Patent Abstract:
The method of making thin glass panes includes conveying a glass melt through a vertical inlet to a drawing tank with a slit nozzle; drawing a glass ribbon downward from the slit nozzle; setting a total throughput by setting length and cross section of the inlet and by heating and cooling the inlet to control glass melt viscosity so that pressure in the inlet decreases; and setting a throughput per unit of length in a lateral direction along the glass ribbon via nozzle system geometry and by heating and cooling of the drawing tank and slit nozzle to control melt viscosity, so that glass does not wet an underside of the slit nozzle near a breaking edge. The setting of the total throughput and the throughput per unit length are largely decoupled to simplify process control. An inventive apparatus for performing the method is also disclosed.

Full Description:
CROSS-REFERENCE 
   This is a continuation of U.S. patent application Ser. No. 10/451,559 filed Nov. 14, 2003 now abandoned, which is the U.S. National Stage of PCT/EP 01/15654, filed on Dec. 13, 2001, in Europe, which claims the priority of German Patent Application 100 64 977.7, filed on Dec. 23, 2000, in Germany, under 35 U.S.C. 119 (a) to (d) and 35 U.S.C. 365 (b). 

   BACKGROUND OF THE INVENTION 
   The invention relates to a method for producing glass panes, in particular glass panes with a thickness of less than 3 mm, by drawing a thin glass ribbon vertically downward, in which a glass melt is conveyed from a tank furnace, through an inlet, and to a drawing tank with a nozzle system that has at least one debiteuse. The invention also relates to a device comprising a tank furnace, a homogenization system, an inlet and a drawing tank, wherein the drawing tank has a nozzle system having at least one slit nozzle. 
   Thin glass panes of this kind are used as substrate glass in electronic devices, for example displays (portable telephones, flat screens, etc.) and digital mass storage devices of computers. Therefore extremely high demands are placed on the internal quality of the glass that is essentially determined by bubbles and inclusions, on the cleanness, on the quality of the surface geometry that is essentially determined by the fine corrugations (waviness) and by deviations from flatness (warp), on the breaking strength, and possibly also on the low weight. 
   When the glasses are used as substrate glass for displays, clients subject them to thermal manufacturing processes at temperatures that approach the transition temperature of glass. The dimensional stability of the glass substrates must be maintained in the course of this. Therefore special glasses or even glass ceramics with increased glass transition temperatures are used as substrate glasses, most of which have an increased tendency to crystallize and an atypical viscosity curve as a function of the temperature. These glasses consequently require higher processing temperatures than standard glasses. 
   Since these applications are mass produced products, it is necessary to manufacture the glass panes as inexpensively as possible. It is thus desirable to achieve a high degree of process stability with short set-up times and down times, with high throughputs and low waste, which are caused by glass defects and in the border region. In addition, it is necessary to largely fulfill the client requirements with regard to the cleanness of the surface and the quality of the surface geometry, thus allowing the high-cost processing steps of finishing work, e.g. grinding and polishing, to be reduced or eliminated. 
   In the drawing methods that have been known up to this point, the above-mentioned requirements with regard to the product quality and economy are only partially fulfilled. 
   Among the manufacturing methods, a distinction is drawn between drawing methods with and without a debiteuse. 
   In the drawing method without a debiteuse, which is described for example in U.S. Pat. No. 3,338,696, a trough is used into which the glass melt is conveyed. The glass melt runs over the upper edge of the trough walls and travels downward along the outsides of the wedge-shaped trough. At the vertex point, the glass films thus produced flow together and are drawn downward. The ceramic trough is clad with platinum to reduce corrosion. 
   In this method, the total throughput is essentially determined by the inlet between the tank furnace and the trough. The so-called linear throughput, which is understood to be the throughput per unit of length lateral to the drawing direction of the glass ribbon, is adjusted by means of the glass flow in the trough, the glass level in the vicinity of the overflow lip of the trough, the geometry of the overflow lip, and the viscosity of the glass. A very exact temperature regulation is required, which must be to within 0.1° C. 
   Format or throughput changes require the geometry of the drawing trough to be adapted, in particular for the glass flow in the trough. Since it takes up to a week or more to start up a new drawing trough, the desired flexibility of production is only achieved to a limited degree. 
   The corrosion of the trough edges cannot be compensated for by tilting or stretching the trough. The trough must be replaced and the process must then be started up again. 
   In order to produce a wide glass ribbon with narrow borders, the glass ribbon must be stretched lateral to the ribbon direction, by means of so-called border rollers in the edge region. The border rollers increase the complexity of the manufacturing process. 
   Among the drawing methods with a slit nozzle, a distinction is drawn between those with and without draw bars or spreaders. 
   In the method with a slit nozzle, but without a draw bar described for example in SU 617,390, the glass from the refiner or conditioner runs over the opposing walls on both sides of an overflow weir made of a fireproof material. The two glass films thus produced flow together above a nozzle and are then drawn downward. The throughput is regulated by means of the glass level at the overflow lips. This can occur either by changing the glass level in the refiner or by more deeply immersing the overflow block. 
   The method with a slit nozzle but without a draw bar cannot fulfill the increased demands for surface quality, in particular with regard to fine corrugations (waviness). Because of the short dwell time of the glass in the onion region and because of the high viscosity of the glass, the irregularities do not heal. 
