Abstract:
An apparatus ( 10 ) for forming the outer layers of a glass laminate sheet comprises a reservoir ( 12 ), individual first ( 14   a ) and second ( 14   b ) distributors extending below and in fluid communication with the reservoir, and first ( 30   a ) and second ( 30   b ) slots positioned respectively at the bottom of the first and second distributors. The slots have a length, the distributors have sides and a middle, and the length of the slots on the sides of the distributors is desirably decreased relative to the length of the slots in the middle of the distributors. The apparatus is useful with a trough or isopipe ( 100 ) to provide clad glass streams to contact an over-flowing core glass on respective sides of the trough or isopipe.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/884,985 filed on Sep. 30, 2013, the content of which is relied upon and incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present disclosure relates to devices and methods for forming laminated glass sheets having a core glass layer surrounded by one or more outer glass layers (“clad” glass layers), and particularly to devices and methods adaptable to a wide range of glass compositions and properties. 
       BACKGROUND 
       [0003]    Laminated glass sheets typically include a core glass layer surrounded by first and second clad glass layers. The composition of the core glass and the clad glass may be selected to have different properties in order to provide desired advantages in the resulting laminate. One significant beneficial property obtainable in the laminate is increased strength and damage resistance: by properly selecting the clad glass and the core glass in connection and the process conditions for forming the laminate (such as by choosing a core glass having a higher CTE than the clad glass), the clad layers in the final laminate sheet will be in compression, resulting in a glass laminate sheet that significantly resists damage and breaking. These and other desirable properties can be obtained from glass laminate sheets. 
         [0004]    An isopipe is a convenient apparatus for production of two thin glass sheets and an isopipe may beneficially be used to form the cladding of a glass laminate sheet. However, an isopipe is typically best suited for a rather narrow range of flow rates and viscosities. If flow rates and/or viscosities are changed to accommodate different clad glass compositions to produce sheets optimized for various differing uses, it will typically be the case that the isopipe needs to be tilted to maintain a flat flow profile over the width of the resulting sheet(s). Providing a mechanism for such tilting may be difficult within the constraints of the equipment space available in the manufacturing environment. Furthermore tilting may be only allow for a relatively limited range of different viscosities and flow rates. A new clad forming device and method able to accommodate a wide range of glass viscosities and flow rates within a small equipment footprint would accordingly be useful. 
       SUMMARY 
       [0005]    The present disclosure provides for an apparatus for forming the outer layers of a glass laminate sheet comprising a reservoir, individual first and second distributors extending below and in fluid communication with the reservoir, and first and second slots positioned respectively at the bottom of the first and second distributors. The slots have a length, and a width, and the length of the slots is greatest at a center of the width thereof 
         [0006]    The resulting apparatus provides the ability to produce twin glass sheets for the outer layers of a glass laminate by gravity feed over a wide range of viscosities and flow rates, allowing a wide range of glass compositions to be employed. 
         [0007]    Additional features and advantages 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 embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
         [0008]    It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagrammatic cross-section of an embodiment of an apparatus according to the present disclosure; 
           [0010]      FIG. 1A  is a diagrammatic cross section illustrating the use of an embodiment of an apparatus according to the present disclosure to produce a glass laminate; 
           [0011]      FIG. 2  is a three-dimensional cut-away view of an embodiment similar to that of  FIG. 1  of an apparatus according to the present disclosure; 
           [0012]      FIG. 3  is a three-dimensional representation of the shape of one of the distributors (shown with half of the shape cut away). 
           [0013]      FIG. 4  is a graph of pressure as a function of height above the slot exits of an apparatus like that of  FIG. 1  or of  FIG. 2 . 
           [0014]      FIG. 5  is a graph of the data of  FIG. 4  with the gravitational force acting on the glass is subtracted from the pressure shown in  FIG. 4 . 
           [0015]      FIG. 6  is a graph of the free surface level above distributor exit as a function of flow rate at a typical glass viscosity of 8000 poise with a representative distributor geometry. 
           [0016]      FIG. 7  is the flow rate at which the distributor is just filled completely (at the bottom of reservoir) represented as a function of glass viscosity for a typical glass density of 2,400 kg/m 3 . 
