Abstract:
A process produces a glass sheet. The process includes down-drawing a molten glass into a sheet-like glass ribbon, in which the molten glass is fed to a forming trough arranged in a forming furnace and the molten glass is caused to flow down from the forming trough through a conveyance passage extending vertically. The process also includes removing an internal strain in the glass ribbon in an annealing furnace, cooling the glass ribbon to around room temperature in a cooling chamber, and cutting the glass ribbon into a given size, in which the cooling chamber is provided with a gas exhausting passage, thereby exhausting air in the cooling chamber to an outside.

Description:
TECHNICAL FIELD 
     The present invention relates to a process and apparatus for producing a glass sheet, the process involving causing a molten glass to flow down from a forming trough, and vertically down-drawing a glass ribbon. 
     BACKGROUND ART 
     As a process for producing a glass sheet for various electronic instruments, in particular, a flat panel display such as a liquid crystal display, there is known a down-draw method, in which a glass sheet is produced by causing a molten glass to flow down from a forming trough, and vertically down-drawing a glass ribbon. 
     The down-draw method includes two methods, i.e., an overflow down-draw method and a slot down-draw method. In particular, the overflow down-draw method is widely known as a method, with which a glass sheet having very small waviness and roughness on its surface and being excellent in surface quality can be obtained. 
     The overflow down-draw method is a method, which involves continuously feeding a molten glass to a top portion of a forming trough having a wedge-shaped cross-section, causing the molten glass to flow down from the top portion of the forming trough along both side surfaces of the forming trough, and allowing the molten glasses to fuse at a lower end portion of the forming trough to form a sheet-like glass ribbon, and causing the glass ribbon to flow down through a conveyance passage extending vertically while holding the glass ribbon at both edge portions with a plurality of pulling rollers, thereby down-drawing the molten glass into a glass ribbon. With this process, the glass ribbon is gradually solidified, and a glass sheet having a given width and thickness is obtained. Further, atmospheric temperature in the conveyance passage is strictly controlled, and the internal strain (thermal strain) in the glass sheet is sufficiently reduced thereby. Then, the glass sheet is cooled to around room temperature. 
     In a case of a glass sheet for a liquid crystal display, in particular, if even only a minute internal strain remains in the glass sheet, birefringence occurs, and as a result, a uniform image is not provided. Thus, there have conventionally been proposed various ideas for uniformly cooling a glass ribbon as much as possible at a given temperature gradient. 
     For example, JP-A-5-124826 discloses a structure including supporting a roller on a single side for preventing occurrence of an internal strain in a glass sheet caused by the influence of the cooling of a roller shaft so as to prevent the deformation of the glass sheet, and also discloses a preventive plate against convection for separating a conveyance passage horizontally for preventing the internal strain in a glass ribbon caused by thermal convection occurring in the conveyance passage. 
     Further, JP-A-10-53426 discloses a process for producing a glass sheet having a small internal strain, the process involving forming a plurality of chambers by separating an internal space of a forming furnace or an annealing furnace horizontally, and allowing each chamber to have a room temperature-controlling function to carry out sufficient annealing. 
     Still further, JP-A-2001-31435 discloses a technology by which temperature distribution in the annealing furnace is also formed in a width direction of a glass ribbon, to thereby prevent a minute internal strain or deformation.
     Patent Document 1: JP-A-5-124826   Patent Document 2: JP-A-10-53426   Patent Document 3: JP-A-2001-31435   

     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In recent years, high definition and high quality are increasingly demanded for a liquid crystal display, and it is demanded for a glass sheet used for the liquid crystal display to have a maximum value of an internal strain of 1.0 MPa or less. Further, a glass sheet for a liquid crystal display is rapidly moving toward a large-size plate. For example, even a glass ribbon having a width of 2000 mm or more as the width size of a portion serving as a final glass product (effective width) is produced. However, as the size of a glass sheet produced is becoming larger, an internal strain in the glass sheet also tends to become larger, and hence it is becoming difficult to control the internal strain to 1.0 MPa or less. 
