Method and device for thickness control of a material strip

Abstract

Method for producing a material strip (1) in which a material (2) is drawn into a material strip (1) in a heated and softened state and then cools, wherein the material (2) is heated during the forming process in which the material strip (1) is formed and drawn, wherein thermal energy is supplied to the material (2) during the forming process at least partially in the form of thermal radiation, which is emitted from the surface (50) of a heated heating element (5) arranged opposite the material (2), wherein the heating of the heating element (5) is partially effected by the energy of a laser beam (70) which is directed at the heating element (5) and locally heats the heating element (5), wherein the laser beam (70) is moved in a direction transverse to the longitudinal direction of the material strip (1).so that the energy of the laser beam is distributed on the heating element (5) in a direction transverse to the longitudinal direction of the material strip (1), and the heating element (5) is additionally heated electrically or with a burner.

Description

Field of invention The invention relates generally to a method for thickness control of a material strip, for example a substrate made of glass or plastic. In particular, the invention relates to a method and a device, Background of the invention Methods for manufacturing substrates, such as glass or plastic substrates, have been known for a long time. However, these processes, particularly when manufacturing glass or plastic strips, often result in thickness variations across the strip's transverse coordinate and a high total thickness variation (TTV). These inhomogeneous thickness distributions and the high TTV are the result of temperature differences that occur across the strip's width during the manufacturing process. To counteract the thickness variations and the high TTV, a number of different solutions are proposed. For example, DE 101 28 636 C1 discloses a method in which the thickness of a flat glass is selectively influenced, as well as a device for carrying out such a method. In this process, a flat glass, immediately after forming, is passed through a device extending across the entire width of the glass strip. This device provides controlled cooling of the glass and allows for targeted and adjustable heating across its entire width, with the heat being supplied locally by means of a laser beam. This laser beam is guided across the width of the glass strip at a high frequency, and its power is precisely adjusted to achieve a spatially resolved heating output. In this way, a flat glass is obtained that has a thickness that is as constant as possible across its width. Furthermore, DE 10 2008 063 554 A1 describes a method and a device for manufacturing flat glass in which the thickness can also be selectively controlled across the width of the glass. In this process, a glass ribbon is drawn through a slot die and then guided into a drawing chamber, which is designed such that at least part of the wall across the width of the glass ribbon exhibits locally varying radiation absorption and / or thermal conductivity. To further assist in thickness control, a laser beam can be used to locally influence the glass ribbon. Additionally, a gas flow can also be used to selectively influence the thickness of the ribbon. In this way, a glass ribbon is obtained in which a desired thickness profile can be set across its width, for example, with a greater glass thickness in the center of the ribbon than towards its edges. US Patent 8,904,822 B2 discloses a method for obtaining a substrate of controlled thickness made of glass or plastic. In this method, a strip of glass or plastic is drawn by applying tension to the edges of the strip. The thickness of the strip is then determined and monitored. If a deviation in thickness is detected, the area of ​​this deviation is selected, which is in a viscous state. This selected area, which is in a viscous state, is then heated by directing a laser beam onto it. The heating process causes the area to reach the specified thickness. This heating process involves controlling the laser power, the laser's dwell time on the selected area, and / or adjusting the laser's wavelength. From WO 2015 / 080 897 A1, a device for manufacturing glass and a method for manufacturing a glass ribbon are known. The device comprises a heating module for heating a surface by means of thermal radiation. For this purpose, the heating elements are designed in the form of finely segmented heating coils, so that a homogeneous, ideally 100%, heating coverage of the glass ribbon is achieved. This is a delicate, very failure-prone structure. US 9,290,403 B2 describes heating systems for use in glass production. The distance between the heating element and the glass sheet is adjustable to achieve the desired temperature of the glass ribbon or glass surface. WO 2014 / 098 209 A1 describes an optimization of the temperature distribution during the production of a glass ribbon. The temperature distribution is achieved by means of an offset arrangement of the heating elements of two heaters, which are positioned