   It is also impermissible to exceed certain processing temperatures due to the increased instabilities in the onion region when the glass melt falls below critical viscosities. Consequently, the known drawing methods without a draw bar cannot be used to produce special glasses with an increased tendency to crystallize. 
   The disadvantages described above are partially avoided through the use of a slit nozzle with an internal draw bar. 
   U.S. Pat. No. 1,759,229 describes a drawing method for producing flat glass, in which glass flows through a slit nozzle that is provided in the bottom of a refiner or conditioner, onto a spreader with a rhomboid cross section, and is drawn downward over this spreader. Underneath the slit nozzle, the spreader is fitted into a recess that widens out toward the bottom. The glass bath depth of the refiner or conditioner increases toward the ends of the slit nozzle. Both the nozzle and the spreader can be contoured. It is essential here that the nozzle slit widens out toward the edges and that the alignable spreader that is used can be bowed upward in the middle. 
   The lower, wedge-shaped part of the spreader is provided with an enclosed region that can contain heating or cooling elements extending lateral to the drawing direction. The temperatures of the spreader can be adjusted by means of flows of temperature control mediums. 
   The glass flow is influenced by the geometry of the nozzle and spreader, by the temperature level and profile in the refiner or conditioner, and by the position of the spreader in relation to the nozzle. Among other tasks, the spreader must homogenize the temperatures of the glass lateral to the drawing direction so that flat glass is produced with the predetermined dimensions and a uniform appearance. 
   DE-AS 15 96 484 has disclosed a device, which includes a homogenizing receptacle that is connected to a drawing furnace by means of a closed, heatable channel. The drawing furnace is equipped with a nozzle, which is provided with a slit made of platinum at the bottom. Underneath the nozzle, a draw bar is provided in the form of a vertically situated plate. The glass melt emerging from the nozzle travels downward on both sides of the draw bar and comes together at the bottom end to form a glass ribbon. 
   For heat dissipation from the inside, bores are provided in the draw bar, through which coolant can be conveyed. In addition, cooling bodies are attached to the outside of the draw bar at the bottom. The height of the draw bar can be adjusted by means of adjusting screws. 
   JP 2-217,327 has disclosed a device for producing flat glass in which the glass is drawn downward through a slit that is provided in the bottom of a feeder or a refiner. The slit nozzle can be heated indirectly. The glass flow is shut off with the aid of a plunger disposed over the slit. In order to stabilize the glass ribbon, the debiteuse has a plate-shaped internal draw bar disposed in it, which is secured at the sides and can be adjusted in position, whose upper section protrudes into the debiteuse and whose lower part is encapsulated. 
   U.S. Pat. No. 1,829,639 describes a method and a device with which the total throughput and the linear throughput are adjusted only by means of the geometry of the nozzle system, the viscosity of the glass melt in the vicinity of the nozzle, and the pressure of the glass melt through the nozzle. Upon emerging from the nozzle, the glass melt has only a slight excess pressure due to the open storage system with a low glass fill level. The essential disadvantage of this device and this method lies in the coupling of the total throughput and the linear throughput. 
   The known methods with a slit nozzle and draw bar cannot achieve the excellent surface qualities that can be achieved with the known drawing methods without the slit nozzle. Particularly at high linear throughputs and high processing temperatures, the dwell times of the glass film on the draw bar are not sufficient to heal the waviness of the surface, which is caused by the wetting of the nozzle in the vicinity of the breaking edge. 
   In the known methods with the slit nozzle and without a draw bar, the adjustments to the total throughput and the adjustment of the linear throughput (thickness distribution lateral to the drawing direction) are coupled to each other. 
   In format or throughput changes, the nozzle geometry and the temperature control in the vicinity of the nozzle must be constantly renewed in order to be able to make adjustments in a generally empirical fashion. The starting processes last a long time and the desired flexibility of production is only achieved to a limited degree. 
   In order to adjust the specified thickness distribution, defined temperature profiles must be set lateral to the drawing direction in the vicinity of the nozzle. The temperature profiles imparted to the glass ribbon can only be partially balanced up to the fine annealing zone. This can lead to an impermissible deformation of the glass ribbon (warp) during the cooling to room temperature. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide a method that can be used among other things to process glasses with a more pronounced tendency to crystallize or to form glass ceramics, wherein the quality of the thin glass panes is improved and the productivity is increased. Another object of the invention is to provide a device for carrying out this method. 
   This object is attained with a method of producing thin glass panes, in particular glass panes with a thickness of less than 1 mm, by drawing a thin glass ribbon vertically downward. This method comprises the steps of:
         a) conveying a glass melt from a tank furnace, through a vertical inlet and to a drawing tank with a nozzle system, which has at least one slit nozzle;   b) setting a total throughput by means of a length and cross section of the vertical inlet, by definite heating and cooling of the vertical inlet, and by means of glass melt viscosity in the vertical inlet so that a pressure of the glass melt in the vertical inlet, which is due to a level difference between a free surface height of the glass melt and a nozzle system height, significantly decreases; and   c) setting a throughput per unit of length in a direction lateral to the glass ribbon by means of nozzle system geometry, by definite heating and cooling of segments of the drawing tank and the at least one slit nozzle, by means of glass melt viscosity in the nozzle system, so that, upon emerging from the nozzle system, glass does not wet an underside of the at least one slit nozzle in the vicinity of a breaking edge.       

