Abstract:
A method and apparatus are provided to flame polish a thin glass sheet which includes cooling the thin glass sheet followed by heating the thin glass sheet with an intense flame to substantially reduce if not eliminate surface roughness of the glass sheet.

Description:
[0001]    The present inventive embodiments relate to polishing a glass sheet with heat. 
         [0002]    Typically, flat sheet glass is made by one of two processes: a float glass or a rolled glass process. Float glass is formed by floating molten glass on a bath of molten tin and typically has a higher surface quality than rolled glass. Rolled glass is formed by passing glass between a series of rollers. As there is physical contact between the glass and the rollers in the rolled glass process, there is the propensity for the glass surface to be marred, distorted or imprinted. The surface quality of glass sheets formed by the rolled glass process is not suitable for the manufacture of photovoltaic cells used in solar panels. 
         [0003]    One method of manufacturing display glass, e.g. liquid crystal display (LCD) or plasma display panel (PDP) glass, is by a float glass process. A thin (−0.5-0.8 mm thick) glass sheet is formed by the process of floating molten glass on a bath of molten tin. The glass sheet is then fed through a lehr to carefully cool the glass prior to cutting and polishing the sheet. The lehr is a long, heated oven through which the glass sheet moves so gradual cooling will properly anneal and remove stress from the glass. A polishing stage follows, which is an expensive, but essential, large-scale operation that contributes significantly to the cost of the final glass product and is responsible for a significant amount of product loss. The justification to polish LCD or PDP glass is to reduce roughness of the separate glass sheets, so that when placed together form a useable LCD or PDP display. Any roughness remaining on the individual sheets creates uneven distribution of liquid crystals. This uneven crystal distribution creates differences in the color of the LCD cell and hence impacts product quality and usability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    For a more complete understanding of the present embodiments, reference may be had to the following detailed description taken in conjunction with the drawings, of which: 
           [0005]      FIG. 1  shows a system for flame polishing of glass; 
           [0006]      FIG. 2  shows other features of the system of  FIG. 1 ; and 
           [0007]      FIG. 3  shows a graph of heat transfer with respect to types of flames. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    The present embodiments eliminate the need for a tin bath to produce high quality sheet glass for PV applications and substantially reduce if not eliminate the need for mechanical polishing of a glass sheet for display panel manufacture. The system embodiments apply high intensity flame polishing (or “fire-polishing”) to the moving glass sheet. The flame polishing involves the direct impingement of intense high temperature flames onto the glass surface. The application of the high temperature flame causes the surface layer of the glass to become molten and flow so as to reduce visible and microscopic surface roughness and imperfections. It is important to reduce the amount of heat transferred to the underlying bulk of the glass sheet so that the bulk glass temperature does not approach the softening point OT the glass and thereby cause sagging of the sheet. Were the softening point to be reached, the glass sheet could deform and stripes also may be created in the sheet as it moves through the flame. 
         [0009]    The surface layer of the glass sheet is heated, in the present embodiments, as rapidly as possible so as to minimize heat transfer into the bulk of the glass sheet below the sheet&#39;s surface. As the flame is applied to the surface of the glass sheet, heat will begin to penetrate into the bulk of the glass sheet moving away from the heated surface. Were the flame to heat the glass sheet at a slow rate, the temperature distribution within the glass sheet would be more uniform than where the glass is heated at a faster rate. The embodiments heat the surface of the glass sheet with a high intensity flame so that heat penetrates into the glass to a depth at which the heated surface of the glass sheet is molten and only up to one-third (⅓ rd ) of the glass sheet thickness is above its softening point, with the remaining bulk glass strong enough to resist softening. For example, heat is applied to the surface of the sheet for a total period of 0.5-4 seconds. 
         [0010]    The rate of heating the surface of the glass sheet is controlled by the nature of the flame impinging the glass sheet, including the flame temperature, composition and velocity. Heat transfer rates have been determined for a range of flames and increase in the order oxy-methane tip-mixed; oxy-hydrogen tip-mixed; oxy-methane premixed; and oxy-hydrogen premixed. 
         [0011]    “Tip-mixed” refers to the fuel and oxidant coming into contact only at a tip of the burner; also referred to as a diffusion flame. The flame forms at an interface of regions where the fuel and oxidant come together and so the rate of reaction is affected by the rates of mixing and not just the chemical kinetics. 
