DEVICE AND METHOD FOR PRODUCING GLASS STRIPS

Description

[0001] The invention generally relates to glass production. In particular, the invention relates to an apparatus and a method for producing glass ribbons, in particular with a thickness of up to 3000 µm (micrometers), preferably from 15 to 1100 µm, from a glass melt using the down-draw process.

[0002] In the down-draw process, a drawing tank ensures uniform glass distribution up to a nozzle opening through which the glass emerges at hot forming temperature. A simple slotted nozzle can be located at the drawing tank outlet, while a separate flow resistance, such as a blade or a sword body, can also be located in the drawing tank itself or at the drawing tank outlet. All forming tools are preferably made of precious metal alloys, preferably with a high platinum content. This makes the down-draw process highly flexible, as the precious metal tools can be changed and subsequently reused after the process is stopped. This is not economical, for example, with a ceramic trough used in overflow fusion processes, since the heating and cooling times are correspondingly long.For example, the start-up process for an overflow fusion process takes more than a week, whereas for a down-draw process it takes less than a day.

[0003] In addition, the same precious metal tool can also be used for several types of glass that differ greatly in their chemical and physical properties, such as corrosion behavior, conductivity, density, viscosity curve, glass transition temperature, hot forming temperature, etc. This means that with a down-draw process, it is also easy to remelt glass types flexibly without the need to change the hot forming tools.

[0004] DE 10 2004 007 560 B4 is based on the object of providing a device and a suitable drawing tank for the production of thin glass panes, which allows the production of a glass ribbon that meets high requirements with regard to thickness consistency and flatness. This is achieved by a drawing tank that has at least two sections with different surface cross-sections across its entire width, wherein the sections are dimensioned such that the total pressure drop over the distance traveled by the glass melt in both sections is constant and equal at every point on the slot nozzle. Furthermore, DE 10 2004 007 560 B4 describes the advantage of equipping the drawing tank with heating elements in order to achieve optimal temperature distribution, whereby the drawing tank is only indirectly heated.

[0005] In the past, attempts have also been made to improve the quality of the glass panes by using special temperature profiles perpendicular to the drawing direction in the nozzle area. For example, DE 100 64 977 C1 proposes designing the inlet, drawing tank, and nozzle system as a closed system. The inlet comprises a circular pipe with symmetrical pipe sections, and the drawing tank has a heating system segmented vertically and transversely. The heating can be implemented directly or indirectly as electrical heating.

[0006] DE1596484 B1 discloses a device for producing thin glass ribbons, wherein the platinum wall of the drawing tank and its nozzle are heated independently of each other directly by a current flowing through the wall. In addition, the drawing tank is also indirectly heated by heating coils.

[0007] The drawing tank is a complex component in which the glass distribution, homogenization and temperature setting must be finely adjusted in order to optimize the geometric properties of the glass ribbon to be produced, so that thickness variations, warping in the glass ribbon and glass defects, such as streaks, etc., are avoided.

[0008] The invention is therefore based on the object of providing a device for producing glass ribbons that allows the production of glass ribbons that meet high requirements with regard to dimensional accuracy of the glass ribbon and glass quality while avoiding glass defects and with regard to mechanical stability to extend the service life. Furthermore, a correspondingly improved, particularly flexible, process with more stable processes and increased yield is to be provided.

[0009] This object is achieved by the subject matter of claims 1 and 13. Advantageous further developments are specified in the dependent claims.

[0010] Accordingly, the invention according to claim 1 provides a device for drawing glass ribbons, in particular with a thickness of less than or equal to 3000 µm, preferably from 15 to 1100 µm, from a glass melt. The device comprises a drawing tank having a lower elongated nozzle opening through which the glass melt can exit downwards, as well as a direct heating device and an indirect heating device. In particular, at least one direct heating device and at least one indirect heating device are provided in the region of the nozzle opening.

[0011] The direct heating device comprises at least one heating circuit with which at least one area of the drawing tank can be heated in the form of at least one first heating zone, wherein the direct heating device comprises a power source for each heating circuit, wherein each heating circuit has connections with which each heating circuit is connected to a wall of the drawing tank, so that the current from the power source flows through at least part of the wall and heats the wall, and each heating circuit of the direct heating comprises parts of the wall of the drawing tank through which current flows. Preferably, the direct heating device comprises a plurality of heating circuits with which at least one area of the drawing tank can be heated in the form of a plurality of first heating zones. The indirect heating device has heating elements for at least one second heating zone, preferably for a plurality of second heating zones.

[0012] Indirect heating of the drawing tank creates a base temperature and direct heating of the drawing tank creates the target temperature.