           [0017]      FIG. 8  is a graph of an outflow velocity profile achievable using an embodiment of the apparatus of the present disclosure with distributors as disclosed. 
           [0018]      FIG. 9  is a comparative graph of a calculated outflow velocity profile for an apparatus similar to that disclosed herein but having distributors with constant length slots. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Reference will now be made in detail to the present preferred embodiment(s), examples of which is/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. 
         [0020]    As shown in the diagrammatic cross section of  FIG. 1  and in the corresponding three-dimensional cut-away of  FIG. 2 , the present disclosure provides an apparatus  10  and corresponding method for forming the outer layers of a glass laminate sheet, the apparatus  10  comprising a reservoir  12 , below which there are two distributors  14   a ,  14   b , as shown in  FIGS. 1 and 2 . Glass  16  is fed into the reservoir  12  from the top thereof at a particular flow rate, q. The flow of glass  16  is divided into two by flowing through the entrances  20   a ,  20   b  of the two distributors  14   a ,  14   b . The flow in each distributor  14   a ,  14   b  then passes through a respective reshaping section  22   a ,  22   b  in which the flow is thinned and widened before coming out at the bottom of the apparatus  10  through respective slots  30   a ,  30   b . A three-dimensional representation of the shape of one of the distributors  14   a  (shown with half of the shape cut away) is given in  FIG. 3 . 
         [0021]    Under operating conditions, the glass  16  fills the distributors  14   a ,  14   b  completely, and the free surface  18  of the glass  16  floats at some position within the reservoir  12 . The glass  16  upon entering into the reservoir  12  pours onto the free surface  18  to refill the reservoir  12  continuously, and the free surface  18  is desirably maintained consistently at a given level appropriate to the composition of the glass  16  and the needs of the laminate forming process. The free surface level  18  (shown as a dashed line perpendicular to a dashed vertical reference line) in the reservoir  12  may be maintained as needed at any location within the reservoir, depending on the viscosity, flow rate, and density of the glass  16 . This allows the use of glasses of widely varying properties and characterstics. The glass flow coming out of each respective exit  32   a ,  32   b , of each respective slot  30   a ,  30   b  at the bottom of the respective distributors  14   a ,  14   b , desirably has uniform velocity throughout. The slots have a length L (in the vertical or “flow” direction) (see  FIG. 1 ) and a width W (indicated in  FIG. 2  as “½W” since the distributors are shown with one half cut away). 
         [0022]    As shown in  FIG. 1A , glass  16  leaving the slots  32   a ,  32   b , according to an embodiment of a process or method of the current disclosure, desirably contacts another different glass  17  as glass  17  is overflowing from a trough or isopipe  100 . The spacing of slots  32   a ,  32   b  is selected accordingly to match the desire contact points on the trough or isopipe, such that the emerging glass from the slots  32   a ,  32   b  is positioned above the glass overflowing from or flowing down along the trough or isopipe  100 .  8 . Molten clad glass  16  is fed into the apparatus  10  so as to maintain a selected free surface level within the reservoir of the apparatus, and to cause first and second clad glass streams  52   a ,  52   b  to emerge from the first and second slots  32   a ,  32   b  of the apparatus  10 . Molten core glass  17  is supplied to a trough or isopipe  100  sufficiently so as to allow the core glass  17  to overflow the trough or isopipe  100 , with the trough or isopipe  100  being positioned below the first and second slots  32   a ,  32   b  of the apparatus  10 . 
         [0023]    The core glass  17  overflowing a first side of the trough or isopipe  100  is contacted with the first clad glass stream  52   a  while the core glass overflowing a second side of the trough or isopipe  100  is contacted with the second clad glass stream  52   b.    
         [0024]    The core glass  17  overflowing the first side of the trough or isopipe  100  (now flowing with a layer of clad glass from the first clad glass stream  52   a ) is then merged with the core glass overflowing the second side of the trough or isopipe  100  (flowing with a layer of clad glass from the second clad glass stream  52   b ) to form a glass laminate  200  having a core comprising the core glass  17  and a clad comprising the clad glass  16 . 
       Free Surface Level in the Reservoir 
       [0025]    A mass and momentum balance equation gives the relation between the pressure drop and the geometry. In its simplest form, where subscript “1” denotes the conditions at the entrances  13   a ,  13   b  to the distributors  14   a ,  14   b  and subscript “2” denotes the conditions at the exits  32   a ,  32   b  of the slots, we can write along a given streamline: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         V 
                         1 
                         2 
                       