   The method according to the invention fulfills the above-described requirements with regard to product quality and economy. 
   The method combines the advantages of the drawing method with a nozzle and the advantages of the drawing method without a nozzle. This means that glass thicknesses of 20 μm to 3000 μm can be adjusted, with a good thickness distribution in the lateral direction and in the drawing direction, with thickness fluctuations of &lt;20 μm and a small border width. Moreover, an excellent surface quality is achieved, i.e. with a waviness of less than 20 nm. In addition, the warp is extremely slight. High processing temperatures and low processing viscosities (5×10 3  to 3×10 4  dPas) are also possible for the production of glasses or glass ceramics that are susceptible to crystallization. In addition, a large degree of flexibility with regard to formats is possible, whose widths can be adjusted between 300 mm and 2000 mm. Furthermore, high linear throughputs (mass of glass per ribbon width per unit time&gt;5 g/(mm×min) can be achieved with a high degree of process stability. By contrast, this largely avoids the disadvantages of the drawing method with and without nozzles. 
   The inlet, the drawing tank, and the nozzle system preferably constitute a closed system. The total throughput is set by means of the length arid cross section of the inlet and by means of the viscosity of the glass melt disposed in the inlet. On the other hand, the throughput per unit of length in a direction lateral to the glass ribbon (linear throughput) is set to the definite thickness setting by means of the geometry of the nozzle system as well as by means of the viscosity of the glass melt along the nozzle. The pressure of the glass melt, which is generated by the level difference between the height of the free surface of the glass melt and the height of the nozzle system, is reduced by the steps taken according to the invention, essentially in the inlet. The pressure drop in the vicinity of the nozzle system is low so that the glass melt widens only to an insignificant degree upon emerging from the slit nozzle. The wetting of the nozzle in the vicinity of the breaking edge achieves improved surface qualities in comparison to the known drawing methods. 
   According to the invention, the adjusting of the total throughput is largely decoupled from the adjusting of the linear throughput. The total throughput is adjusted through the definite heating and cooling of the inlet. The linear throughput and therefore the thickness distribution lateral to the ribbon direction, is adjusted through the definite heating and cooling of drawing tank segments, the slit nozzle, and the draw bar. This means that changes in the pressure drop in the vicinity of the drawing tank and the nozzles e.g. caused by temperature changes in the region, have only an insignificant influence on the total throughput. This increases the stability of the total throughput and therefore the stability of the thickness distribution of the glass ribbon in the Z-direction (see the coordinate system in  FIG. 1 ). The decoupling of the different process steps simplifies the process control. The times for initiation processes are reduced and the flexibility of production, e.g. changes to the total throughput or changes to the glass ribbon thickness, are increased in comparison to the known drawing methods due to the broadening of the process window. The equipment does not have to be adapted to the changed geometries of the glass ribbon or to changed process parameters. 
   The following temperature ranges for the inlet (ZL), drawing tank (ZT), and slit nozzle (SD) have turned out to be advantageous for an alkali-free borosilicate glass with Tg of approx. 700° C., a density of approx. 2.5 g/cm 3 , and a thermal expansion of approx. 37×10 −7 /K:
 
 T   ZL1   =Tg+ 670 K to  T   ZL2   =Tg+ 590 K
 
 T   ZT1   =Tg+ 590 K to  T   ZT2   =Tg+ 570 K
 
 T   SD   =Tg+ 570 K to  Tg+ 550 K,
 
Tg here stands for the glass transition temperature.
 