     One of the causes for the internal strain in a glass sheet is flow of air climbing along the surface of a glass ribbon (hereinafter referred to as low-temperature airflow). That is, in a conveyance passage of a glass ribbon, low-temperature airflow always climbs along the surface of the glass ribbon, causing easy variation of the atmospheric temperature in an annealing furnace. JP-A-5-139766 discloses formation of a preventive plate against convection in an annealing furnace, but because low-temperature airflow climbs near the surface of a glass ribbon, the preventive plate against convection cannot cut off the low-temperature airflow sufficiently. Further, when a preventive plate against convection is used to fully block low-temperature airflow, it is necessary for the distance between a glass ribbon and the preventive plate against convection to be very small, but in this case, the glass ribbon contacts the preventive plate against convection, thereby causing possible formation of flaws on the surface of the glass ribbon. 
     The present invention has been made in view of the above-mentioned circumstances. A technological object of the present invention is to provide a process for obtaining a glass sheet of high quality with good productivity by avoiding a problem with an internal strain that becomes serious as the move toward a large-size glass sheet progresses. 
     Means for Solving the Problem 
     The inventors of the present invention have made various studies to solve the above-mentioned problem. As a result, the inventors have found that the climb of low-temperature airflow can be suppressed in the conveyance passage of a glass ribbon by providing a cooling chamber with a gas exhausting passage, thereby reducing the amount of the low-temperature airflow flowing from the cooling chamber to the annealing furnace. Thus, the present invention has been proposed. 
     A first aspect of the invention made for solving the above-mentioned problem relates to a process for producing a glass sheet including: a forming step of down-drawing a molten glass into a sheet-like glass ribbon, in which the molten glass is fed to a forming trough arranged in a forming furnace and the molten glass is caused to flow down from the forming trough through a conveyance passage extending vertically; an annealing step of removing an internal strain in the glass ribbon in an annealing furnace; a cooling step of cooling the glass ribbon to around room temperature; and a cutting step of cutting the glass ribbon in a given size, in which the cooling chamber is provided with a gas exhausting passage, thereby exhausting air in the cooling chamber to an outside. 
     A second aspect of the invention made for solving the above-mentioned problem relates to the process for producing a glass sheet according to the first aspect further including exhausting the air in the cooling chamber into a chamber surrounding a forming furnace and/or an annealing furnace through the gas exhausting passage. 
     A third aspect of the invention made for solving the above-mentioned problem relates to the process for producing a glass sheet according to the first or second aspect, in which the forming step includes a step of forming a glass ribbon by an overflow down-draw method or a slot down-draw method. 
     A fourth aspect of the invention made for solving the above-mentioned problem relates to the process for producing a glass sheet according to any one the first to third aspects, in which a length of a short side of the glass sheet is 2000 mm or more. 
     A fifth aspect of the invention made for solving the above-mentioned problem relates to the process for producing a glass sheet according to any one of the first to fourth aspects, in which a maximum value of the internal strain of the glass sheet is 1.0 MPa or less. 
     A sixth aspect of the invention made for solving the above-mentioned problem relates to the process for producing a glass sheet according to any one of the first to fifth aspects, in which the glass sheet contains, in terms of mass %, 40 to 70% of SiO 2 , 2 to 25% of Al 2 O 3 , 0 to 20% of B 2 O 3 , 0 to 10% of MgO, 0 to 15% of CaO, 0 to 10% of SrO, 0 to 15% of BaO, 0 to 10% of ZnO, 0 to 10% of ZrO 2 , and 0 to 2% of an fining agent. 
     A seventh aspect of the invention made for solving the above-mentioned problem relates to an apparatus for producing a glass sheet including: a forming furnace for down-drawing a molten glass into a sheet-like glass ribbon, in which the molten glass is fed to a forming trough and the molten glass is caused to flow down from the forming trough through a conveyance passage extending vertically; an annealing furnace for removing an internal strain in the glass ribbon; a cooling chamber for cooling the glass ribbon to around room temperature; and a cutting chamber for cutting the glass ribbon in a given size, in which the cooling chamber is provided with a gas exhausting passage. 
