Abstract:
Apparatus and method for balancing the mass flow rate of molten glass flowing over a forming body to form a glass ribbon. The apparatus comprises upper and lower transition members that surround the glass ribbon as the glass ribbon is drawn from the forming body. The upper and lower transition members are independently moveable, and separated by a gap less than 8 cm across. The narrow gap minimizes disruption to the thermal environment contained within the upper and lower transition members, and is positioned a sufficient distance above the setting zone of the ribbon to minimize any influence on the dimensional consistency of the ribbon.

Description:
CLAIMING BENEFIT OF PRIOR FILED U.S. APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/155,740, filed on Feb. 26, 2009. The content of this document and the entire disclosure of publications, patents, and patent documents mentioned herein are incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to the drawing of molten glass into a glass sheet, and in particular, to controlling the mass flow rate of the molten glass as it flows over a forming body and is drawn through downstream appliances. 
         [0004]    2. Technical Background 
         [0005]    One method of producing high quality sheets of glass is by the fusion downdraw method. Molten glass is flowed over converging forming surfaces of a forming body, wherein the molten glass fuses at the line where the converging forming surfaces meet to produce a glass ribbon. Drawing equipment positioned downstream of the forming body pull the ribbon downward, and individual glass sheets are cut from the continuous ribbon. 
         [0006]    Maintaining the dimensional stability of the glass ribbon involves an intricate relationship between the mass flow distribution of the molten glass flowing over the forming body, the temperature control of the molten glass and the ribbon 
       SUMMARY 
       [0007]    In one embodiment, an apparatus for forming a sheet of glass is described including a forming body comprising a trough and converging forming surfaces that join at a root such that molten glass overflowing the trough forms multiple streams of molten glass that flow over the converging forming surfaces and combine at the root to form a glass ribbon, and wherein the forming body can be tilted relative to a horizontal plane. The apparatus further comprises an upper transition member coupled to the forming body defining a first interior space through which the glass ribbon descends, and wherein the upper transition member can be tilted and a lower transition member positioned below the upper transition member, the lower transition member defining a second interior space through which the glass ribbon descends. The upper transition member and the lower transition member are separated by a gap less than 8 cm and wherein the gap is positioned such that a viscosity of the glass ribbon adjacent to the gap is equal to or less than about 10 7.3  poise, and preferably also greater than about 10 5.7  poise. Preferably the gap between the upper and lower transition members is equal to or less than about 8 cm, and more preferably less than about 3 cm. 
         [0008]    The forming body and the upper transition member may be tilted in unison, and may be rigidly coupled. 
         [0009]    To prevent air from leaking into the interior spaces of the transition members and disrupting the thermal environment within the transition members, an insulating blanket disposed between the upper and lower transition members. A flexible sealing member or membrane may be coupled to the upper and lower transition members and covering the gap to further prevent air leakage through the gap and to help retain the insulating blanket in position. 
         [0010]    In addition to movement of the upper transition member, the lower transition member may be vertically movable relative to the horizontal plane. However, the upper transition member and lower transition member are independently moveable from each other. Thus, tilting of the upper transition member and vertical translation of the lower transition member may be conducted independently from each other. 
         [0011]    In another embodiment, a method of balancing the mass flow rate of molten glass flowing over the surfaces of a forming body is disclosed. 
         [0012]    The forming body comprises a trough and converging forming surfaces that join at a root such that molten glass overflowing the trough forms multiple streams of molten glass that flow over the converging forming surfaces and combine at the root to form a glass ribbon. 
         [0013]    The method is characterized by the steps of tilting the forming body relative to a horizontal plane in response to a change in a mass flow rate of the molten glass flowing over the converging forming surfaces, tilting an upper transition body relative to the horizontal plane, the upper transition member positioned below and coupled to the forming body. 
         [0014]    The glass ribbon is drawn through a first interior space defined by the upper transition member and a second interior space defined by a lower transition member positioned below the upper transition member. The upper transition member and the lower transition member are separated by a gap less than 8 cm and wherein the gap is positioned such that a viscosity of the glass ribbon horizontally adjacent to the gap is equal to or less than about 10 7.3  poise. For example, the gap may be positioned such that a viscosity of the glass ribbon horizontally adjacent to the gap is equal to or less than about 10 7  poise, equal to or less than about 10 6.5  poise, or equal to or less than about 10 6  poise. 
         [0015]    According to the present embodiment, the upper and lower transition members may be independently moveable, wherein the lower transition member may be translated vertically relative to the horizontal plane, but not tilted while drawing the glass ribbon. In some instances the gap may be made equal to or less than 3 cm. In another embodiment, the lower transition member can be tilted. 
         [0016]    The invention will be understood more easily and other objects, characteristics, details and advantages thereof will become more clearly apparent in the course of the following explanatory description, which is given, without in any way implying a limitation, with reference to the attached Figures. It is intended that all such additional systems, methods features and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective view and partial cross section of an exemplary forming body for use in a fusion downdraw glass sheet forming apparatus. 