   Although the method also furnishes good results with regard to the surface quality even without a draw bar, a further improvement can be achieved through the use of a draw bar. The draw bar splits the glass melt flow as it passes through the slit nozzle. The glass melt flows downward on both sides of the draw bar. The two glass films melt back together again to form one glass ribbon at the bottom end of the draw bar. 
   Preferably, the dwell time and the viscosity of the glass films on the draw bar are adjusted in such a way that the deviations from the ideal surface contour heal almost completely. In order to achieve this, the glass films are preferably selectively heated and/or cooled on the draw bar. 
   Due to the long dwell time of the two glass films on the draw bar while the viscosity of the glass is low, deviations from the ideal surface contour due to surface tensions heal almost completely. A surface quality is achieved, which is comparable to the surface quality of the drawing method without the nozzle (waviness&lt;20 mm). 
   In this connection, preferably the following temperature is set in the draw bar (LK):
 
 T   LK   =Tg+ 560 K to  Tg+ 540 K.
 
This corresponds to a glass viscosity of 1×10 4  dPas.
 
   The slit width between the slit nozzle and the draw bar can, for example, be 10 mm. The thickness of the two glass films is 8 mm, for example. This means that the two glass films do not wet the slit nozzle from underneath. 
   The quality demands on the geometry of the breaking edge of the slit nozzle are low in comparison to the known drawing methods with nozzles. Therefore in contrast to the known drawing methods, the nozzles can be used several times, even after stopping. Manufacturing costs are consequently reduced. 
   Due to the stabilization of the glass films by means of the draw bar and the rapid cooling after the fusing of the two glass films in the onion, the processing temperatures can be selected as significantly higher (&gt;/=100 K) than in the drawing methods that use a nozzle without a draw bar, and are thus comparable to the processing temperatures in the drawing methods without a nozzle. The increase in the processing temperatures permits the production of special glasses with pronounced crystallizing tendencies or of glass ceramics. 
   The glass ribbon is preferably cooled selectively in the vicinity of the onion. 
   Due to the stabilization of the glass films by means of the draw bar and the rapid cooling of the glass in the vicinity of the onion, the glass mass is low in the vicinity of the onion. As a result, linear throughputs of up to 5 g/(mm×min) can be set with a high degree of process stability. At the above-indicated temperature settings, for example linear throughputs of 3.5 g/(mm×min) are achieved. This decisively improves the productivity. 
   The temperature deviation ΔT SD  of the glass melt along the slit nozzle is preferably set to ΔT SD &lt;/=20 K. By contrast with the known drawing method with a nozzle, the temperature in the vicinity of the nozzle is distinctly more homogeneous. The shrinkage rate of the glass ribbon is consequently more homogenous during the cooling process in the drawing direction lateral to the glass ribbon than in the previously known drawing methods with a nozzle. The deformation of the glass ribbon (warp) during the cooling to room temperature is thus distinctly reduced. As a result, thin glass panes can be produced that meet high quality demands with regard to flatness. 
   Preferably the glass films are laterally guided along their side edges. Through the side limiter of the draw bar and the temperature guidance in the vicinity of the cooling systems of the nozzle furnace, the process can be guided so that the width of the borders and consequently the glass losses are adequately small. It is thus possible to eliminate a non-homogeneous temperature control in the nozzle region and on the border rollers in order to set the desired width. Improving the yield increases the productivity. 
   In the nozzle furnace, the main cooler cools the glass in the vicinity of the onion to a temperature of for example approx. Tg+290 K. 
   Slight irregularities in the linear throughput can lead to slight thickness fluctuations lateral to the drawing direction of the glass ribbon. According to the invention, the thickness distribution can be corrected by means of the cooling system of the nozzle furnace underneath the main cooler, which cooling system is segmented in the X-direction. To that end, different zones of the glass ribbon can be cooled slightly differently as needed so that the stretching of the glass ribbon in the Z-direction is greater for hotter glass zones than it is for cooler glass zones. Therefore in the stretching of the glass ribbon, the reduction in the thickness of the cooler zones is moderately reduced in comparison to the reduction in the thickness of the warmer zones. In nozzle furnaces, the segmented heating and cooling elements can consequently be used to set definite temperature profiles in the ribbon direction and lateral to the ribbon direction. This consequently improves the results with regard to the quality criterion of thickness distribution. 