on the two opposing surfaces of the glass ribbon. Finally, WO 2011 / 066 064 A2 describes a thickness control of a glass ribbon using a heat sink / heater combination. However, the aforementioned methods and devices have a number of disadvantages. For example, if a gas stream is applied to the strip via nozzles for thickness control, this can cause waviness across the net width of the strip. This waviness results from the nozzle width and the distance between the surface cooled by the nozzle and the glass. For instance, the resolution of such nozzles is approximately 30 mm, meaning that fine waviness in the strip with a periodicity or wavelength of less than 30 mm cannot be eliminated in this way. Here, the net width refers to the portion of the glass strip where its properties fall within the specified parameters. The net width of a glass strip is therefore the width of the quality range of the glass strip and is generally obtained from the drawn glass strip after removing the edges. Furthermore, measures to influence the thickness of a glass or plastic strip target a region where the strip is in a viscous state. This viscous state is defined, as can be seen, for example, in US 8,904,822 B2, as a region where the viscosity is greater than 105 dPas. To ensure sufficient heating of the material to make the strip sufficiently fluid to achieve a reduction in thickness, very high power levels must be applied. This not only makes the process expensive, but also results in insufficiently precise thickness control due to the high energy or power of the laser beam, meaning that certain variations in thickness across the net width of the glass or plastic strip persist. WO 2016 / 085778 A1 discloses a device for producing a thin glass ribbon from a glass preform, in which the glass can be heated with a laser. A thermal distributor may also be provided. US 2009 / 0217705A1 describes a device in which a glass pane is formed with a refractory body that can be heated with a laser. From DE 100 64 977 C1 a device for the production of thin glass sheets is known in which the local temperature profile can be set with segmented heating and cooling elements. In the case of very finely adjustable positioning or very delicate structures, such as the WO 2015 / 080 879 A1 or the US 9 290 403 B2, the resulting structures are very susceptible to interference. In the case of the heater arrangement according to WO 2014 / 098 209 A1, only locally acting temperature corrections can be set, and not arbitrarily. With thickness control, as proposed in WO 2011 / 066 064 A2, the combination of heat sink and heater results in a high control complexity. Locally acting temperature corrections are also difficult to achieve here. Therefore, there is a need for a method for the controlled adjustment of the thickness of a strip of glass or plastic, which reduces the existing weaknesses of the prior art. Object of the invention The object of the invention is to provide a flexible method for controlling the thickness of a material strip, in particular a glass strip, and a device suitable for carrying out such a method. Summary of the invention The problem is solved by the subject matter of the independent claims. Preferred embodiments are found in the dependent claims. Accordingly, the invention provides a method for producing a material strip in which a material is drawn into a strip in a heated and softened state and then cools, wherein the material is heated during the forming process in which the strip is formed and drawn, wherein thermal energy is supplied to the material during the forming process at least partially in the form of thermal radiation radiated from the surface of a heated heating element arranged opposite the material, wherein the heating of the heating element is effected at least partially by the energy of a laser beam directed at the heating element and locally heating the heating element.For the purposes of the invention, the term "heated and softened" also includes states of the material with low viscosity, in particular also melting of the material. Unlike DE 101 28 636 C1, the material is not heated directly with the laser. Instead, indirect heating is used, in which a heating element is heated, which then emits thermal radiation that is absorbed by the material of the strip, thus heating it. This offers the advantage that the heating element has a certain heat capacity and can transfer the applied energy evenly to the material of the strip. In contrast, a laser beam shining directly onto the material strip can only heat the material at the moment and at the point of impact. If the laser beam is then passed over the surface to heat a larger area, the heating occurs essentially along the path swept by the point of impact and is therefore somewhat inhomogeneous. These inhomogeneities can subsequently manifest themselves as variations in the thickness of the material strip after drawing.Another point is that a laser beam directed locally onto the glass can cause changes in the structure or composition due to the high local heat output. To carry out the method, the invention further provides a device for producing a material strip, comprising: a