     An eighth aspect of the invention made for solving the above-mentioned problem relates to the apparatus for producing a glass sheet according to the seventh aspect, in which the gas exhausting passage of the cooling chamber is communicated with a chamber surrounding a forming furnace and/or an annealing furnace. 
     Effects of the Invention 
     In the process for producing a glass sheet, which include a forming step of down-drawing a molten glass into a sheet-like glass ribbon, in which the molten glass is fed to a forming trough arranged in a forming furnace and the molten glass is caused to flow down from the forming trough through a conveyance passage extending vertically, an annealing step of removing an internal strain in the glass ribbon in an annealing furnace, a cooling step of cooling the glass ribbon to around room temperature, and a cutting step of cutting the glass ribbon in a given size, the invention according to the first aspect provides the cooling chamber with a gas exhausting passage to exhaust the air in the cooling chamber to the outside, to thereby allow the air in the cooling chamber to be exhausted while being dispersed into both of the conveyance passage of the glass ribbon and the gas exhausting passage, with the result that the climb of low-temperature airflow can be suppressed in the conveyance passage. As a result, the variation of the atmospheric temperature in the annealing furnace can be suppressed to a minimum extent, leading to a sufficient reduction in the internal strain in a glass sheet even if the size of the glass sheet becomes large. The cooling chamber is preferably provided with the gas exhausting passage at its ceiling portion because the air-exhausting effect becomes larger. 
     The invention according to the second aspect allows the air in the cooling chamber to be exhausted to a chamber surrounding a forming furnace and/or an annealing furnace through the gas exhausting passage, to thereby elevate the pressure in the chamber surrounding a forming furnace and/or an annealing furnace. As a result, an effect of suppressing the climb of the low-temperature airflow in the conveyance passage of the glass ribbon becomes larger. That is, the low-temperature airflow that has climbed from the cooling chamber into the annealing furnace is heated in the annealing furnace, and then part of the low-temperature airflow leaks into the outside atmosphere through the gaps of furnace walls of the forming furnace and/or an annealing furnace. However, the elevation of the pressure in the chamber surrounding a forming furnace and/or an annealing furnace suppresses the leakage of the inside air of the forming furnace and/or annealing furnace. As a result, the effect of suppressing the climb of the low-temperature airflow in the conveyance passage of the glass ribbon becomes larger. 
     By using the invention according to the third aspect, the forming step is a step for forming a glass ribbon by an overflow down-draw method or a slot down-draw method and hence a thin plate glass can be efficiently formed. In order to obtain a plate glass particularly excellent in surface quality, it is desirable to employ the overflow down-draw method rather than the slot down-draw method. It should be noted that the slot down-draw method is a method involving supplying a molten glass to a forming trough having an aperture in a long-hole shape (slot shape), then pulling the molten glass out of the aperture of the forming trough to form a sheet-like glass ribbon, and vertically extending and forming the glass ribbon into a glass sheet. 
     Further, in the present invention, when the glass ribbon is cut in a given size, the glass ribbon flowing down vertically may be cut in the width direction of the glass ribbon (direction perpendicular to the flowing-down direction of the glass ribbon) from the cooling step, or the glass ribbon may be bent from the vertical direction to the horizontal direction and cut in the width direction while being moved in the horizontal direction. 
     By using the invention according to the fourth aspect, the glass sheet has a length of the short side of 2000 mm or more, and hence many glass sheets for a display panel can be cut out from one piece of the glass sheet (original sheet), enabling the improvement in the production efficiency. The larger the size of the glass sheet, the larger the internal strain of the glass sheet tends to be. One of the reasons for that is considered to be that when the size of the glass sheet becomes larger, the apparatus for producing a glass sheet has to be larger, resulting in the easiness for low-temperature air to flow in the conveyance passage of the glass ribbon, with the result that the variation of the atmospheric temperature in the annealing furnace easily occurs. Thus, the present invention becomes useful when the invention is used for producing particularly large glass sheets, specifically, a glass sheet having a length of the short side of 2000 mm or more, preferably 2500 mm or more, or still more preferably 3000 mm or more. 