   With regard to the device, when in the viscoelastic state, the glass ribbon is stretched as needed lateral to the ribbon direction by means of rollers underneath the nozzle furnace so that the glass ribbon is conveyed flat and without tension to the heat sinks disposed in the drawing shaft. This permits the correction of possible deviations from flatness caused in the hot formation region. In addition, the cylinders or rollers prevent the deformation in the elastic region from having an effect on the hot forming region. The cylinders or rollers are disposed in a rolling furnace or in the drawing shaft in which segmented heating and cooling elements can be used to set definite temperature profiles in the ribbon direction lateral to the ribbon direction. This improves the results with regard to the quality criterion of warp. 
   According to the invention, the glass ribbon is drawn vertically downward with drawing rollers underneath the nozzle furnace. The drawing rollers draw the glass ribbon at a speed of e.g. 1.6 m/min so that with a linear throughput of 3.5 g/(min×mm) and a gross range of approx. 800 mm, a glass ribbon with a thickness in the net range of 0.7 mm is produced. Preferably, the thickness of the glass ribbon is continuously measured, wherein the drawing speed is controlled by means of the measured thickness values. 
   The drawing speed can optionally be regulated as a function of the on-line thickness measurement executed after the cooling of the glass. This improves the results with regard to the quality criterion of average thickness. 
   With large ribbon thicknesses, i.e. a heavy ribbon weight and a light drawing force, if the drawing forces needed to set the required thickness are lighter than the ribbon weight, then the drawing rollers can compensate for a part of the ribbon weight. They must then preferably be disposed in the rolling furnace or in the drawing shaft. 
   According to another embodiment, the glass ribbon is selectively heated and/or cooled in the rolling furnace and/or in the drawing shaft, both in the ribbon direction and lateral to the ribbon direction. 
   In the first rapid cooling zone, the glass ribbon is preferably cooled to Tg+50° K, then in the fine annealing zone it is annealed to for example Tg−50 K in order to adjust the heat sink tension, and in the rapid cooling zone, is rapidly cooled for example to 450 K. 
   Preferable uses of the glass panes produced according to this method include substrate glasses in electronic devices, e.g. displays (portable telephones, flat screens, etc.) or substrate glasses for mass storage devices of computers. 
   Accordingly the device of the present invention for producing thin glass panes, in particular glass panes with a thickness of less than 1 mm, comprises a tank furnace, a homogenization system, an inlet and a drawing tank, wherein the drawing tank has a nozzle system, which has at least one slit nozzle. 
   In this instance, the inlet, the drawing tank, and the nozzle system constitute a closed system. The inlet can be comprised of a platinum alloy or of a fireproof material and can have segmented, directly or indirectly heated or cooled tube sections. The tube or tube system has a length of 2 m to 5 m and a round cross section with a diameter of between 50 mm and 80 mm. 
   The tube is preferably situated vertically. 
   The required viscosity curve of the glass is set in the tube through a combination of heating and cooling. At the end of the inlet, the glass is fed into the drawing tank. 
   In the drawing tank, the glass melt is uniformly distributed lateral to the drawing direction. The drawing tank is a receptacle preferably made of a platinum alloy in which the required viscosity curve of the glass is set through a combination of heating and cooling. The glass is supplied to the nozzle system underneath the drawing tank. 
   The nozzle system preferably has one or two slit nozzles and possibly a draw bar made of a platinum alloy. The slit nozzles preferably have a heating unit. The draw bar can be a platinum alloy plate that tapers off to a point toward the bottom and preferably has a heating unit and if need be a cooling unit. The setting of the homogeneous linear throughput is executed through the definite heating and cooling of the drawing tank, the slit nozzle, and the draw bar. 
   The draw bar is situated vertically in the slit nozzle and preferably protrudes into the drawing tank through the slit of the slit nozzle. The draw bar splits the glass flow in the Y-direction while it is still in the drawing tank (see the coordinate system in  FIG. 1 ). Equal glass film thicknesses are assured by means of this and also by means of an external adjusting system with which the draw bar can be adjusted in the X-Y-direction and possibly also in the Z-direction in relation to the position of the nozzle slit. It is necessary for the glass film to be as equal as possible in thickness in order to assure low warp values. 
   In addition, the straightness of the draw bar, in particular for large ribbon widths and long processing times, can be assured through exertion of a tensile force in the X-direction. As a result, large ribbon widths with long manufacturing duration can be produced, which once again improves the productivity decisively. 