                       2 
                     
                     + 
                     
                       
                         P 
                         1 
                       
                       ρ 
                     
                     + 
                     gH 
                     - 
                     
                       F 
                       loss 
                     
                   
                   = 
                   
                     
                       
                         V 
                         2 
                         2 
                       
                       2 
                     
                     + 
                     
                       
                         P 
                         a 
                       
                       ρ 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0026]    Here the pressure at the exit of the distributor P 2  is taken to be equal to the atmospheric pressure Pa. Rearranging this expression, we have: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         1 
                       
                       - 
                       
                         P 
                         a 
                       
                     
                     ρ 
                   
                   = 
                   
                     
                       - 
                       gH 
                     
                     + 
                     
                       
                         1 
                         2 
                       
                        
                       
                         ( 
                         
                           
                             V 
                             2 
                             2 
                           
                           - 
                           
                             V 
                             1 
                             2 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       F 
                       loss 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0027]    From mass conservation for an incompressible fluid, we can write: 
         [0000]      V 1 A 1 =V 2 A 2   (3)
 
         [0028]    Combining (2) and (3), we can write: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         1 
                       
                       - 
                       
                         P 
                         a 
                       
                     
                     ρ 
                   
                   = 
                   
                     
                       - 
                       gH 
                     
                     + 
                     
                       
                         1 
                         2 
                       
                        
                       
                         
                           V 
                           2 
                           2 
                         
                          
                         
                           ( 
                           
                             1 
                             - 
                             
                               
                                 ( 
                                 
                                   
                                     A 
                                     2 
                                   
                                   
                                     A 
                                     1 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                           ) 
                         
                       
                     
                     + 
                     
                       F 
                       loss 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0029]    Now the condition for the pressure to decrease in the flow/gravity direction, we have to ensure P 1 &gt;Pa that leads to certain design requirements: First, the sum of the second and the third term on the right side of the expression (4) above has to be larger than gH. Second, the second term will have positive contribution only if the area of the distributor at the exit (A 2 ) is smaller than the area at the inlet (A 1 ). This second condition introduces a constraint on the cross-sectional area and thus the thickness of the slots  30   a ,  30   b , which must be small enough such that the second term, when combined with the third term (the loss during the flow from 1 to 2 denoted by F loss ) (combined with the second term) is to be large enough so that P 1 &gt;Pa is satisfied. 
         [0030]    For the distributor entrance sections  20 , which in this embodiment have essentially circular cross section, the Fanning friction factor for laminar flow in round tubes is often taken to be: 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                     
                       loss 
                        
                       
                         ( 
                         circ 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       16 
                        
                       
                           
                       
                        
                       μ 
                     
                     
                       
                         V 
                         av 
                       
                        
                       
                           
                       
                        
                       D 
                        
                       
                           
                       
                        
                       ρ 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0031]    However, the resistance of the entrance section, whether circular or oval or other shape is negligible compared to the two main sources of resistance: shape change from distributor entrance to the land sections or slots  30   a ,  30   b , designated R 2 ; and the resistance offered by the slots  30   a ,  30   b , designated R 1 . 
         [0032]    Computational Fluid Dynamics (CFD) was used to identify the resistance to flow by a given distributor geometry similar to that of  FIGS. 1 and 2 , without considering gravitational effects, for a typical glass viscosity of 4000 poise and 3.6 kg/h flow rate. Pressure (in Pascal) as a function of height (in meters) above the slot exits is shown in the graph in  FIG. 4 . As can be seen from the figure, the resistance R 1  is considerably higher than the resistance R 2 , essentially dominating the total resistance or total pressure drop. 
         [0033]    The value of R 1  (or the pressure drop due to R 1 ) can be obtained analytically from the Poiseuille flow equation for rectangular channels with width&gt;&gt;thickness (as in the case of slots  30   a ,  30   b ) as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     P 
                   