           [0018]      FIG. 2  is a side elevation view of a sheet forming apparatus according to an embodiment of the present invention. 
           [0019]      FIG. 3  is a perspective view of the upper transition member of the apparatus of  FIG. 2 . 
           [0020]      FIG. 4  is a perspective view of the lower transition member of the apparatus of  FIG. 2   
           [0021]      FIG. 5  is a side elevation view of a fusion downdraw apparatus comprising a single transition member, wherein tilting of the forming body results in a tilting of the entire apparatus, a possible flow disruption of the molten glass. 
           [0022]      FIG. 6  is a close up cross sectional view of a portion of the upper and lower transition members showing a two layer insulating blanket. Disposed between the transition members. 
           [0023]      FIG. 7  is a side elevation view of the apparatus of  FIG. 2  showing tilt of the muffle, forming body and upper transition member without tilting of the lower portions of the apparatus (e.g. the pulling rollers and the lower transition member). 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of the present invention. Finally, wherever applicable, like reference numerals refer to like elements. 
         [0025]    As used here, unless otherwise defined, “above” and “below” are referenced in absolute terms to the surface of the earth. The terms may also relate relatively to objects in the same reference system. Thus, a first object one meter above the ground vertically beneath the object would be 1 meter above the ground surface, and an object  2  meters above the ground vertically over the first object would also be above the ground surface, but the first object would be below the second object. 
         [0026]    One method of making glass sheet is by a fusion downdraw process, so called because a glass flow is caused to separate into two separate streams of molten glass that flow over a forming body. The streams are then rejoined, or fused, at the bottom of the forming body to produce the glass sheet. This can be more clearly understood with the benefit of  FIG. 1  illustrating an exemplary forming body that may be used in a fusion downdraw glass making process. 
         [0027]      FIG. 1  depicts forming body  10  comprising a channel or trough  12  formed in an upper portion of the body framed by side walls  14 , and converging forming surfaces  16   a  and  16   b  that meet at root  18  extending along a length of the forming body. Molten glass  20  is introduced into trough  12  and overflows side walls  14  on both sides of the forming body, creating two separate streams of molten glass that flow down and over the forming body. The two separate molten glass streams flow over converging forming surfaces  16   a  and  16   b,  and meet at root  18 , where root  18  is a line formed by the convergence of the converging forming surfaces. The streams rejoin, or fuse, at the root to produce a single stream that flows downward from the root as pristine glass ribbon  22 . The molten glass that has been in contact with the surfaces of the forming body (e.g. the converging forming surfaces) is disposed within the interior portion of the resultant ribbon, whereas the outer surfaces of the ribbon have not contacted the forming surfaces. 
         [0028]      FIG. 2  illustrates fusion draw apparatus  30  comprising forming body  10  of  FIG. 1 . In addition to forming body  10 , fusion draw apparatus  30  comprises muffle  32  enclosing forming body  10 , upper transition member  34  defining first interior space  36  ( FIG. 3 ), lower transition member  38  defining second interior space  40  ( FIG. 4 ) and rollers for drawing the sheet downward represented by pulling roller set  42  (pulling roller set  42  constitutes two pair of the pulling roller sets shown on opposite sides of the ribbon, such that the ribbon is pinched between the opposing rollers). Muffle  32 , upper transition member  34  and lower transition member  38 , in combination with various heating and cooling appliances (not shown) disposed within the muffle and transition bodies, serve to regulate the thermal environment surrounding glass ribbon  22  as it is drawn from forming body root  18  and down through the upper and lower transition members. The ability to control the thermal environment of the ribbon, and particularly the thermal environment in the temperature range over which the glass ribbon cools and transitions from a visco-elastic material to an elastic material, allows the manufacture of thin sheets of glass of very high quality. The temperature range where 90% of the high temperature stresses will be retained at room temperature. It is calculated considering the effect of cooling rates and glass visco-elastic properties. That is, for higher cooling rates (e.g. draw velocities) the setting zone goes hotter and smaller. 
         [0029]    According to the embodiment of  FIG. 2 , muffle  32 , upper transition member  34  and forming member  10  are physically coupled so that the components can move as a unit. Moreover, muffle  32 , upper transition member  34  and forming body  10  can be tilted relative to horizontal plane  46  to balance the mass flow rate of molten glass flowing over forming body  10  as a function of position along the length of the forming body. Tilting can be performed, for example, by supporting at least one end of the coupled muffle—forming body—transition member assembly via jack screws. For example, in response to a change or disruption in mass flow rate of molten glass over the forming body, the forming body can be tilted as needed to redistribute the molten glass flowing over the length of the forming body (from one end to the other end) to maintain a consistent thickness of glass ribbon  22 . 