   The nozzle furnace is connected beneath the slit nozzle. The draw bar preferably protrudes into the nozzle furnace. The nozzle furnace preferably has a heating device and two cooling systems. 
   The draw bar can be directly heated and cooled in order to definitely set the dwell time of the glass film on the draw bar. To this end, the draw bar preferably has a heating device and/or a cooling device. In addition, the glass films can be heated on both sides by the segmented heating device of the nozzle furnace. 
   The nozzle furnace preferably has radiation plates on opposite sides in the vicinity of the draw bar. 
   According to another embodiment, the nozzle furnace has at least one dividing wall that can be moved into the furnace shaft underneath the radiation plates. 
   Through the provision of a small onion, heat is drawn from the glass directly underneath the draw bar by means of two cooling systems, a main cooling unit and a segmented cooling unit underneath the main cooling unit. The main cooling unit dissipates the main portion of the heat quantity. The cooling unit that is finely segmented in the X-direction assures the correction of slight thickness fluctuations caused by the nozzle system. 
   The glass ribbon is stretched vertically downward, and possibly also lateral to the ribbon direction. The drawing rollers and warp rollers required for this are situated underneath the nozzle furnace, possibly in the rolling furnace, in the drawing shaft, or in the drawing machine. 
   The drawing rollers are used to adjust the thickness. The warp rollers assure the required flatness of the glass ribbon and of the later glass panes. 
   During the starting process, the glass ribbon is preferably drawn in by means of drawing rollers underneath the nozzle furnace. This significantly reduces the duration of the starting phase. 
   In order to facilitate the starting process, the nozzle furnace can be opened lateral to the ribbon direction. This facilitates the starting process. 
   The rolling furnace can also be opened lateral to the ribbon direction in order to facilitate the starting process. 
   In order to facilitate the starting process, the drawing shaft can be telescoped downward or to the side. During production, the drawing shaft is docked against the rolling furnace from underneath. This also facilitates the starting process. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Exemplary embodiments of the invention will be explained in detail below in conjunction with the drawings. 
       FIG. 1  shows a vertical section through the device and 
       FIG. 2  shows an enlarged vertical section through the region of the slit nozzle. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  have a coordinate system sketched into them to establish the directions in space. The X-direction here indicates direction of the glass ribbon width, the Y-direction is the direction perpendicular to the ideal ribbon surface (glass ribbon thickness), and the Z-direction is the direction in the drawing direction. 
     FIG. 1  shows a vertical section through the device, including the schematically depicted sections of the inlet  1 , drawing tank  6 , nozzle furnace  18 , rolling furnace  25 , and the drawing shaft  32 . 
   The glass melt is fed from a homogenization system that is not shown, through the inlet  1 , and into the drawing tank  6 . The inlet  1  is comprised of a tube  2  disposed in the vertical direction. The tube  2  is comprised of a platinum alloy or a fireproof material, which is segmented and has symmetrical tube sections  2   a, b, c  that are directly or indirectly heated or cooled. In accordance with the tube sections  2   a, b, c , a correspondingly segmented heating device  3  and a segmented cooling device  4  are provided, which are shielded from the outside by means of insulation  5 . The heating device  3  is embodied by a direct or indirect electric heating unit. The cooling device  4  is comprised of cooling tubes through which a cooling medium is conveyed. The cooling tubes encompass the segments of the tube  2  and are connected to corresponding cooling units. 
   The drawing tank  6  that adjoins the inlet channel  1  from underneath has a glass distributor  7  made of a platinum alloys which transitions vertically into the glass inlet  8  toward the bottom, which is also made of a platinum alloy. The glass distributor  7  is used to distribute the glass melt over the entire length of the slit nozzle  11 , which will be described in further detail below. The drawing tank  6  has a heating system  9 , which is likewise segmented in the vertical direction and in the lateral direction. The heating unit can be embodied as either a direct or indirect electric heating unit. On the outside, the drawing tank  6  is enclosed by an insulation  10 . 
   Underneath the drawing tank  6  is the nozzle system with the slit nozzle  11 , the seals  12  and  13 , and the draw bar  16 . The slit nozzle and the draw bar are made of a platinum alloy. The nozzle holder  14  presses the slit nozzle  11  against the drawing tank  6  from underneath. The seal  12  and the flexible seal  13  assure the frictional and positive engagement. 