drawing device for drawing out a material in a heated and softened state into a strip; and a heating element for heating the material during the forming process in which the strip is formed and drawn out, wherein the heating element is designed for heating by thermal radiation, which is emitted from the surface of the heating element arranged opposite the heated or to-be-heated material; and wherein the device comprises a laser which is arranged such that the laser beam of the laser is directed at the heating element and heats the heating element locally. Typically, the heating element also heats the material. However, it is also possible that heat is drawn from the material strip at the heating element, causing the strip to cool down. The heating element can then be used to selectively slow down and thus control this heat dissipation. Local heating of the heating element with the laser beam therefore does not necessarily lead to local heating of the material strip, but can instead locally slow its cooling. A typical embodiment in which, despite the heating element, there is a net heat dissipation from the material strip and thus cooling, is a cooling oven. Such an oven is used particularly in glass manufacturing to bring the glass from a softened state to temperatures below its cooling point in a controlled manner. In principle, the invention can be used in all processes where direct laser irradiation of the material being processed is disadvantageous or not feasible, or to improve thickness homogeneity compared to direct irradiation. It is particularly intended for the production of glass ribbons, but the process is also applicable in the plastics, metal, and semiconductor industries. With regard to glass as the material, all hot forming processes for shaping glass ribbons, in which the glass is drawn, are suitable. The temperature of the material strip is preferably controlled on both sides using heating elements. Accordingly, heating elements are arranged on both sides of the material strip and heated by a laser beam. However, for very thin glass strips, such as those less than 300 µm thick, heating on one side may also be sufficient. Brief description of the drawings Fig. 1 shows a schematic view of a first embodiment of the invention. Fig. 2 shows diagrams of the voltage drop along two electrically operated heating elements. Fig. 3 shows details of a device for producing a material strip. Fig. 4 is a view of an embodiment of a heating element. Fig. 5 shows an embodiment with multiple heating elements. Fig. 6 shows an embodiment of the invention with a plate-shaped heating element. Fig. 7 shows a variant of the example in Fig. 6. Fig. 8 shows measured values ​​of the position of the center of a material strip. Fig. 9 shows measured values ​​of the thickness of a glass strip before and after an adjustment of the heating power, as well as the heating power as a function of the transverse coordinate of the strip. Fig. 10 shows a device for producing glass strips with a float tray. Fig. 11 is an embodiment of a device for down-drawing a glass strip. Fig. 12 shows an embodiment of a heating element. Detailed description of the invention The device 10 shown in Fig. 1 is used to produce a strip of material 1 using the method according to the invention. The device comprises a drawing device 12 for drawing a material 2 into a strip in a heated and softened state, and a heating element 5 for heating the material 2 during the forming process in which the strip of material is formed and drawn. The heating element 5 heats the material 2 by thermal radiation. For this purpose, the surface 50 of the heating element 5 is arranged at a distance from the material 2, so that the thermal radiation is emitted from the surface 50 of the heating element 5 opposite the heated material and is then absorbed by the material 2. Due to inhomogeneous heating, or also due to inhomogeneous temperature radiation from the strip of material 1, different temperatures can occur along the strip's transverse coordinate during the forming process.This leads to inhomogeneous thickness distributions and a higher TTV (total thickness variation). Using a laser, the temperature distribution of the heating element is corrected to achieve a lower TTV. The laser beam delivers additional heat to specific locations within the heating element. According to the invention, a laser 7 is provided to supply at least part of the heating power of the heating element 5. For this purpose, the laser 7 is arranged such that its laser beam 70 is directed towards the heating element 5, so that the heating element 5 heats up locally through the absorption of the laser radiation. Suitable laser sources include, in particular, gas lasers, such as a CO2 laser, as well as solid-state lasers and diode lasers. In order to achieve a sufficiently local effect of heating with the laser, it is generally preferred, without limitation to the example shown, if the distance of the surface of the heating element facing the material strip in the direction transverse to the material strip 1 is smaller than its dimension perpendicular to the longitudinal direction of the material strip. Due to its heat