wherein the first thickness Tl, representative of the thickness originating from the first side wall 70 is less than the thickness T2 originating from the second side wall 72. [0049] The table below shows modeled data of mass flow change for a forming body with an overall length of 103 inches (261.6 cm) as a result of rotating (rolling) the forming body through an angle a relative to vertical plane 78.

Table

[0050] FIG. 4 is a graph showing modeled data for a forming body receiving a 10.0 pounds/inch/hour total flow of molten glass (in units of pounds of molten glass per inch length of the forming body per hour) showing the variation of flow over each side wall for a roll angle a between 0 degrees and 1 degree. In FIG. 4, the line 88 represents the side wall in which direction the roll occurs (representative of the "B" side of the glass ribbon drawn from the forming body), and that therefore receives an increased flow, and line 90 represents molten glass flow for the opposing side wall. The data show that rotating the forming body, in the present example, in direction toward the "B" side, increases the flow of molten glass over the "B" side wall. Concurrently, the mass flow of molten glass over the "B" side wall is increased and the mass flow of molten glass over the "A" side is decreased. The last column of the Table shows an increase in cord in the "B" side layer, which does not indicate a percent increase in overall cord for the glass sheet, but rather an increase in cord in the "B" side layer of the glass sheet as a function of the percent change in mass flow rate of molten glass from a baseline flow rate.. Overall thickness is maintained, with only a change in thickness of the layers 80, 82 comprising the glass ribbon.

[0051] Cord measurement may be performed using a near-infrared light source, optical fiber and discrete free-space optics to launch a free-space collimated beam. The collimated beam is transmitted through the finished product, e.g., flat glass substrate, and into a detector assembly on the opposite side, where the transmitted light is focused by a lens and captured by a sensing element with an oriented slit aperture. A coherence length of the collimated beam is less than the substrate thickness, with a uniform phase front across the beam width. As the beam passes through a substrate with cord, the beam phase is weakly modulated by the thickness variations. The optical effect is similar to that of a diffraction grating, and to the production of the zero-order and the two first-order diffracted fields. These diffracted fields interfere as they continue to propagate to give intensity maxima and minima as a function of distance from the substrate. A focusing lens is used to enhance contrast and to shorten the optical path length to the sensing element, and the slit aperture is used to achieve an appropriate amount of spatial resolution and insensitivity to vibration. The cord measurement is made by moving the sensor element in an across-the-substrate direction, all while recording the amount of power received by the detector. Digital filtering of the detector signal profiles may be performed to extract out the cord content, recorded as "contrast".

[0052] It should be apparent from the foregoing that flow can be used to redistribute cord within the glass ribbon (and hence the glass sheet) drawn from forming body 42. It should also be apparent that variations in pitch can also redistribute cord. By pitch what is meant is rotation of the forming body about a lateral axis 92 orthogonal to longitudinal axis 86, as illustrated by FIG. 5. Lateral axis 92 may be placed at any position along the length of the forming body, as necessary. That is, the pitch can be affected by lifting or lowering one end of the forming body (i. e. , wherein the lateral axis is positioned at an end of the forming body rather than intermediate between the opposing ends of the forming body as shown in FIG. 5), although in further embodiments, the same effect can be obtained by lifting one end of the forming body and lowering the opposite end. By configuring the forming body to be raised, or lowered, at only one end of the forming body, the mechanisms needed to effect the change in position (e.g., jack screws, etc.) can be simplified. The following provides a brief description of the effects of pitching the forming body. [0053] As molten glass flows from exit conduit 44 through inlet 50, the molten glass moving through the center of the conduits travels faster than molten glass traveling along the inner surfaces of the conduits. This flow profile is largely maintained as the molten glass enters into and flows along trough 52 from inlet end 100 to the opposite end 102, with the exception that the trough is open at the top. Consequently, the flow through the center of the trough is faster than the flow along the inner surfaces of the trough (i. e. , inner surfaces of side walls 70, 72), and the molten glass within the center of the trough is newer and less stretched than the molten glass at the side wall inner surfaces. It should be noted that in some embodiments the trough floor slopes upward as the molten glass flows from the inlet end of the forming body to the opposite end to produce a consistent flow of molten glass over the side walls as a function of distance from the inlet end along the length of the forming body. This floor slope accounts for impedance experienced by the molten glass from contact with the trough surfaces. As a consequence, the molten glass at the inlet end of the forming body is deeper and slower moving that the molten glass farther along the length of the forming body. In the context of the molten glass residence time in the trough, molten glass at inlet end 100 of forming body 42 experiences greater residence time than molten glass farther from the inlet end in a direction toward the opposite end 102. At the same time, opposite end 102 of the forming body trough is blocked such that the molten glass cannot flow out from the opposite end. Thus, the flow of molten glass tends to stagnate at the end of the trough opposite the inlet end, again increasing the residence time. Accordingly, molten glass flowing over the side walls at the ends of the forming body experience greater residence time within trough 52 that the molten glass overflowing the side walls in the middle portion of the trough. Moreover, the slow-moving molten glass at the ends of the forming body provide greater time for stretching of cord (stretching of inhomogeneous molten glass) within the molten glass flow, and, as earlier described, increased stretching of the cord can increase the effects of the cord, particularly the detrimental visual effects. As a result, troublesome well- stretched cord tends to be more prevalent along the longitudinal edges of the ribbon, representative of the molten glass flowing over the side walls at the ends of trough 52 rather than within the middle portion of the ribbon.