[0013] In a preferred embodiment, the direct heating device comprises at least four separately controllable heating circuits with which four different areas of the drawing tank can be heated in the form of first heating zones. The direct heating device comprises a power source for each of the heating circuits, wherein the heating circuits have connections with which the heating circuits are connected to the wall of the drawing tank, so that the current from the power sources flows through at least part of the wall and heats the wall, and the heating circuits of the direct heating comprise parts of the wall of the drawing tank through which current flows, in particular wherein the heating circuits are formed with metal sheets of the drawing tank through which current flows. The indirect heating device preferably has heating elements for at least three separately controllable second heating zones that are spatially different from one another.The glass ribbon thickness is very sensitive to temperature deviations, therefore the optimization and fine adjustment of the temperature of the glass melt in and around the drawing tank is particularly important for a stable production process.

[0014] If a heating system is implemented only indirectly, meaning the heating elements are arranged around the drawing tank, temperature control is sluggish, and fine-tuning or optimizing the temperature profile in the drawing tank both transversely and longitudinally is difficult. Gradient control, and thus better and faster controllability to compensate for flow inhomogeneities to improve melt quality and adjust the thickness profile, is difficult. Furthermore, start-up and shutdown processes can only be carried out slowly in a sluggish system.

[0015] If a heating system is implemented solely directly, excessively high direct heating currents can promote electrochemical reactions between the glass melt and the drawing tank sheet, which can lead to bubble formation in the glass and unwanted particle formation from the drawing tank sheet. Furthermore, with only direct heating of the component, it is not possible to implement finely spatially resolved temperature settings, which hinders process control.

[0016] The special combination of the direct heating device and the indirect heating device according to the invention allows the disadvantages of each heating type to be reduced or mutually compensated. Thus, the direct heating device enables rapid control of the glass temperature by ± 0.5 K and rapid start-up and stop-up of the process. At the same time, the indirect heating device can prevent or at least reduce excessive heating currents from direct heating and thus undesirable bubble and particle formation in the glass. By separating the heating zones into different first and second heating zones, the linear throughput of the glass melt, i.e., the throughput of the glass melt per unit length in the transverse direction, can be better adjusted.

[0017] Thus, the device according to the invention for producing thin glass ribbons offers a heating system that is improved compared to the prior art, with a particularly flexible temperature and throughput adjustment in the drawing tank, and thus ensures the production of glass ribbons with improved dimensional accuracy and glass quality.

[0018] In order to further improve the controllability of the indirect heating device, the indirect heating device preferably has heating elements for five or more separately controllable second heating zones.

[0019] In an advantageous embodiment, the first heating zones of the direct heating device and the second heating zones of the indirect heating device are arranged vertically and / or horizontally distributed around and on the drawing tank. The vertical and / or horizontal distribution into different heating zones allows for optimal spatial control of the temperature profile and line throughput of the glass melt, which promotes controllability and thus stability of the process. This also prevents, for example, negative pressure in the drawing tank, which can lead to deformation and even process failure.

[0020] Aging of the material of the drawing tank, forming tools, or other components can also be problematic over long periods of use and at high temperatures exceeding 1200°C, which can negatively impact stability, for example, due to grain size growth in a component, even one made of precious metal. These disadvantages can be avoided by the combined direct and indirect heating system with vertical and / or horizontal distribution into different heating zones.

[0021] Preferably, the heating currents applied to the sheets of the drawing tank for direct heating are less than 2500 A, preferably less than 1000 A. Heating currents of less than 2500 A, preferably less than 1000 A, reduce particle formation, in particular the formation of precious metal particles from precious metal components, and bubble formation in the glass.

[0022] Preferably, the indirect heating device is designed such that, as a base load, it contributes more than 50% to the total power. If the heat supply via the indirect heating device predominates over that of the direct heating device, the control current of the direct heating device can be adjusted to currents below those critical for particle and bubble formation.

[0023] In order to improve the thermal performance of the direct heating device, the drawing tank advantageously comprises flanges or sleeves as connections for the power sources for direct heating.

[0024] It was found that the thickness of the current-carrying sheets of the drawing tank, when locally adjusted, is advantageously in the range of 0.5 to 5 mm in order to optimally adjust the local current densities, i.e., the heat input as a function of current intensity and conductor cross-section. By adjusting the material thicknesses for the respective ideal temperature distribution, critical areas such as local hot spots, which can occur with direct heating at bends or flanges of the drawing tank, can be reduced at set current intensities, thus preventing the formation of bubbles and foreign particles.

[0025] Because preferably several temperature measuring points are positively connected to the drawing tank, in particular welded on, a particularly quick and direct adjustment can be achieved.

[0026] In a further advantageous embodiment, at least two temperature measuring points are provided, which form control measuring points for controlling the heating output of the direct heating device or the indirect heating device for a control device for controlling the heating output.

[0027] For particularly even heat distribution, the indirect heating system can include heating tiles or meander heaters as heating elements.

[0028] Heating elements of the indirect heating device that include platinum, stainless steel or SiC as resistance heating material have proven to be particularly advantageous.

[0029] To prevent creep deformation under component stress, for example, due to high temperatures, and to increase process stability and the service life of the device, the invention provides for the drawing tank to comprise sheets with at least one fine-grain stabilized precious metal or a fine-grain stabilized precious metal alloy. Platinum and platinum alloys, in particular Pt, PtRh, PtAu, PtRhAu, PtIr, are particularly suitable as metals, and nanoparticles, such as ZrO2 particles, are particularly preferably included.