capacity, the heating element 5 acts as a buffer for the laser energy. This energy is continuously released through the heating element, even if the laser's point of impact 71 is moved and / or the laser power is varied. In general, without being limited to the example shown, it is advantageous not to rely solely on a laser for heating, so that a high heating power can be provided easily. The laser beam then serves as an additional heater to create a desired temperature profile on the heating element or to compensate for local or temporal inhomogeneities in the temperature distribution. Thus, it is planned that the heating element 5 will also be heated electrically or with a burner. According to one embodiment of the invention, the material 2 can be provided in the form of a preform 6, from which the material strip 1 is then drawn by applying tension to the strip 1 using the drawing device 12, while the preform 6 is heated and softened by means of the heating element 5 and optionally further heating elements. This method is particularly suitable for producing a glass strip from a glass preform. However, temperature variations in the direction transverse to the direction of drawing manifest themselves directly as an inhomogeneous thickness of the strip. According to the invention, local heating with the laser beam 70 can now be used to selectively heat the heating element locally in order to counteract such temperature variations. It would be conceivable to focus the laser beam 70 precisely on a specific point, provided that the position in question on the material strip 1 is cooler than adjacent areas.A particularly good and flexibly adjustable compensation is generally achieved by moving the laser beam 70 in a direction transverse to the longitudinal direction of the material strip 1, so that the energy of the laser beam is distributed on the heating element 5 in a direction transverse to the longitudinal direction of the material strip 1. For this purpose, as also implemented in the example of Fig. 1, a beam deflection device 9 can be provided to move the point of impact of the laser beam 70 on the heating element 5 in a direction transverse to the longitudinal direction of the material strip 1, so that the energy of the laser beam is distributed on the heating element in a direction transverse to the longitudinal direction of the material strip 1. Generally, it is preferred if the distribution of the laser power on the heating element can be controlled. According to one embodiment of the invention, a control device 15 is provided for this purpose, which is configured to change the position of the point of impact 71 of the laser beam 70 of the laser 7 in order to influence the temperature distribution on the heating element 5. In general, such a control device 15 can also control other parameters. In particular, control of the laser power should be mentioned here. Accordingly, in the example of Fig. 1, the control device 15 is connected to both the beam deflection device 9 and the laser 7. According to an embodiment of the invention, and without being limited to the illustrated example, at least one sensor is advantageously provided which acquires measured values ​​and is connected to the control device, wherein the control device 15 is configured to control at least one of the parameters position, irradiation duration, and radiant power of the laser beam 70 based on the acquired measured values. As in the example of Fig. 1, the sensor 8 can acquire measured values ​​of the material strip 1. These can include the local temperature of the material strip 1, but also its thickness or position.An example where the position of the material strip is influenced by the laser beam 70 is explained in more detail below. A galvanometer scanner or a polygon mirror is particularly suitable as a beam deflection device 9. According to a further embodiment, the laser light is transmitted via a fiber optic cable or a light guide. It is possible to move the exit end of the light guide, and thus also the exiting laser beam, in order to adjust the point of impact. Fig. 2 shows two diagrams of the measured voltage drop across electrically heated silicon carbide heating tubes. The bars around the measured values, represented as lines, represent the respective error bars. Generally, the voltage drops from left to right, but the voltage values ​​do fluctuate measurably. The heating power and temperatures also vary accordingly. These measured values ​​can, for example, be used to create a motion profile of the point of impact 71 of the laser beam 70, which counteracts the inhomogeneous temperature distribution. In general, without limiting oneself to the example of Fig. 2, one embodiment of the invention provides that the temperature distribution along the heating element 5 is determined and the point of impact 71 of the laser beam 70 is moved depending on this temperature distribution. The temperature distribution can be determined directly in the form of temperature measurements, or indirectly using a parameter that is influenced by or affects the temperature. In the example of Fig. 2, this parameter is the voltage drop along the heating tube. An example of