     By using the invention according to the fifth aspect, the glass sheet has a maximum value of an internal strain of 1.0 MPa or less, and hence the image of a liquid crystal display is prevented from becoming nonuniform due to birefringence. By using the present invention, the variation of the atmospheric temperature in the annealing furnace can be suppressed to a minimum extent, leading to the suppression of the internal strain in the glass sheet even if the size of the glass sheet becomes large. To be specific, it becomes possible to control the maximum value of the internal strain to 1.0 MPa or less, 0.8 MPa or less, or further 0.7 MPa or less. 
     By using the invention according to the sixth aspect, the glass sheet contains, in terms of mass %, 40 to 70% of SiO 2 , 2 to 25% of Al 2 O 3 , 0 to 20% of B 2 O 3 , 0 to 10% of MgO, 0 to 15% of CaO, 0 to 10% of SrO, 0 to 15% of BaO, 0 to 10% of ZnO, 0 to 10% of ZrO 2 , and 0 to 2% of a fining agent, and hence the glass sheet satisfies characteristics such as chemical resistance (good acid resistance, good alkali resistance, and good buffered-hydrofluoric-acid resistance), thermal resistance (strain point of 630° C. or more), meltability (1600° C., temperature corresponding to a viscosity of 10 2.5  poise), formability (liquidus temperature of 1150° C. or less), and thermal expansion coefficient (25 to 45×10 −7 /° C. at a temperature of 30 to 380° C.). Thus, it is possible to obtain a glass sheet for a liquid crystal display in which the internal strain after formation can be easily suppressed. 
     The reason why the above-mentioned glass composition is preferred is as follows. 
     SiO 2  is a component for forming the network of glass and has effects of reducing the thermal expansion coefficient of glass, making the internal strain smaller, improving the acid resistance of glass, and elevating the strain point of glass to make the thermal contraction of a glass sheet smaller. However, when the content of SiO 2  becomes large, the viscosity of glass becomes higher, with the result that meltability tends to deteriorate and devitrified stones of cristobalite tends to precipitate. Thus, the content of SiO 2  is 40 to 70%, preferably 50 to 67%, or more preferably 57 to 64%. 
     Al 2 O 3  is a component for reducing the thermal expansion coefficient of glass and for making the internal strain of a glass sheet smaller. Further, Al 2 O 3  has effects of elevating the strain point of glass and suppressing the precipitation of devitrified stones of cristobalite as well. However, when the content of Al 2 O 3  becomes large, the buffered-hydrofluoric-acid resistance of glass deteriorates and liquidus temperature rises, resulting in a difficulty in formation. Thus, the content of Al 2 O 3  is 2 to 25%, preferably 10 to 20%, or more preferably 14 to 17%. 
     B 2 O 3  is a component, acting as a melting accelerate component, for reducing the viscosity of glass and for improving the meltability. Further, B 2 O 3  is a component for reducing the thermal expansion coefficient of glass and for making the internal strain of a glass sheet smaller. However, when the content of B 2 O 3  becomes large, the strain point of glass easily lowers and the acid resistance easily deteriorates. Thus, the content of B 2 O 3  is 0 to 20%, preferably 5 to 15%, or more preferably 7.5 to 12%. 
     MgO is a component for improving the meltability of glass by reducing only the viscosity of the glass without reducing the strain point. However, when the content of MgO becomes large, devitrified stones easily precipitate in glass, and buffered-hydrofluoric-acid resistance lowers, with the result that when a glass sheet is treated with a buffered hydrofluoric acid, the surface of the glass sheet is corroded, a reaction product attaches to the surface, and the surface easily becomes clouded. Thus, the content of MgO is 0 to 10%, preferably 0 to 5%, or more preferably 0 to 3.5%. 