[0030] The use of fine-grain stabilized precious metal alloys is preferred to increase the stability of the drawing tank under high temperature loads and during long-term use. Fine-grain stabilized alloys, optionally with the addition of nanoparticles, melt-metallurgically or powder-metallurgically produced precious metal alloys made of Pt x Rh y Au z (0%<=x<=100%, 0%<=y<=20%, 0%<=z<=20%) or Pt u Ir v alloys (0%<=u<=100%, 0%<=v<=20%) are particularly advantageous for reducing the creep effect when components are subjected to high temperatures.

[0031] Suppliers of these stabilized materials include Umicore, Furuya, Heraeus, Tanaka, each with their own versions of these materials (Umicore, for example, PtRh10 FKS Rigilit, PtRh10 FKS Saeculit - Heraeus, for example, PtRh10 DPH or DPH-A).

[0032] Highly creep-resistant precious metals such as pure iridium are less suitable because they are not resistant to oxidation.

[0033] With the arrangement described here, it is generally possible, without limitation to the aforementioned special alloys, to draw glass ribbons from glasses with forming temperatures above 1100 °C. According to this disclosure, the forming temperature is defined as a temperature at which the glass has a viscosity of 10 4 < dPa s.

[0034] Of course, not only the sheets of the drawing tank can comprise at least one fine-grain stabilized precious metal or a fine-grain stabilized precious metal alloy, but also other components or tools of the device.

[0035] To avoid negative pressure in the drawing tank, which can cause mechanical deformation of the drawing tank and lead to inhomogeneous thicknesses of the glass ribbon and glass defects, an inlet upstream of the drawing tank preferably flows into a distributor pipe of the drawing tank, which is connected to a shaft. The shaft has a narrower cross-section than the distributor pipe and has the nozzle opening at its lower end. This ensures that the total pressure drop over the distance traveled by the glass melt is constant and the same at every point of the nozzle opening.

[0036] In a preferred embodiment, the power sources of the direct heating device are connected to the drawing tank in such a way that at least three first heating zones are formed, which are distributed in the upper part of the drawing tank transversely to the drawing direction of the glass ribbon, that is to say in particular along the longitudinal direction of the nozzle opening, wherein one of the upper first heating zones is preferably arranged centrally, and wherein at least one further first heating zone heats the lower part of the drawing tank with the nozzle opening.

[0037] This arrangement allows the temperature in the drawing tank up to the nozzle opening to be controlled locally, both horizontally and vertically, and thus the viscosity of the glass melt can be adjusted as required.

[0038] To further improve temperature control, according to a further preferred embodiment, the heating elements of the indirect heating device for the at least three separately controllable, spatially different second heating zones are arranged such that the second heating zones are distributed next to one another transversely to the pulling direction of the glass ribbon, that is to say in particular along the longitudinal direction of the nozzle opening.

[0039] Consequently, a glass ribbon can be produced that meets high requirements regarding thickness consistency and flatness.

[0040] For an extended residence time of the glass melt and for improved controllability of the temperature to influence the thickness distribution (longitudinal and transverse distribution) of the glass melt, the drawing tank of the device preferably has a guide body that projects downwards from the nozzle opening of the drawing tank. The guide body is held at a distance from the edges of the nozzle opening, so that two nozzle gaps are formed between the guide body and the edges of the nozzle opening. The guide body is preferably heated, with the guide body having its own separately controllable connections and at least one heating circuit independent of the drawing tank being formed, with grounding being provided, for example, via a connection.

[0041] The aforementioned structural designs of the device enable higher yields of glass ribbons with homogeneous thicknesses and improved surface quality to be achieved, which also improves the mechanical stability and thus also the service life.

[0042] With one of the devices described above, a method for producing thin glass ribbons, in particular with a thickness of equal to or less than 3000 µm, can be carried out according to claim 13, in which the glass melt is fed to the drawing tank via an inlet, and wherein the glass melt emerges from the nozzle opening and is drawn off to form the glass ribbon, wherein the drawing tank is heated simultaneously both with the direct heating device and with the indirect heating device.

[0043] In a preferred method, the glass melt is distributed through the inlet to the drawing tank into a distributor pipe of the drawing tank, to which a shaft is connected, wherein the shaft has a narrower cross-section than the distributor pipe, wherein by means of the temperature profile in the drawing tank a constant line throughput of the glass melt along the nozzle opening, i.e. a constant throughput per unit length in the transverse direction of the glass melt, is set.

[0044] In a preferred method, the line throughput of the glass melt, i.e. the throughput per unit length in the transverse direction of the glass melt, is controlled by means of the direct heating device and the indirect heating device by the temperature setting and temperature distribution of the glass melt in the drawing tank so that the total throughput remains constant.

[0045] In the circular drawing tank inlet, the flow rate is adjusted as a function of the pipe diameter and temperature. The temperature setting of the drawing tank should preferably only influence the longitudinal distribution of the line flow rate along the outlet and not change the total flow rate.