         [0012]    “Premixed” means that the fuel and oxidant are mixed together into a flammable mixture. In this case, as the reactants are already intermingled at the molecular scale, the formation of the flame is largely dependent on the chemical kinetics; i.e. once a flame is initiated it moves as a wave through the reactive mixture. The speed at which the wave or flame front moves is very much dependent on local composition, temperature, pressure and turbulence. In a jet type flame, the flame front is located where the local burning velocity (how fast the wave propagates) is equal to the local velocity. Premixed flames are more intense than diffusion (tip-mixed) flames and accordingly lead to a higher heat flux. Oxy-hydrogen flames have higher flame speeds than oxy-methane flames and lead to higher heat fluxes. The heat transfer rates increase from oxy-methane tip-mixed to oxy-hydrogen tip mixed to oxy-methane premixed to oxy-hydrogen premixed. 
         [0013]    As the intent is to soften only the heated side of the glass sheet which is relatively thin (approximately 2.8-5 mm thick) and in particular as a sheet of LCD/PDP glass is very thin (˜0.5-0.8 mm thick), a high intensity, premixed, oxy-hydrogen flame is preferred over other types of flames in order to address issues of softening and deformation. Such other types of flames such as premixed oxygen+methane\butane\propane\acetylene or substantive blends thereof with hydrogen, would be suitable for thicker glass sheets where a lower intensity flame would be acceptable. In order to allow the surface to melt and the bulk glass below the surface to remain below its softening point, the system embodiments cool the glass sheet before the application of the flame polishing. 
         [0014]    Such flame polishing by the embodiments may occur between the glass furnace and the lehr so that final stress removal of the sheet can take place. Polishing can take place in general between the furnace and the lehr. The glass may be polished when it is as hot as possible (save energy) and cold enough to maintain its shape. Minimum temperature for glass to be polished normally is above  5000 C. It might be possible to integrate the polishing into a front part of the lehr (if no space is available), as well as directly at an outlet of the furnace, when the glass has the required temperature (500-550° C.). A short distance to the lehr is preferred, to prevent glass from breaking by thermal shock as the polishing can increase tensile forces within the glass. Furthermore, it has been found that the glass plate should be supported after flame polishing to ensure that as the thermal distribution within the plate equalizes and the heat within the glass plate penetrates to a greater depth, that the plate will not sag during this post heating stage. It has been found that supporting the plate on a ceramic plate will provide the necessary support during the process. 
         [0015]    Referring to  FIGS. 1 and 2 , there is shown a system  10  of the present embodiments for flame polishing a glass sheet  20  such as for example a thin glass sheet. System  10  includes a cooling assembly  12  and a heating assembly  14  for use upon the glass sheet  20 , the glass sheet having opposed surfaces  20 A, 20 B. 
         [0016]    The system  10  is disposed between a melter shown generally at  16 , which can be for example a glass melting furnace, and a lehr  18  disposed at a downstream end of the glass sheet  20  processing. Transport assembly conveyors shown generally at  22 ,  24  transport the glass sheet  20  from the melter  16  to the system  10  (cooling assembly  12  and heating assembly  14 ), and from the system  10  to the lehr  18 , respectively. The conveyors  22 , 24  can include rollers, a mesh belt, conveyer belts, moving beds, moving plates such as for example a ceramic plate, etc. The transport mechanism needs to be of such construction that it can withstand the elevated temperatures generated in the system  10  and during support of the flame heated sheet  20 . One embodiment to transport the glass sheet  20  includes a smooth ceramic plate which provides uniform support for the sheet in order to minimize mechanical stresses in the sheet. 
         [0017]    As shown in  FIG. 2 , the glass sheet  20  upon entry into the system  10  has a side  20 A thereof exposed to the cooling assembly  12  and thereafter the heating assembly  14 . 
         [0018]    The cooling assembly  12  consists of a source  26  of a cooling fluid connected by a conduit  28  to a cooling discharge assembly  30 . The source  26  of the cooling fluid may include air or a cryogen, such as nitrogen or carbon dioxide gas. The conduit  12  is connected to the discharge assembly  30  which preferably is a nozzle or other atomizer discharge member for disbursing the cooling medium in a uniform, even flow along the surface  20 A of the glass sheet  20 . The cooling discharge assembly  30  provides as uniformly as possible a flow of cooling medium onto the glass surface  20 A. Cooling is performed uniformly, for the glass sheet being heated at a uniform temperature and as such reaches a uniform temperature across its upper surface  20 A. Deviations in the thermal treatment of the upper surface  20 A would lead to differential stresses, that could rebuilt in breakage and visible impairment of the surface. 
         [0019]    Cooling can be achieved for example by the discharge of a multitude of tiny jets from the assembly  30  arrayed across the upper surface  20 A, or by an air-knife type arrangement (not shown) that consists of a continuous convergent slot through which high velocity air is blown directly on to the surface  20 A at a steep angle with respect to the general direction of movement of the glass sheet  20 . Arrows  32  approximate the flow stream of the cooling medium, whether by jets or air-knife, for contacting the surface  20 A of the glass sheet  20 . 