     CaO is a component for improving the meltability of glass by reducing only the viscosity of the glass without reducing the strain point. However, when the content of CaO becomes large, buffered-hydrofluoric-acid resistance easily deteriorates. Thus, the content of CaO is 0 to 15%, preferably 0 to 12%, or more preferably 3.5 to 9%. 
     SrO is a component for enhancing the chemical resistance and denitrification resistance of glass. However, when the content of SrO becomes large, the thermal expansion coefficient of glass easily becomes large and the internal strain of a glass sheet tends to become large. Thus, the content of SrO is 0 to 10%, preferably 0 to 8%, or more preferably more than 0.5 to 8%. 
     BaO is a component for enhancing the chemical resistance and denitrification resistance of glass similar to SrO. However, when the content of BaO becomes large, the density and thermal expansion coefficient of glass tends to become large, and meltability of glass tends to remarkably deteriorate. Thus, the content of BaO is 0 to 15%, preferably 0 to 10%, or more preferably 0 to 8%. 
     ZnO is a component for improving the buffered-hydrofluoric-acid resistance and meltability of glass. When the content of ZnO becomes large, the denitrification resistance and strain point of glass easily lowers. Thus, the content of ZnO is 0 to 10%, preferably 0 to 5%, or more preferably 0 to 1%. 
     ZrO 2  is a component for raising the strain point of glass. When the content of ZrO 2  becomes large, the density of glass remarkably increases, and devitrified stones derived from ZrO 2  easily precipitate. Thus, the content of ZrO 2  is 0 to 10%, preferably 0 to 7%, or more preferably 0 to 5%. 
     As 2 O 3 , Sb 2 O 3 , SnO 2 , SO 3 , F, Cl, or the like can be used as a fining agent up to 2%. It should be noted that the use of As 2 O 3  and Sb 2 O 3  should be avoided because they are environmental load substances, and when the use of As 2 O 3  and Sb 2 O 3  is avoided, SnO 2  is contained preferably at 0.01 to 2%. 
     In addition, in the present invention, it is possible that, other than the above-mentioned components, for example, up to 3% each of Y 2 O 3 , La 2 O 3 , Nb 2 O 3 , and P 2 O 5  can be contained for reducing the liquidus temperature of glass to improve the formability. It should be noted that alkali metal oxides (R 2 O) such as Na 2 O, K 2 O, and Li 2 O should not be contained because when those components are contained, the characteristics of various films and TFT devices formed on the glass sheet for a liquid crystal display may be degraded. To be specific, the content in terms of R 20  must be regulated at 0.1% or less. 
     By using the invention according to the seventh aspect, in which an apparatus for producing a glass sheet includes a forming furnace for down-drawing a molten glass into a sheet-like glass ribbon, in which the molten glass is fed to a forming trough and the molten glass is caused to flow down from the forming trough through a conveyance passage extending vertically, an annealing furnace for removing an internal strain in the glass ribbon; a cooling chamber for cooling the glass ribbon to around room temperature; and a cutting chamber for cutting the glass ribbon in a given size, the cooling chamber being provided with a gas exhausting passage to exhaust the air in the cooling chamber to the outside, to thereby allow the air in the cooling chamber to be exhausted while being dispersed into both of the conveyance passage of the glass ribbon and the gas exhausting passage, with the result that the climb of low-temperature airflow can be suppressed in the conveyance passage. As a result, the variation of the atmospheric temperature in the annealing furnace can be suppressed to a minimum extent, leading to a sufficient reduction in the internal strain in a glass sheet even if the size of the glass sheet becomes large. 