[0046] In a further preferred method with a device in which the drawing tank comprises a shaft in which a guide body is arranged and which has the nozzle opening at its lower end, the temperature of the glass melt in the drawing tank is preferably adjusted so that the following relationship is satisfied with its temperature-dependent viscosity η: 48 v ˙ B ∫ H L η D S − D L 3 dz + 12 v ˙ B ∫ H S η D S 3 dz > ρ ⋅ g ⋅ h − p u where v is the volume flow of the glass melt, B is the width of the drawing tank in the direction along the nozzle opening, DS is the local width of the shaft, DL is the local thickness of the guide body, ρ is the density of the glass melt, g is the acceleration due to gravity, h is the height of the shaft and pu is a pressure of 2000 Pa.

[0047] In order to produce glass ribbons with uniform thicknesses and as wide as possible, a temperature gradient in the drawing tank along the nozzle opening from its edges to the center of the nozzle opening is advantageously set from T grad =T middle -T edge = 0 to 50 K, preferably from T grad = 20 to 40 K.

[0048] The aforementioned process versions, with both direct and indirect heating, allow for flexible, particularly stable processes and thus higher yields of glass ribbons with homogeneous thicknesses and improved surface quality. This also increases the cost-effectiveness of the process. Short description of the characters

[0049] The invention is explained in more detail below with reference to the attached figures. Fig. 1is a cross-sectional view through a drawing tank with a direct heating device, an indirect heating device and with a guide body. Fig. 2 shows schematically a section of a drawing tank with a direct heating device in cross section. Fig. 3 shows schematically a section of a drawing tank with an indirect heating device in cross section. Fig. 4 shows the influence of the temperature curve at the drawing tank outlet on the thickness profile and the width of the glass ribbon. Fig. 5 . is a perspective view of a draw tank with a single pair of sleeves as connections for electricity for direct heating. Fig. 6 shows a cross-sectional view of a drawing tank to illustrate the parameters for calculating the pressure drop in the drawing tank. Detailed description of the invention

[0050] Fig. 1shows parts of a device 1 for producing, in particular for drawing, glass ribbons 3, in particular with a thickness of equal to or less than 3000 µm, preferably from 15 to 1100 µm, from a glass melt 5. The device 1 has a drawing tank 7 for receiving a glass melt 5, which has an elongated nozzle opening 9 at its lower end through which the glass melt 5 can exit downwards, and according to the invention combines a direct heating device 2 and an indirect heating device 4.

[0051] A guide body 11 can preferably be arranged in the drawing tank 7, which can protrude downwards from the nozzle opening 9 of the drawing tank 7. The guide body 11 is preferably held at a distance from the edges 90, 92 of the nozzle opening 9, so that two nozzle gaps 94, 96 are formed between the guide body 11 and the edges 90, 92 of the nozzle opening 9. The glass melt 5 thus preferably exits through the nozzle gaps 94, 96 in two partial streams 50, 52. These partial streams 50, 52 can run downwards along the guide body 11 and merge at the lower end of the part 100 of the guide body 11, which preferably protrudes from the nozzle opening 9. This area, in which the two partial streams 50, 52 merge and from which the glass ribbon 3 is formed by drawing, is referred to as the drawing bulb 15. During drawing, the thickness of the glass ribbon 3 decreases due to the drawing out of the glass.At the same time, the glass becomes colder and correspondingly more viscous with increasing distance from the nozzle opening 9 until it solidifies.

[0052] In general, it is preferred if the guide body 11 protrudes at least 30 mm, preferably at least 80 mm, from the nozzle opening 9. In this way, a good distribution of the glass melt 5 on the guide body 11 is enabled, so that thickness fluctuations in the glass ribbon 3 are suppressed.

[0053] Without being limited to a specific embodiment, a further development provides that the guide body 11 has a greater thickness within the drawing tank 7 than at the nozzle opening 9. The thickening is advantageous for mechanical reasons, but not mandatory.

[0054] The guide body 11 may comprise a resistance body 101 and a sword or blade 103 arranged below the resistance body 101, wherein the resistance body 101 preferably has a greater width than the blade 103 in order to narrow the flow cross-section in the drawing tank 7.

[0055] The distance from the nozzle opening 9 to the lower edge of the flow resistance, or the resistance body 101, is generally preferably at least 3 mm, preferably at least 8 mm.

[0056] Not in Fig. 1 shown, but it is also technically conceivable to heat the guide body 11, in particular via its own separately controllable connections, whereby at least one separate heating circuit independent of the drawing tank 7 is formed and earthing is effected, for example, via a connection.

[0057] To pull off the glass ribbon 3, a pulling device 17 may be provided. This may, for example, comprise one or more pairs of driven rollers.

[0058] Fig. 1 shows a heating circuit 206 of the direct heating device 2, which has two connections 26, 27, with which the heating circuit 206 is connected to a wall 700 of the drawing tank 7, so that the current of the current source 201 flows through at least part of the wall 700 and heats the wall 700, whereby at least one area of the drawing tank 7 can be heated in the form of a first heating zone 34.