[0054] To overcome the residence time differences between molten glass at the inlet end of the forming body, the middle portion of the forming body and the end of the forming body opposite the inlet end, the forming body may be rotated about a lateral axis 84 perpendicular to longitudinal axis 82. This rotation will be referred to herein as pitch. Thus, to decrease the residence time of molten glass at the inlet end of the forming body, the forming body may be rotated about lateral axis 84 such that the end 102 of the forming body opposite inlet end 100 is moved downward relative to inlet end 100. As represented in FIG. 5, the forming body may be rotated clockwise through an angle of β degrees about lateral axis 84 relative to horizontal plane 94. To decrease the residence time of molten glass at the end of the forming body opposite the inlet, the forming body may be rotated about lateral axis 84 such that inlet end 100 of the forming body is moved downward relative to opposite end 102. It should be apparent that the same effects could be achieved by rotation in the opposite direction. That is, in the former case, the forming body could be rotated about lateral axis 84 such that the inlet end 100 of the forming body is moved upward relative to opposite end 102, and in the latter case, the forming body may be rotated about lateral axis 84 such that opposite end 102 of the forming body is moved upward relative to inlet end 102.

[0055] Pitch rotation and roll rotation may be combined to effect variations in the manifestation of cord in the glass sheet resulting from the foregoing processes. For example, roll rotation about longitudinal axis 82 can be used to shift cord from one side of the glass sheet to the opposite side, while pitch rotation can be used to shift cord from the inlet end of the forming body to the opposite end of the forming body by lifting the inlet end such that the height of the inlet end is greater than a height of the opposite end, as measured from the bottom edge of the forming body relative to a horizontal plane. Of course, as noted supra, the same effect than be achieved by lowering the end of the forming body opposite the inlet end.

[0056] In other embodiments, a viscosity of the molten glass overflowing side walls 74 and 76 may be modified in addition to roll and/or pitch changes. For example, thermal elements 96a, 96b positioned above the forming body, for example above the top surfaces 74, 76 of side walls 70, 72, respectively, may be used to change a viscosity of the molten glass (see FIG. 3A). Thermal elements 96a may be heating elements, for example resistive heating elements, or thermal elements 96a, 96b may be cooling elements, for example cooling elements through which a cooling fluid is flowed. In some embodiments, at least one of each thermal element 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. A relative increase in flow can be obtained by heating the molten glass flowing over the appropriate side wall, or by cooling the molten glass flowing over the opposite side wall. For example, in the embodiment of FIG. 3 A, the heat output of a heating element 96b above second side wall 72 can be increased to reduce the viscosity of the molten glass flowing over the second side wall and thereby increase the flow of molten glass over the second side wall, or a cooling element 96a above first side wall 70 can be used to increase the viscosity of the molten glass overflowing first side wall 70 and thereby reducing the flow of molten glass over the first side wall.

[0057] 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.