     In the invention according to the eighth aspect, because the gas exhausting passage of the cooling chamber is communicated with the chamber surrounding a forming furnace and/or an annealing furnace, the air in the cooling chamber is allowed to flow into the chamber surrounding a forming furnace and/or an annealing furnace, causing the elevation of the pressure in the chamber, to thereby provide a rare possibility for the inside air in the forming furnace and/or annealing furnace to leak into the outside through the gaps of furnace walls of the furnaces. As a result, an effect of suppressing the climb of the low-temperature airflow in the conveyance passage of the glass ribbon becomes larger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic front view illustrating an apparatus for producing a glass sheet of the present invention. 
         FIG. 2  is a schematic front view illustrating an apparatus for producing a glass sheet of a comparative example. 
     
    
    
     DESCRIPTION OF SYMBOLS 
     
         
           10  forming trough 
           11  forming furnace 
           11   a  furnace wall of forming furnace 
           12  annealing furnace 
           12   a  furnace wall of annealing furnace 
           13  cooling chamber 
           14  cutting chamber gas exhausting passage 
           16  forming chamber 
           17  peripheral wall portion 
           18  conveyance passage 
           19  cooling roller (edge roller) pulling roller (annealing roller) 
           21  supporting roller 
           22  air communication hole 
         A molten glass 
         B glass ribbon 
         C glass sheet 
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention is described in detail in reference to a figure attached. 
       FIG. 1  is a schematic front view illustrating an apparatus for producing a glass sheet of the present invention. The production apparatus is for producing a glass sheet (glass substrate) for a liquid crystal display by an overflow down-draw method. In the order from the top, the production apparatus is provided with a forming furnace  11  for forming a glass ribbon B by overflowing a molten glass A supplied to a forming trough  10  having a wedge-shaped cross-section from the top portion of the forming trough  10  and allowing the molten glass A to fuse at the lower end portion of the forming trough  10 , an annealing furnace  12  for removing the internal strain in the glass ribbon B while annealing the glass ribbon B, a cooling chamber  13  for sufficiently cooling the glass ribbon B annealed, and a cutting chamber  14  for cutting the glass ribbon B cooled in a given size. In addition, the cooling chamber  13  is provided with a gas exhausting passage  15  at its ceiling portion. The forming furnace  11  and the annealing furnace  12  are surrounded by a forming chamber  16 . The cooling chamber  13  and the forming chamber  16  are communicated with each other via the gas exhausting passage  15 . The cooling chamber  13 , the cutting chamber  14 , and the forming chamber  16 , which are neighboring in the vertical direction, are surrounded by a peripheral wall portion  17  having airtightness. The forming furnace  11 , the annealing furnace  12 , the cooling chamber  13 , and the cutting chamber  14  are communicated with each other via a conveyance passage  18  through which the glass ribbon B flows down. Further, the cutting chamber  14  is provided with another conveyance passage for conveying a glass sheet C to a subsequent step (for example, edge-polishing step) which is not shown. 
     Next, a production process for a glass sheet using the above-mentioned apparatus for producing a glass sheet is described. 
     In the production apparatus, the molten glass A is first supplied to the top portion of the forming trough  10  provided in the forming furnace  11 , the molten glass A is caused to overflow from the top portion of the forming trough  10 , and the molten glass A is fused at the lower end portion of the forming trough  10 , to thereby form into a sheet-like glass ribbon B. In the vicinity of the forming trough  10 , a pair of cooling rollers (edge rollers)  19  are provided, and the cooling rollers  19  hold both edges of the glass ribbon B, thereby suppressing its contraction in the width direction to a minimum extent. 
     Next, annealing the formed glass ribbon B in the annealing furnace  12  removes its internal strain. The annealing furnace  12  is provided with a plurality of pairs of pulling rollers (annealing roller)  19  in the vertical direction, and the glass ribbon B is pulled downward while the pulling rollers  20  are pulling the glass ribbon B in the width direction to prevent the glass ribbon B from contracting in the width direction because of surface tension or the like. In addition, the annealing furnace  12  is set so as to have a given temperature gradient controlled by a heater (not shown). Thus, the temperature of the glass ribbon B is gradually lowered as the glass ribbon B flows down through the annealing furnace  12 , thereby removing the internal strain. 