         [0020]    The heating assembly  14  includes a source  34  of combustible fuel connected by a conduit  36  to a burner assembly  38 , such as for example an oxy-fuel burner. Burner  38  provides a flame  40  with a footprint  42  to contact and thus heat the surface  20 A of the glass sheet  20 . Although the burner  38  could be constructed in a number of ways, a typical burner for use in this application will be a long body of rectangular or circular cross section containing an internal plenum (not shown) such that a uniform pressure is delivered across the plenum from the premixed fuel and oxygen entering the plenum. 
         [0021]    A plenum discharge assembly having small holes or slots arranged across a side of the plenum oriented towards the glass sheet  20  may be used. Premixed fuel and oxygen are discharged through these holes at a velocity greater than a velocity that the flame  40  can propagate back into the plenum. Were the velocity of the premixed fuel and oxygen issuing through the holes to be less than a velocity at which the flame can move through the flammable premixed fuel and oxygen, then the flame would move towards the holes, as the flame would be moving faster than the premixed fuel and oxygen. This situation where the flammable mixture is moving slower than the velocity at which the flame would propagate through said mixture could ultimately result in the flame passing through the holes and into the plenum chamber. This phenomena is commonly termed as “flash-back” or “back-fire” and could result is the violent release of energy with potentially destructive consequences. Such occurrences are controlled by adopting of a combination of preventative measures such as but not limited to: ensuring the mixture cannot “flash-back”; the inclusion of flame arrestors or flash-back arrestors which are designed to extinguish the flame; pressure or explosion relief vents; minimizing of the volume of flammable gases enclosed by the system, i.e. the plenum and any mixing assembly; and by purging of the enclosure with inert gases. The burner assembly is lowered close to the glass surface  20 A such that the peak heat flux as indicated and discussed below with respect to  FIG. 3  is located on the glass surface  20 A and provides the maximum heating rate. 
         [0022]    In operation, the cooling or perhaps chilling effect of the cooling medium spray  32  compensates for the high intensity flame  40  so that only the surface  20 A is polished. This avoids the bulk of the glass sheet  20  underlying the surface  20 A from being heated above its softening point so that the smooth planar characteristics of surface  20 A and the glass sheet  20  are not compromised. 
         [0023]    The cooling and heating elements are operated such that the elevated temperatures generated within the glass sheet only penetrate to a depth of a approximately one-third (⅓ rd ) of the glass sheet thickness. 
         [0024]      FIG. 3  is a graphical representation shown generally at  50  of the heat transfer rates from different types of flames under consideration. The distance between a discharge end  44  of the burner  38  and the glass surface  20 A is represented on the horizontal axis  52  (Axial distance) and the magnitude of the heat transferred is shown on the vertical axis  54  (Heat transfer).  FIG. 3  shows the heat transfer compared to flame distance from the glass for a premixed oxy-hydrogen flame  56 , a postmixed oxy-hydrogen flame  58 , a premixed oxy-methane flame  60  and a postmixed oxy-methane flame  62 . As shown, a peak or maximum heat flux occurs at a certain distance from the glass surface  20 A which is different in both magnitude and location for each type of flame. At closer or greater distances from the glass surface  20 A than the peak heat flux, a smaller amount of heat would be transferred. For example, the premixed oxy-hydrogen flame generates a peak heat flux of nearly  1600  kW/m2 at a distance of approximately two (2) cm from the glass surface  20 A. This is twice as great as the heat flux from the post mixed oxy-hydrogen flame, over five (5) times greater than a premixed oxy-methane flame and eight (8) times greater than a postmixed oxy-methane flame. The premixed oxy-hydrogen flame generates a noticeably higher heat flux than the other types of flames and as such, would be more effective in melting just the surface layer  20 A of the glass  20  exposed to such a flame. 
         [0025]    This improvement in glass surface quality allows a glass sheet produced with an unacceptably rough surface to be subsequently used in a number of processes that require smoother surfaces. Examples of this are improving the surface quality of rolled sheet glass to allow photovoltaic cells to be applied and of float glass to be used in LCD or PDP display glasses. 
         [0026]    By application of flame polishing with the present embodiments, any irregularities in the surface of the rolled glass sheet are reduced to the point where the glass surface is suitably smooth for the photovoltaic cells. By using the rolled glass process with subsequent flame polishing the cost and complexity of operating the tin bath in the float process is avoided. Furthermore by avoiding the need for a tin bath a smaller glass production facility becomes financially feasible. 
         [0027]    It will be understood that the embodiments described herein are merely exemplary, and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. It should be understood that embodiments described above are not only in the alternative, but may also be combined.