                   = 
                   
                     
                       12 
                        
                       
                           
                       
                        
                       μ 
                        
                       
                           
                       
                        
                       LQ 
                     
                     
                       Wh 
                       3 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ΔP is the pressure drop, μ is the viscosity, L is the length of the land, Q is the flow rate, W is the width of the distributor, and h is the thickness. 
         [0034]    When the gravitational force acted on the glass is subtracted from the pressure obtained from model, we get the predicted actual pressure (P−density·g·height), again in Pascal as a function of height position in meters in  FIG. 5 . For a typical glass viscosity of 4000 poise and 3.6 kg/h with a typical distributor geometry, the free surface is obtained at 0.1 m above the distributor exit,as desired, since in this embodiment the top-to bottom length of the distributors is 0.1 m. 
         [0035]    The free surface level above distributor exit in meters is calculated for different flow rates (here given in lbs/h) at a typical glass viscosity of 8000 poise with a typical distributor geometry, as shown in  FIG. 6 . If the flow rate is 0.9 kg/h (or 2 lbs/h), the free surface level is 0.03 in above distributor exit, and as the flow rate increases the free surface level increases for the gravity driven flow regime. Alternately, the flow rate at which the distributor is just tilled completely (bottom of reservoir) may be represented (given here in lbs/h) as a function of glass viscosity for a typical glass density of 2,400 kg/m 3  in  FIG. 7 . 
         [0036]    By considering the parameters mentioned above that effect the free surface level, the shape for the distributors can be designed, by changing the slot (land) length, slot thickness, and even by adjusting the properties of the shape change from distributor entrance to the slot, to be capable to deliver glass at a desired range of flow rates, glass viscosities and densities under gravity feed, with a free surface  18  of the glass  16  positioned within the reservoir  12 . Desirably, the distributors deliver glass with 8000 poise from 0.3 kg/h to 0.6 kg/h (reservoir almost empty to reservoir full) and for glass with 6000 poise, from 0.4 kg/h to 0.8 kg/hr. In other words, by using embodiments of the apparatus according the present disclosure, a device is provided that is able to deliver glass with 8000 poise at rates in the range of from 0.3·S kg/h to 0.6·S kg/h under gravity feed with a free surface of the glass positioned in the reservoir, and to deliver glass with 6000 poise at rates in the range of from 0.4·S kg/h to 0.8·S kg/hr under gravity feed with a free surface of the glass positioned in the reservoir, where S is an arbitrary scaling constant scaled with the size of the apparatus and with the glass sheets to be produced. 
       Uniform Glass Flow at Slot Exits 
       [0037]    It is desirable to have uniform glass flow at the slot exits. Generally, for a given fluid at a given flow rate, the flow resistance is dependent on the length and cross-sectional area of the flow path. A longer flow path leads to higher flow resistance than a short one. Meanwhile, for the given fluid at a given flow rate, a flow path with a greater cross-sectional area results in lower shear strain rate and leads to lower flow resistance. Thus, if the length (which here is the vertical length) is greatest at a center of the width of the slots, or in other words, at the middle of the distributor, the flow resistance difference between flow path to the center of the slot and the flow path to the sides of the slot can be largely eliminated, so that glass is more uniformly distributed from the middle toward the sides. Accordingly, the distributor according to present disclosure desirably has slots with a length and a width, wherein the length of the slots is greatest at a center of the width. According to one alternative, this may be achieved by introducing an angle “A” to the top of the slot, as shown in  FIG. 3 , which makes the flow resistance balanced throughout distributor, delivering a more uniform flow out of the slot. Also, an overall increase in the slot length will help for better uniformity of the flow profile at the exit. Plotted in  FIG. 8  is the outflow velocity profile achievable using this embodiment of distributor. A comparative outflow velocity profile for a distributor with constant length slot is plotted in  FIG. 9  as an example predicted by CFD modeling, showing the relatively high non-uniformity of flow that results. 
         [0038]    It will be apparent to those skilled in the art that other various modifications and variations can be made without departing from the spirit or scope of the claims.