         [0030]      FIG. 5  illustrates a hypothetical fusion draw apparatus  50  comprising a single transition member  52 , forming body  10 , muffle  32  and pulling rollers  42 , and wherein forming body  10 , muffle  32 , pulling rollers  42  and single transition member  52  are rigidly coupled together through a frame (not shown) so that to tilt forming body  10  the entire apparatus  50  must be tilted. In the example of  FIG. 5 , a tilted condition results in two nonparallel drawing forces act on the glass ribbon, a vertical gravitation force g and the pulling force n applied by pulling rollers  42 . The result of the two nonparallel drawing forces can be flow instability of the molten glass, and a horizontal shift of the ribbon. In extreme cases, the horizontal shift may cause contact with single transition member  52 . 
         [0031]    In accordance with the embodiment of  FIG. 2 , to facilitate tilting of the forming body and upper transition member without disturbing the flow dynamics of the ribbon, and to prevent contact with the transition housing, a two-piece transition housing for apparatus  30  comprising upper transition member  34  and lower transition member  38  is provided. Upper transition member  34  and lower transition member  38  are separated from each other by gap δ. Because of the difficulty of positioning heating and/or cooling appliances within gap δ and still accommodate tilting of upper transition member  34 , gap δ is made as narrow as possible. To that end, the maximum separation between the upper and lower transition members (i.e. gap δ) is preferably less than 8 cm, but may vary depending on the degree of tilt by the upper transition member. For example, when upper transition member  34  is in a nominal position (horizontally level), gap δ is preferably equal to or less than 3 cm, typically about 2.5 cm. 
         [0032]    An insulating blanket  54  (shown in a close-up cross section detail of upper and lower transition members  34 ,  38  in  FIG. 6  as two layers) may be disposed in gap δ between upper transition member  34  and lower transition member  38  to prevent ambient air from leaking through the gap that might disrupt the thermal environment within the transition bodies. The insulating blanket forms a flexible, and preferably compressible barrier between the upper and lower transition members that can accommodate movement between the upper and lower transition bodies, while continuing to prevent airflow through the gap δ. Insulating blanket  54  may, for example, comprise one or more layers of an inorganic fiber material, such as those sold by the Unifrax Corporation under the trade name Fiberfrax® Durablanket®. However, any high temperature-resistant insulating material having sufficient flexibility and compressibility to withstand the rigors of the process described above may be used. An optional flexible sealing member  56  may be coupled to the upper transition member and the lower transition member to form an additional barrier over gap  6  that prevents air flow and heat loss through the gap.  FIGS. 2 and 7  are shown without insulating blanket  54 , and portions of sealing member  56  so as to not obstruct other portions of the figure. For example, in practice sealing member  56  may be extended around the entire perimeters of upper and lower transition members  34 ,  38  over gap  6 . 
         [0033]    In some embodiments, lower transition member  38  can move vertically, but is not configured to tilt as is the upper transition body. Preferably, pulling rollers  42  are also capable of vertical movement, and more preferably are capable of moving in unison with lower transition member  38 , when coupled through supporting framework (not shown). For example, in some instances the positions of lower transition member  34  and pulling rollers  42  may be vertically adjusted to accommodate the tilt applied to upper transition member  38 . 
         [0034]    In other embodiments, the lower transition member can be tilted independently from the upper transition member (that is, both the upper and lower transition members are independently tillable). This can be desirable if a different order of operation is performed. For example, the upper and lower transition members may be connected, such as by jack screws, so that the upper and lower transition members are first tilted in unison. Once the initial tilt is performed, the jack screws are temporarily released and the lower transition is tilted (re-leveled) so that the lower transition is again vertical (or the upper edge of the lower transition member adjacent gap δ is back in a horizontal plane). The upper and lower transition members may then be reconnected in the new configuration. 
         [0035]    To minimize any disruption to the glass ribbon as it descends through the setting zone, the length of upper transition member  34  is configured such that gap δ is positioned where the viscosity of the glass ribbon horizontally adjacent to the gap is equal to or less than a viscosity V max  of about 10 7.3  poise. For example, the gap may be positioned such that the viscosity of the glass ribbon horizontally adjacent to the gap is equal to or less than a V max  of about 10 7  poise, equal to or less than a V max  of about 10 6.5  poise, or equal to or less than a V max  of about 10 6  poise. Preferably, gap δ may further be positioned where a viscosity of the glass ribbon horizontally adjacent to the gap is equal to or greater than a viscosity (V min ) of about 10 5.7  poise. Thus, for example, the gap may be positioned horizontally adjacent a location of the ribbon where the viscosity of the ribbon is between V min  and V max  e.g. 10 7.3  poise and 10 5.7  poise, between 10 7  poise and 10 6.5  poise, between 10 6.5  poise and 10 5.7  poise, or between 10 6  poise and 10 5.7  poise. 
         [0036]    It should be emphasized that the above-described embodiments, particularly any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. For example, although the example embodiments described herein relate to an organic light emitting diode display device, other embodiments are contemplated and within the scope of the present invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.