[0059] Preferably, the direct heating device 2 has at least four separately controllable heating circuits 206, 207, 208, 209, with which four different areas of the drawing tank 7 in the form of first heating zones 34, 35, 36, 37 can be heated, as for example in Fig. 2 shown.

[0060] The direct heating device 2 further comprises a power source 201, 202, 203, 204 for each of the preferably at least four heating circuits 206, 207, 208, 209, wherein the heating circuits 206, 207, 208, 209 each have connections 26, 27 with which the heating circuits 206, 207, 208, 209 are connected to the wall 700 of the drawing tank 7, so that the current of the power sources 201, 202, 203, 204 each flows through at least a part of the wall 700 and heats the wall 700.

[0061] The heating currents of a heating circuit are preferably less than 2500 A, especially less than 1000 A. However, current sources with a minimum power of 200 A are preferred in order to provide sufficient heating power when using direct heating for temperature adjustment. In particular, the heating circuits 206, 207, 208, and 209 of the direct heating system are formed with current-carrying sheets 78 of the drawing tank 7. Favorable thicknesses of the sheets 78 for direct heating are in the range of 0.5 to 5 millimeters.

[0062] According to the invention, the device 1 comprises, in addition to the heating device 2, also an indirect heating device 4, which has heating elements for at least a second heating zone, as shown in the Fig. 1 In particularly preferred embodiments, the indirect heating device 4 has heating elements for at least three or for five or more ( Fig. 3) separately controllable second heating zones, spatially different from each other. In the embodiment of the device 1 according to Fig. 1 A thermal insulation 14 from the external environment is also arranged on the indirect heating device 4.

[0063] As in Fig. 1As shown schematically, in an advantageous further development, at least two temperature measuring points 18, 19 are provided, which can form control measuring points for controlling the heating output of the direct heating device 2 and / or the indirect heating device 4 for a control device 20 for controlling the heating output. As illustrated by the arrows, the control device 20 accordingly reads the sensor values of the temperature measuring points 18, 19 and controls the heating output of the heating devices 2, 4. During direct heating, the current flow is adjusted for this purpose. The indirect heating device 4 also preferably comprises conductive heating, but can also comprise burners, for example. The temperature measuring points 18, 19 can be positively connected to the drawing tank 7, in particular welded, for rapid feedback of the temperature changes.

[0064] In Fig. 2For reasons of clarity, a section of a drawing tank 7 with a direct heating device 2 is shown, preferably with at least four separately controllable heating circuits 206, 207, 208, 209, with which four different areas of the drawing tank 7 in the form of the first heating zones 34, 35, 36, 37 can be heated. The direct heating device 2 according to Fig. 2 comprises for each of the heating circuits 206, 207, 208, 209 an associated power source 201, 202, 203, 204, each with the connections 26, 27, with which the heating circuits 206, 207, 208, 209 are connected to the wall 700 of the drawing tank 7.

[0065] In an advantageous embodiment, the first heating zones 34, 35, 36, 37 of the direct heating device 2 are arranged vertically and horizontally distributed on the drawing tank 7. This arrangement allows the temperature in the drawing tank up to the nozzle opening 9 to be locally variably controlled, and thus the viscosity of the glass melt 5 can also be adjusted as desired.

[0066] In a particularly preferred embodiment of the direct heating device 2, which is Fig. 2As shown, the power sources 201, 202, 203, 204 are connected to the drawing tank 7 in such a way that at least three first heating zones 34, 35, 36 are preferably distributed in the upper part of the drawing tank 7 transversely to the drawing direction of the glass ribbon 3, that is to say in particular along the longitudinal direction of the nozzle opening 9. Of these three first heating zones 34, 35, 36 in the upper part of the drawing tank 7, one heating zone 35 is preferably arranged centrally in the region of an inlet 74 of the glass melt 5 into the drawing tank 7, and the other two heating zones 34, 36 are each preferably located in the region of a distributor pipe 76 for the glass melt 5 arranged downstream of the inlet 74. As in Fig. 2 As shown, advantageously at least one further first heating zone 37 heats the lower part of the drawing tank 7 with the nozzle opening 9.

[0067] Fig. 3 For reasons of clarity, shows a section of a drawing tank 7 with an indirect heating device 4.

[0068] Without being limited to the embodiments, it is provided in one embodiment of the invention that, as in Fig. 3 shown, the indirect heating device 4 preferably has six heating elements 41, 42, 43, 44, 45, 46 for six separately controllable second heating zones 61, 62, 63, 64, 65, 66, wherein the second heating zones 61, 62, 63, 64, 65, 66 of the indirect heating device 4 can be arranged next to one another horizontally with a vertical longitudinal extension of the heating zones 61, 62, 63, 64, 65, 66 on the drawing tank 7, more preferably transversely to the drawing direction of the glass ribbon 5, that is to say in particular along the longitudinal direction of the nozzle opening 9.