   The slit nozzle  11  is electrically heated directly or indirectly by means of the heating unit  15  disposed in the nozzle holder  14 . In order to optimize the glass flow, another slit nozzle made of a platinum alloy (not shown) can, if needed, be disposed above the slit nozzle  11 . 
   As shown in  FIG. 2 , the slit nozzle  11  has a draw bar  16  made of a platinum alloy disposed in it, which is comprised of a plate-shaped element that tapers down to a point at its lower end. The draw bar  16  can have a side limiter  17  and is disposed so that it can be adjusted in the X-, Y-, and Z-direction, which is not shown in  FIGS. 1 and 2 . The draw bar  16  also has a direct electrical heating unit, and possibly also has a cooling unit. 
   The two glass films, which flow vertically downward out of the slit nozzle  11  on the outsides of the draw bar  16 , come together to form a common glass ribbon at the lower end that tapers down to a point. The glass films on the draw bar, the onion, and the stretching region of the glass ribbon are disposed between the radiation plates  19  of the nozzle furnace  18 , which has an electric heating unit  20  in the upper part and has cooling systems  23  and  24  in the lower part. The radiation plates  19  shield the heating unit from the glass ribbon in order to improve the temperature homogeneity. 
   A dividing wall  22  that can move in the Y-direction and disposed at the same height as the onion can be used to regulate the amount of heat that the cooling systems  23 ,  24  withdraw from the glass ribbon. Below the dividing wall  22  is the main cooling unit  23  for adjusting the temperature of the glass ribbon in the vicinity of the onion. The cooling system  24  that is segmented in the X-direction is used to adjust the thickness distribution in the lateral direction of the glass ribbon. The nozzle furnace  18  is insulated in relation to the outside by means of the insulation  21 . In order to facilitate the starting process, the nozzle furnace can be opened in the Y-direction if need be. 
   Underneath the nozzle furnace  18  is the rolling furnace  25  with the rolling shaft  26 . The rolling furnace  25  has an electric heating unit  27  that is segmented in the X- and Y-direction and a cooling unit  28  that is also segmented in the X- and Y-direction. In order to improve the temperature homogeneity, the rolling shaft  26  shields the heating unit from the glass ribbon. The warp rollers  29  are disposed in the rolling furnace  25  and, by means of their inclined position or a special profiling, stretch the glass ribbon in the X-direction in order to set the required flatness. The rolling furnace  25  also contains one or more drawing roller pairs  30 , which stretch the ribbon in a definite fashion in the Z-direction in order to set the required thickness. The rolling furnace  25  is insulated in relation to the outside by means of the insulation  31 . In order to facilitate the starting process, the rolling furnace  25  can be opened in the Y-direction, if need be. 
   Underneath the rolling furnace  25  is the drawing shaft  32 . The drawing shaft  32  has an electric heating unit  35  that is segmented in the X- and Y-direction, and has a cooling unit  36  that is also segmented in the X- and Y-direction. In order to improve the temperature homogeneity, the radiation shaft  33  shields the heating unit  35  from the glass ribbon. In order to prevent indefinite convection, movable dividing walls  34  are provided in the radiation shaft. The drawing shaft  32  can contain stabilization rollers  37 , which stabilize the position of the glass ribbon in the Y-direction. The drawing shaft  32  is insulated in relation to the outside (insulation  38 ). 
   In order to facilitate the starting process, drawing shaft  32  can be opened in the Y-direction if need be or doors disposed at the end can be opened. In addition, the drawing shaft  32  can be telescoped downward in order to facilitate setup activities. For production, the drawing shaft  32  is docked from underneath against the rolling furnace  25 . 
   The rolling furnace  25  can also be omitted. If the rolling furnace is not provided, then the warp rollers  29  or the drawing rollers  30  can be contained in the drawing shaft  32 . With low wall thicknesses, i.e. with a light ribbon weight, and with light drawing forces, in short drawing shafts  32 , it is possible to eliminate the drawing rollers  30  and the stabilizing rollers  37  in the drawing shaft. The drawing rollers  30  are then disposed underneath the drawing shaft  32 . 
   All heating systems are operated electrically. The cooling systems are cooled by means of circulating fluids or gaseous mediums. 
   An example is given below for the production of substrate glass for TFT displays. 
   Glass: 
   Alkali-free borosilicate glass with Tg&gt;700° C., density&lt;2.5 g/cm 3 , and thermal expansion&lt;37×10 −7 /K 
   Quality Criteria with Regard to Geometry: 
   