the production of a glass ribbon using the method according to the invention is described below with reference to these two diagrams. Fig. 3 shows a schematic view of a device according to the invention, looking towards the edge of a material strip 1, and further embodiments of the invention. As in the example shown in Fig. 1, the material strip 1 is drawn from a preform 6, with the tension being exerted by a pulling device 12. The pulling device 12 can, for example, comprise driven rollers, as shown. According to one embodiment of the invention, the heating element 5 comprises an electrically heated heating tube 51. In particular, two spaced-apart heating tubes 51 can also be provided, as shown, between which the material 2 to be processed, or the material strip 1, is passed. The heating tube 51 is generally preferably arranged transversely to the longitudinal direction of the material strip 1, or the longitudinal direction of the heating tube runs transversely to the longitudinal direction of the material strip, in order to heat the material strip across its width.On the other hand, inhomogeneities in the temperature distribution of the material strip 1 can also occur across its width. Such irregularities are counteracted by the additional local heating of the heating tube 51 with the laser beam 70. However, with a tube, the problem arises that its side facing the material strip 1 is also covered by the material strip. In order to nevertheless direct the laser beam 70 onto this area of ​​the heating tube, a further development of the invention provides that the side of the outer surface 52 of the heating tube 51 facing away from the material strip 1 has an opening 53. In this way, the laser beam 70 can pass through the opening 53 and the heating tube 51 and strike the inner side of the outer surface 52 facing the material strip 1.If the laser beam 70 is not only to illuminate a specific point on the lateral surface 52, it is advantageous to design the opening 53 as a slot extending in the longitudinal direction of the heating tube 51. In this way, the laser beam can be guided along the slot, so that the point of impact of the laser beam 70 can be selectively positioned in the longitudinal direction of the heating tube 51. As shown in the example of Fig. 3, the heating element 5 can further be surrounded by thermal insulation 17. The thermal insulation 17 advantageously has an opening 18 to allow the laser beam 70 to pass through this opening 18 and shine onto the heating element 5. Fig. 4 shows, for clarification, another view of such a heating element in the form of a heating tube 51 with a slot-shaped opening 53 in the outer surface 52 of the tube, as well as the laser beam 70 passing through the slot-shaped opening and striking the inside. As shown, the laser beam 70 can be moved along the slot to vary the point of impact. Fig. 5 shows an embodiment in which the material strip 1 is produced using the overflow fusion process. In this process, an open-topped vessel 19 is continuously filled with molten material 2, typically a glass melt 3, so that the material 2 eventually overflows the rim 20 of the vessel 19 and flows down its sides. The material flow then merges at the bottom of the vessel 19, typically in a draw-off point 21, from which the material strip 1 is drawn off the vessel by a drawing device (not shown). To prevent premature cooling of the molten material 2 as it flows down the sides of the vessel 19, several heating elements 5, in this case in the form of heating tubes 51, are arranged one above the other on both sides.At least one, preferably several as shown, of the heating elements 5 on each of the sides are each locally heated by means of a laser beam 70 in order to keep the temperature distribution of the molten material 2 as uniform as possible. Fig. 6 shows an example of a further embodiment of the invention. This embodiment is generally based on the fact that the heating element 5 comprises a plate 55, one side 56 of which faces the material 2 of the material strip 1 and the opposite side 57 of which can be heated by the laser beam 70. Here, too, a surface of the heating element 5 facing away from the material strip 1 is generally irradiated with the laser beam 70. This is generally advantageous because, in the case of very strong, short-term heating of the irradiated point, impurities may evaporate from the irradiated surface and precipitate onto the material strip. The plate 55 is preferably aligned parallel to the material strip. Here, a planar, or two-dimensional, temperature distribution can be influenced by one or more laser beams 70.This allows not only the temperature of the plate 55 to be increased locally, but also, in the case of an increase in an area extending in the direction of travel of the material, the duration of the temperature increase to be generally adjusted. The heating element 5, in the form of a plate 55, is heated by heating devices 22, which provide the main heat output. These heating devices 22 can be, for example, combustion heaters or electric heaters. As in the example of Fig. 5, in the example of Fig. 6, the material strip 1 is produced using the overflow fusion