[0069] Because the device 1 for producing, in particular for drawing, glass ribbons 3 according to the invention simultaneously comprises a direct heating device 2 and an indirect heating device 4, the temperature profile and the line throughput of the glass melt 5 can be optimally adjusted in a spatially resolved manner, in particular by separating it into different first and second heating zones, which particularly promotes the controllability and thus also the stability of the process carried out with this device. In particular, the control system can be used to achieve a line throughput that is as constant as possible along the nozzle opening 9.

[0070] Since the glass ribbon thickness is very sensitive to temperature deviations, the optimization and fine adjustment of the temperature of the glass melt 5 is particularly important for a stable production process.

[0071] Fig. 4shows the influence of the temperature curve at the drawing tank outlet on the thickness profile and the width of the glass ribbon 3.

[0072] The glass ribbons 3 have a central region 30, along which the thickness of the glass ribbon 3 changes little or not at all, as well as edge borders 31, 33. The central region 30 forms the so-called quality region from which the glass products to be manufactured are produced. Typically, the borders 31, 33 are separated, and the glass from the borders 31, 33 can be remelted and fed to the drawing tank 7.

[0073] In order to obtain a particularly wide glass ribbon 3 with a uniform thickness in the usable quality range or in the central area 30, a temperature gradient of ideally 40 K should be set between the edges 90, 92 of the nozzle opening 9 and the center of the nozzle opening 9, as shown in Fig. 4 shown by the thinner printed "middle" line of the three lines.

[0074] If the center of the nozzle opening 9 is hotter, i.e. if the temperature gradient is, for example, 50K, the line throughput of the glass melt 5 increases in the center, which is why the thickness profile of the glass ribbon 3 takes on a W-shape, as shown, for example, in the Fig. 4 indicated by the dashed line.

[0075] If the temperature difference is less than or equal to 20K, as in the Fig. 4 As indicated by the bold line throughout, the band width of the glass ribbon 3 decreases and the thickness profile becomes "tub-shaped." The usable quality range, or rather the central area 30, decreases significantly.

[0076] In order to produce glass ribbons 3 that are as wide as possible and have a constant thickness, a temperature gradient is preferably set in the drawing tank 7 along the nozzle opening 9 from its edges 90, 92 to the center of the nozzle opening 9, advantageously from T grad =T mitte -T rand = 0 to 50 K, preferably from T grad = 20 to 40 K.

[0077] A further possibility for adjusting or regulating the thickness of the glass ribbon 3 by adjusting the temperature is achieved in an advantageous embodiment of the device 1 with a drawing tank 7, in which the drawing tank 7 comprises flanges or sleeves 28 as connections 26, 27 for current for direct conductive heating.

[0078] In Fig. 5 A section of a preferred embodiment is shown, in which an inlet 74 for the glass melt is arranged upstream of the drawing tank 7, and in which the upper part of the drawing tank 7 is formed by a tubular section or the distributor pipe 76 for the glass melt 5, to which a shaft 75 is connected. For example, a single pair of sleeves 28 is attached to the distributor pipe 76 as separate connections 26, 27 for electricity. The sleeves 28 preferably each have an electricity connection tab 29.

[0079] According to a further embodiment of the invention, the pressure drop in the drawing tank 7 is specially adjusted to improve the dimensional accuracy of the glass ribbon 3. This adjustment prevents the development of a negative pressure in the drawing tank 7. Such a negative pressure can mechanically deform the drawing tank 7, which can also affect the glass thickness. Furthermore, a local negative pressure can lead to unstable flows of the glass melt 5 in the drawing tank 7, which can also result in inhomogeneous glass thicknesses or glass defects.

[0080] Based on the schematic sectional view of a drawing tank 7 in Fig. 6 The setting according to this embodiment will now be explained in more detail. Fig. 6 shows a cross-sectional view of a drawing tank 7 showing dimensions used to calculate the pressure drop in the drawing tank 7.

[0081] The upper part of the drawing tank 7 is formed here by a tubular section or the distributor pipe 76, which is adjoined by a shaft 75 that opens into a bottom 70 of the drawing tank 7. The shaft 75 preferably has a narrower cross-section than the distributor pipe 76. Accordingly, the width DA of the shaft 75 is smaller than the diameter of the distributor pipe 76. Due to the small cross-section, pressure changes occur primarily along the shaft 75. The sections in which preferably at least one guide body 11 further narrows the shaft 75 contribute in particular.

[0082] According to a preferred embodiment, the temperature in the drawing tank 7 is adjusted so that the relationship already mentioned above applies: 48 v ˙ B ∫ H L η D S − D L 3 dz + 12 v ˙ B ∫ H S η D S 3 dz > ρ ⋅ g ⋅ h − p u

[0083] In the above relationship, ṅthe volume flow of the glass melt 5, B the width of the drawing tank 7 in the direction along the nozzle opening 9, or along the glass ribbon 3 perpendicular to the drawing direction, η the viscosity of the glass melt 5, DS the local width of the shaft 75, DL the local thickness of the guide body 11, ρ the density of the glass melt 5, g the acceleration due to gravity and h the height of the shaft 75. The integration is performed over the sections HL and HS in the vertical direction z. The integration can also be performed over two or more subsections, in which case the partial integrals must be added. This is the case if, in addition to the conductor body 11, there are further conductor bodies that are separated from each other by a distance in the vertical direction, for example.