     In addition, the cooling chamber  13  in the downstream of the annealing furnace  12  is provided with a plurality of pairs of supporting rollers  21 , which pull downward the glass ribbon B solidified in a given width and given thickness. The glass ribbon B is cooled to around room temperature in the cooling chamber  13 . In addition, the air in the cooling chamber  13  flows into both the annealing furnace  12  and the gas exhausting passage  15 , and the air that has flown into the gas exhausting passage  15  flows into the forming chamber  16 . As a result, the amount of the air flowing into the annealing furnace  12  is reduced, suppressing the climb of the low-temperature airflow in the conveyance passage  18  of the glass ribbon. 
     The glass ribbon cooled to around room temperature in the cooling chamber  14  is cut into glass sheets C having a given size in the cutting chamber  14 , and the glass sheets are conveyed to a subsequent step. 
     The above-mentioned apparatus for producing a glass sheet was used to form a glass sheet for a liquid crystal display containing, in terms of mass %, 60% of SiO 2 , 15% of Al 2 O 3 , 10% of B 2 O 3 , 6% of CaO, 6% of SrO, 2% of BaO, and 1% of an fining agent (OA-10, manufactured by Nippon Electric Glass Co., Ltd.). 
     The dimension of the glass sheet obtained was 2360×2030×0 0.7 mm. The maximum strain of the glass sheet was measured and was 0.8 MPa. 
     Further,  FIG. 2  is a schematic front view illustrating an apparatus for producing a glass sheet of a comparative example. The structure of the apparatus is the same as that of the apparatus in  FIG. 1  except that a cooling chamber  13  is not provided with a gas exhausting passage. The apparatus in  FIG. 2  was used to produce a glass sheet in the same conditions as those in the above-mentioned embodiment. The maximum strain of the glass sheet was measured and was 1.1 MPa. 
     The foregoing shows that the glass sheet obtained in the embodiment has a smaller maximum strain than the glass sheet obtained in the comparative example, and hence the present invention has a greater effect of reducing the internal strain of a glass sheet by providing a gas exhausting passage leading from the cooling chamber to a chamber surrounding the annealing furnace. 
     Here, the maximum strain of a glass sheet was determined by measuring strain stress from the birefringence amount of the glass sheet through an optical heterodyne interferometry with a strain indicator manufactured by Uniopt Co., Ltd. The reason why the maximum strain of a glass sheet was determined is that if even only one strong strain is present in the glass sheet, the glass sheet does not meet the product specification for a glass sheet for a liquid crystal display. 
     It should be noted that the present invention is not limited to the above-mentioned embodiment, and may be carried out in any other various embodiments as long as the embodiments do not deviate from the gist of the present invention. 
     For example, the above-mentioned embodiment described the case where the present invention was applied to the production of a glass sheet by an overflow down-draw method. In addition to that, for example, the present invention can be likewise applied to the production of a glass sheet by a slot down-draw method. 
     Further, although the embodiment described the case where the forming furnace and the annealing furnace were surrounded by one chamber (forming chamber), the forming furnace and the annealing furnace may be each surrounded by different chambers (for example, forming chamber and annealing chamber). In that case, the gas exhausting passage of the cooling chamber is provided so as to lead to the annealing chamber. 
     Further, although the embodiment described the case where the gas exhausting passage was provided near the annealing furnace, the gas exhausting passage may be provided apart from the annealing furnace. In addition, it is recommended that the shape and size of the gas exhausting passage be suitably set depending on the size of the cooling chamber and annealing furnace or the like. 
     INDUSTRIAL APPLICABILITY 
     The process and apparatus for producing a glass sheet of the present invention can be used for the production, mainly of a glass sheet for a liquid crystal display, of a glass sheet used for various flat panel displays, for example, a plasma display, an electroluminescence display such as an OLED display, and a field emission display, and of a glass sheet used as a substrate on which various devices with an electronic display function or various thin films are formed.