     
       
             
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               length: 
               670 
               mm 
             
             
                 
               width: 
               590 
               mm 
             
             
                 
               thickness: 
               0.7 
               mm 
             
             
                 
               thickness distribution: 
               &lt;0.025 
               mm 
             
             
                 
               warp: 
               &lt;0.5 
               mm 
             
             
                 
               waviness: 
               &lt;50 
               nm 
             
             
                 
                 
             
           
        
       
     
   
   Process Management: 
   Inlet: 
   The glass temperatures in the inlet are set to between Tg+670 K and Tg+590 K (falling monotonically along the inlet). The total throughput is then 2.8 kg/min. 
   Drawing tank, nozzle system, and heating of the nozzle furnace: The glass temperatures in the drawing tank  6  are set to between Tg+590 K and Tg+570 K, the glass temperatures in the slit nozzle  11  are set to between Tg+570 K and Tg+550 K, and the glass temperatures at the draw bar  16  are set to between 1-g+560 K and Tg+540 K. This yields a homogeneous linear throughput lateral to the glass ribbon of 3.5 g/(min×mm). The glass films on the draw bar  16  are each 8 mm thick. With a slit width (distance between the slit nozzle  11  and the draw bar  16 ) of 10 mm each, the glass films on the draw bar  16  consequently do not wet the underside of the slit nozzle  11 . 
   Cooling, Nozzle Furnace, and Rolling Furnace: 
   The main cooling unit  23  (cooling medium: water) cools the glass in the vicinity of the onion to approximately Tg+290 K. The glass is stretched by the drawing rollers  30  of the rolling furnace  25  at a drawing speed of 1.6 m/min, producing a glass ribbon with a gross width of 800 mm and a thickness of 0.7 mm. The segmented cooling unit  23 ,  24  (cooling medium: water/air) is set so as to reliably achieve the required thickness distribution. The segmented heating and cooling of the rolling furnace  25  homogeneously cool the glass ribbon to Tg+140 K. 
   Drawing Shaft: 
   With the segmented heating and cooling of the drawing shaft  32 , the glass ribbon is cooled to Tg+50 K in the first rapid cooling zone, and then in the fine annealing zone, the glass ribbon is fine annealed to Tg−50 K in order to set the permanent heat sink tensions, and then in the second rapid cooling zone B, the glass ribbon is rapidly cooled to 450 K. In the drawing shaft  32  and underneath the drawing shaft, additional rollers are provided, which stabilize the position of the glass ribbon in and under the drawing shaft  32 . After the fast quenching and cutting to length, the requirements are met regarding warp. 
   REFERENCE NUMERALS 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               Reference Numerals 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
                1 
               inlet 
             
             
                 
                2 
               tube 
             
             
                 
                2a,b,c 
               tube section 
             
             
                 
                3 
               heatfng unit 
             
             
                 
                4 
               cooling unit 
             
             
                 
                5 
               insulation 
             
             
                 
                6 
               drawing tank 
             
             
                 
                7 
               glass distributor 
             
             
                 
                8 
               glass inlet 
             
             
                 
                9 
               heating system 
             
             
                 
               10 
               insulation 
             
             
                 
               11 
               slit nozzle 
             
             
                 
               12 
               seal 
             
             
                 
               13 
               flexible seal 
             
             
                 
               14 
               nozzle holder 
             
             
                 
               15 
               heating unit 
             
             
                 
               16 
               draw bar 
             
             
                 
               17 
               side limiter 
             
             
                 
               18 
               nozzle furnace 
             
             
                 
               19 
               radiation plates 
             
             
                 
               20 
               heating unit 
             
             
                 
               21 
               insulation 
             
             
                 
               22 
               dividing wall 
             
             
                 
               23 
               main cooling unit 
             
             
                 
               24 
               segmented cooling system 
             
             
                 
               25 
               rolling furnace 
             
             
                 
               26 
               rolling shaft 
             
             
                 
               27 
               heating unit 
             
             
                 
               28 
               cooling unit 
             
             
                 
               29 
               warp rollers 
             
             
                 
               30 
               drawing rollers 
             
             
                 
               31 
               insulation 
             
             
                 
               32 
               drawing shaft 
             
             
                 
               33 
               radiation shaft 
             
             
                 
               34 
               dividing wall 
             
             
                 
               35 
               heating unit 
             
             
                 
               36 
               cooling unit 
             
             
                 
               37 
               stabilizing rollers 
             
             
                 
               38 
               insulation

Technology Classification (CPC): 2