[0084] The symbol pudenotes a pressure of 2000 Pa. This value takes into account a still tolerable negative pressure. On the right-hand side of the relationship, therefore, is the pressure pu reduced hydrostatic pressure of the glass melt 5. This term is a constant. The prefactor ṅ / B determines the thickness of the glass ribbon 3. This is predetermined, so that the prefactor also represents a constant. However, for a given thickness of the glass ribbon 3, the strongly temperature-dependent viscosity η can be controlled via the temperature.

[0085] As shown above for the configurations of the direct heating device 2 and the indirect heating device 4, temperature control can also be performed differently locally. Likewise, the temperature in the drawing tank 7 can also vary along the vertical direction. The viscosity can therefore also be location-dependent, η=η(z). This dependency can also be taken into account during integration.

[0086] According to one embodiment, it is therefore provided that the drawing tank 7 comprises a shaft 75 in which the guide body 11 is arranged and which has the nozzle opening 9 at its lower end, wherein the temperature of the glass melt 5 in the drawing tank 7 is adjusted such that the above-mentioned relationship is satisfied with its temperature-dependent viscosity.

[0087] It will be apparent to those skilled in the art that the invention is not limited to the specific embodiments described in the figures, but can be varied in many ways. List of reference symbols

[0088] 1 Device for producing glass ribbons 2 direct heating device 3 glass ribbon 4 indirect heating device 5 glass melt 7 Pull tank 9 Nozzle opening 11 Guide body 14 thermal insulation 15 Drawing onion 17 pulling device 18, 19 Temperature measuring point 20 Control device for regulating the heating output 26, 27 Connections for heating circuit 28 cuff 29 Power connection tab 30 central area of 3 31, 32 Borders of 3 34, 35, 36, 37 first heating zones 50, 52 Partial streams of 5 41, 42, 43, 44, 45, 46 Heating elements of the indirect heating device 4 61, 62, 63, 64, 65, 66 second heating zones 70 Floor of 7 74 Inflow to 7 75 shaft 76 distribution pipe 78 sheet metal 90, 92 Margins of 94, 96 94, 96 Nozzle gap 100 from 9 outstanding part of 11 101 resistance body 103 Sword or blade 201, 202, 203, 204 Power source 206, 207, 208, 209 Heating circuit 700 Wall of 7

Claims

1. An apparatus (1) for producing thin glass ribbons (3), in particular with a thickness of equal to or less than 3000 µm, from a molten glass (5), comprising a drawing tank (7), wherein said drawing tank (7) has a lower elongated nozzle opening (9) through which the molten glass (5) can exit downwards, and comprises direct heating means (2) and indirect heating means (4), the direct heating means (2) comprising at least one heating circuit (206, 207, 208, 209) that is operable to heat at least one area of the drawing tank (7) in the form of at least one first heating zone (34, 35, 36, 37), wherein the direct heating means (2) comprise a respective power source (201, 202, 203, 204) for each heating circuit (206, 207, 208, 209), wherein each heating circuit (206, 207, 208, 209) has connections (26, 27) through which each heating circuit (206, 207, 208, 209) is connected to a wall (700) of the drawing tank (7) so that the current from each power source (201, 202, 203, 204) flows through at least a portion of the wall (700) and heats the wall (700), and wherein each heating circuit (206, 207, 208, 209) of the direct heating means includes current-carrying portions of the wall (700) of the drawing tank (7), and wherein the indirect heating means (4) comprise heating elements (41, 42, 43, 44, 45, 46) for at least one second heating zone (61, 62, 63, 64, 65, 66); characterized in that the drawing tank (7) comprises metal sheets (78) including at least one fine grain stabilized noble metal or one fine grain stabilized noble metal alloy.

2. The apparatus (1) according to the preceding claim, characterized in that the direct heating means (2) comprise at least four separately controllable heating circuits (206, 207, 208, 209) which are operable to heat four different areas of the drawing tank (7) in the form of four first heating zones (34, 35, 36, 37), wherein the direct heating means (2) comprise connections (26, 27) for each of the four heating circuits, through which the heating circuits (206, 207, 208, 209) are connected to the wall (700) of the drawing tank (7).

3. The apparatus (1) according to any one of the preceding claims, characterized in that the indirect heating means (4) comprise heating elements (41, 42, 43, 44, 45, 46) for at least three, preferably for at least five or more separately controllable, spatially distinct second heating zones (61, 62, 63, 64, 65, 66).

4. The apparatus (1) according to the preceding claim, characterized in that the first heating zones (34, 35, 36, 37) of the direct heating means (2) and the second heating zones (61, 62, 63, 64, 65, 66) of the indirect heating means (4) are arranged on and around the drawing tank (7) in a vertically and / or horizontally distributed manner.