process, although, of course, other methods, such as drawing from a preform, can also be used. For the sake of simplicity, only one plate 55 is shown in Fig. 6. Generally, however, two opposing plates are used, between which the material 2 of the material strip 1 is passed. The heating devices 22 can advantageously also be adjustable in their position to adjust the spatial distribution of the heat output.The combination of conventional heating systems with laser heating is particularly advantageous, as the laser alone may not be able to provide the desired heating power. According to a further embodiment of the invention, the material strip 1 is produced using a downdraw process. An example of this is shown in Fig. 7. In this process, the molten material 2, preferably molten glass 3, emerges from a downward-facing nozzle 25 and is also drawn into a material strip by means of a drawing device (not shown in the figure). As shown, the nozzle 25 can comprise a central nozzle body 26, which projects from the nozzle opening and around which the molten material flows. As in the example of Fig. 6, a plate 55 is provided as a heating element 5, which faces the material strip 1 with one side 56 and heats the nozzle 25 and the emerging material 2. The plate 55 is heated, as in the example of Fig. 6, by heating devices 22 and one or more laser beams 70 pivotable via the side 57 facing away from the nozzle 25 and the material strip 1. Again, only one of the plates 55 arranged opposite the material strip 1 is shown. The following presents examples of the invention and comparative examples for the production of glass ribbons. Comparative example 1 In a redrawing machine, a preform 6 made of optical glass with a refractive index > 1.7, with a thickness of 14 mm and a width of 380 mm, is drawn into a glass ribbon with a thickness of 300µm. The redrawing system comprises a preforming drive, a vertical furnace, and a drawing unit with a drawing device 12. The furnace includes a preheating zone, a hot forming zone, and a cooling zone. The preheating and cooling zones are equipped with segmented coil heaters. The hot forming zone is heated by means of two opposing, horizontally arranged silicon carbide heating tubes 51, through which an electric current flows. The heating tubes 51 have a heatable length of 500 mm and a diameter of 25 mm. To obtain the most homogeneous temperature distribution possible, several heating pipes are measured with regard to their heating power distribution. For this purpose, the pipe to be measured is subjected to an electric current, and the resistance drop is measured every 5 cm along the length of the pipe over a measuring distance of 3 cm using contact pins. Based on the measurements, two tubes are selected and arranged such that the mean power density is constant across the entire width of the furnace. Particular attention is paid to ensuring that the heating power distributions are symmetrical about the vertical central axis of the heating zone. The voltage drops along these two heating tubes are the measured values ​​shown in diagrams (a) and (b) of Fig. 2. The thickness across the glass ribbon and the position of the ribbon edges are determined using confocal chromatic thickness measurement with an optical sensor moved perpendicular to the glass ribbon. The resulting thickness and position signals are electronically recorded and evaluated. The resulting glass ribbon 1 has a net area where the thickness deviates from the target thickness by less than + / - 15 µm. This net area, with a width of 180 mm, is bordered by thicker edges at the periphery of ribbon 1. The deviations from the target thickness in the net area can be reduced by adding cooling nozzles below the SiC tube. However, this leads to increased warping of the glass ribbon, as the nozzle settings also affect cooling and thus the stresses in the glass ribbon. Additionally, a residual deviation from the target thickness of approximately + / - 10 µm remains. The remaining asymmetry of the heating pipes may cause the glass ribbon to run off-center through the cooling oven. This leads to asymmetrical cooling and consequently to warping of the glass ribbon. Example 1: In the redrawing system from Comparative Example 1, a heating tube is slotted on the side facing away from the furnace interior over a width of 420 mm and a height of 20 mm. The furnace insulation is modified so that a laser beam can be directed onto the inside of the SiC heating tube, resulting in an arrangement as shown schematically in Fig. 3. A CO2 laser beam with a nominal power of 3 kW is used. The laser beam 70 is guided across the inside of the tube in an oscillating motion at a speed of 20 m / s by means of optics. The control system allows for targeted control of the laser beam power at any point on the heating tube. The beam has a diameter of < 20 mm at the heating tube. Adjustment of the band position: To guide the ribbon centrally through the cooling zone, energy is supplied asymmetrically at the edge of the heater. A redistribution of 30 W of heating power over a distance of 60 mm results in a 30 mm correction of the glass ribbon's position. The result is illustrated in the diagram in Fig. 8. The