5. The apparatus (1) according to any one of the preceding claims, characterized in that heating currents applied to the metal sheets (78) of the drawing tank (7) for direct heating are less than 2500 A, preferably less than 1000 A; and / or that the indirect heating means (4) are designed so as to contribute more than 50 % to the total power output, as a base load.

6. The apparatus (1) according to any one of the preceding claims, characterized in that the drawing tank (7) comprises flanges or collars (28) as the power connections (26, 27) for direct heating.

7. The apparatus (1) according to any one of the preceding claims, characterized in that a plurality of temperature measurement points (18, 19) are connected to the drawing tank (7) in a form-fitting manner, in particular welded thereto, wherein preferably at least two temperature measurement points (18, 19) are provided, which are feedback control measurement points for regulating the heating power of the direct heating means (2) or of the indirect heating means (4) for a feedback control unit (20) for controlling the heating power output.

8. The apparatus (1) according to any one of the preceding claims, characterized in that the heating elements (41, 42, 43, 44, 45, 46) of the indirect heating means (4) comprise platinum, stainless steel, or SiC as a resistance heating material, and / or comprise heating tiles or meandering heaters.

9. The apparatus (1) according to any one of the preceding claims, wherein the fine grain stabilized noble metal has at least one of the following features: - the fine grain stabilized noble metal is one of metals Pt, PtRh, PtAu, PtRhAu, PtIr; - the fine grain stabilized noble metal contains nanoparticles, such as ZrO2 particles.

10. The apparatus (1) according to any one of the preceding claims, characterized in that an inlet (74) is provided upstream of the drawing tank (7), and wherein said inlet (74) opens into a manifold pipe (76) of the drawing tank (7) which opens into a chamber (75), wherein said chamber (75) has a smaller cross section than the manifold pipe (76) and has the nozzle opening (9) at its lower end.

11. The apparatus according to any one of the preceding claims, characterized in that the power sources (201, 202, 203, 204) are connected to the drawing tank (7) such that at least three first heating zones (34, 35, 36) are defined, which are distributed in the upper portion of the drawing tank (7) transversely to the drawing direction of the glass ribbon (3), wherein one of the first heating zones (35) is preferably arranged centrally, and wherein a further first heating zone (37) heats the lower portion of the drawing tank (7) including the nozzle opening (9).

12. The apparatus according to any one of the preceding claims, characterized in that the drawing tank (7) comprises a guiding body (11) which protrudes out of the nozzle opening (9) of the drawing tank (7) and downward, wherein said guiding body (11) is supported so as to be spaced apart from the edges (90, 92) of the nozzle opening (9) so that two nozzle slots (94, 96) are defined between the guiding body (11) and the edges (90, 92) of the nozzle opening (9); and wherein the guiding body (11) is heated, wherein the guiding body (11) has its own separately controllable connections, and wherein at least one heating circuit independent of the drawing tank (7) is provided.

13. A method for producing thin glass ribbons (3), in particular with a thickness of equal to or less than 3000 µm, using an apparatus (1) according to any one of the preceding claims, wherein the molten glass (5) is fed into the drawing tank (7) via an inlet (74), and wherein the molten glass (5) emerges from the nozzle opening (9) and is drawn off to form the glass ribbon (3), wherein the drawing tank (7) is heated by using both the direct heating means (2) and at the same time also the indirect heating means (4).

14. The method according to the preceding claim, characterized in that the molten glass (5) is distributed via the inlet (74) to the drawing tank (7) and into a manifold pipe (76) of the drawing tank (7), which opens into a chamber (75), wherein said chamber (75) has a smaller cross section than the manifold pipe (76), wherein a consistent linear throughput of molten glass (5) along the nozzle opening (9) is adjusted through the temperature profile in the drawing tank (7).

15. The method according to any one of the preceding claims 13 - 14, characterized in that the drawing tank (7) comprises a chamber (75) in which a guiding body (11) is arranged and which has the nozzle opening (9) at its lower end, wherein the temperature of the molten glass (5) in the drawing tank (7) is adjusted such that with the temperature-dependent viscosity η thereof the following relationship is fulfilled: 48 v ˙ B ∫ H L η D S − D L 3 dz + 12 v ˙ B ∫ H S η D S 3 dz > ρ ⋅ g ⋅ h − p u wherein ν̇ is the volume flow of the molten glass, B is the width of the drawing tank (7) in the direction along the nozzle opening (9), DS is the local width of the chamber (75), DL is the local thickness of the guiding body (11), ρ is the density of the molten glass, g is the gravitational acceleration, h is the height of the chamber (75), and pu is a pressure of 2000 Pa.

16. The method according to any one of the preceding claims 13 - 15, characterized in that a temperature gradient of Tgrad = Tcentre -Tedge = 0 to 50 K, preferably Tgrad = 20 to 40 K, is adjusted in the drawing tank (7) along the nozzle opening (9) from the edges (90, 92) thereof to the centre of the nozzle opening (9).