diagram shows the position of the ribbon's center as a function of its length. At the mark in the form of a vertical line, approximately at length position 424, the laser power was switched. It is clearly visible how, as a consequence, the ribbon's center position shifts to approximately 410 mm at higher values. Net width setting: To increase the net width of the glass ribbon, laser power is selectively applied to the edges. An addition of 22 W of heating power over a distance of 30 results in a 20% increase in the net width of the glass ribbon. Adjusting the thickness deviation: Thickness deviations occurring in the net area are corrected by selectively adding or removing laser power. Adding 22 W of heating power over a distance of 20 mm results in a 3% reduction in the thickness of the glass ribbon in this area. This allows the thickness deviation of the glass ribbon to be adjusted to less than + / - 0.5 µm without negatively affecting the warp. Fig. 9 additionally shows the measurement of the glass ribbon thickness before and after correction. The dashed line represents the measured values ​​of the ribbon thickness in the transverse direction before correction, the solid line the measured values ​​after correction. In the lower half of Fig. 9, the measured values ​​of the heating power along the transverse direction are also shown. On the far right of the diagram, for position values ​​above 25, two additional power values ​​are shown as circles. These are the values ​​after the laser is switched on, which lead to the changed strip thickness. As can be seen, the strip thickness fluctuates less, especially at the edge. Furthermore, the net width of the strip, i.e., the area of ​​constant thickness between the edges, can be widened somewhat. Comparative example 2 To produce an aluminosilicate glass ribbon with a thickness of 700 µm for manufacturing glass sheets suitable for displays, the glass melted in a glass melting tank is guided over a vessel 19 in the form of an overflow channel (isopipe), and the two overflowing glass strands are rejoined under the channel in a drawing bulb 21. The glass ribbon is thus produced using the overflow fusion process, as shown in Fig. 5. The temperature distribution of the glass layer on the isopipe and in the drawing bulb is of particular importance. A homogeneous temperature distribution is achieved by segmented and mechanically adjustable heaters and coolers, as well as radiant plates. Due to the required spatial extent of the heaters, their effect can also be observed in areas on the order of a few centimeters. Example 2 The insulation is modified, and the heater arrangement is replaced, as shown in Fig. 5, by heating elements that are subjected to a scanning laser beam 70 with adjustable laser power, depending on their position, on the side facing away from the furnace interior. This allows the transverse temperature distributions to be precisely controlled along the glass flow during hot forming. Due to the thermal inertia of the heating elements 51, it is generally sufficient to use a single laser 7. Comparative example 3 To produce a glass ribbon with a thickness of 700 µm, again made of aluminosilicate glass, for example for displays, the glass melted in a glass melting tank is guided through a slot die with a blade, or central die body 26, as in the example shown in Fig. 7, and the two glass strands exiting the die body are rejoined in a drawing bulb 21. The temperature distribution of the glass layer on the die body 26 and in the drawing bulb 21 is of particular importance. A homogeneous temperature distribution is achieved by segmented and mechanically adjustable heating elements 22, as well as heating elements 5 in the form of plates 55. Due to the required spatial extent of the heating elements 22, their effect can also be observed in areas on the order of a few centimeters. Example 3 The insulation and heater arrangement are modified so that the radiant plates on the side facing away from the furnace interior are additionally subjected to a scanning laser beam 70, as shown in Fig. 7, with adjustable laser power depending on their location. This allows the transverse temperature distributions to be precisely controlled along the glass flow during hot forming. Due to the thermal inertia of the heaters, it is sufficient to use only one laser. The methods for producing glass ribbons described in the above exemplary embodiments, which involve drawing glass ribbons from molten glass, include the updraw process and, in particular, the float process. Fig. 10 shows a schematic representation of an embodiment of a device 10 with a float tank 29. A melting tank 30 is arranged upstream of the float tank 29, in which a glass melt 3 is produced. The glass melt 3 flows from an outlet 33 onto the tin bath 34 in the float tank 29. The glass melt 3 spreads out over the surface and, supported by drawing rollers 31 as part of the drawing device 12, forms a ribbon of material. As the glass is moved over the tin bath, it slowly cools and solidifies, so that it can be lifted off at the end of the float tank 29. The glass ribbon then passes through a cooling furnace 32. The heating element 5 is arranged above the material ribbon in the superstructure of the float tank 29.The laser beam 70 can, for example, be introduced through a slot in the superstructure of the float tray 29. Here, too, the viscosity of the glass can be locally fine-tuned by the heating element using the laser's heating power to compensate for thickness variations. In general, and without limiting itself to the exemplary embodiment, the forming process of the material strip is thus intended to include the production of a material strip using the float process. Fig. 11 shows details of another embodiment of the invention. In this example, the down-draw process for producing a glass ribbon is also used, as shown in Fig. 7. The nozzle 25 is arranged in a drawing chute 35. The drawing chute 35 is preferably made of metal sheets, for example, stainless steel. The drawing chute prevents lateral airflows and thus supports controlled cooling. The laser beam 70 is introduced through an opening 36 in the drawing chute and then strikes the heating element arranged inside the drawing chute 35. Preferably, the opening 36 is slot-shaped to allow lateral movement of the laser beam over the heating element 5 in the transverse direction of the material ribbon 1. To minimize the impact on airflows inside the drawing chute, the dimensions of the opening 36 are as small as possible.According to one embodiment of the invention, the dimension of the opening 36, without being limited to the specific example shown and also without being limited to the specific drawing method, is at least three times the diameter of the laser beam, at least in the drawing direction. According to an alternative or additional embodiment, the dimension of the window in the drawing direction is at most 10 mm, preferably at most 7 mm, for example between 3 mm and 7 mm. Provided that materials transparent to the wavelength of the laser beam exist, a window 37 can be provided according to a further alternative or additional embodiment of the invention, which closes the opening 36 and through which the laser beam 70 is directed into the draw chute. A window 37 made of quartz glass is given as an example. According to a further alternative or additional embodiment, the opening 36 can itself be shaped differently than shown in the illustration in order to prevent air currents. For example, a collar can be placed on the draw chute 35 and surround the opening 36. In general, without being limited to the drawing process, whether from the melt or from a preform, and without being limited to the specific configuration shown in Fig. 7, a further development of the invention, as described above, provides that the device comprises a drawing shaft 35 in which the material 2 is drawn into a strip of material 1 in a heated and softened state, wherein the heating element 5 is arranged inside the drawing shaft 35, and wherein the laser beam 70 is directed onto the heating element 5 through an opening in the drawing shaft. Preferably, at least one of the following features is implemented: - the drawing shaft is closed with a window transparent to the laser beam 70, - the opening is shaft-shaped, with a collar surrounding the opening 36, - the dimension of the opening 36 in the drawing direction is at most three times the diameter of the laser beam. Many different materials can be used for the heating element 5. One criterion is the coupling to the laser beam. For this purpose, the material of the surface irradiated by the laser beam should have the lowest possible reflectivity for the wavelength of the laser beam 70. In general, ceramic materials are well suited for this. According to one embodiment of the invention, the heating element 5 comprises a ceramic material. This includes a SiC heating element, as mentioned above. Silicon carbide exhibits very high thermal conductivity. This is advantageous for rapidly conducting the laser power, typically directed from the rear, with low inertia to the side of the heating element 5 facing the material strip. However, this can also lead to heat dissipation within the heating element in a direction perpendicular to the material strip.In order to obtain a heat distribution as defined as possible transverse to the material strip, a further development of the invention provides that the heating element has a heat conduction or thermal conductivity in the direction transverse to the longitudinal direction of the material strip 1 (determined by the drawing direction in the forming process), which is lower than the heat conduction or thermal conductivity in the direction from the side facing away from the material strip 1 to the side facing the material strip, i.e. in the direction towards the material strip. Fig. 12 shows an example of how such anisotropic thermal conductivity can be achieved. The heating element 5 in this example is a composite plate 55. The sequence of sections made of different materials with varying thermal conductivities is chosen such that the heat flow in the transverse direction 61 of the material strip is smaller than perpendicular to it, i.e., in the longitudinal direction of the material strip and also in the direction towards the strip. The transverse direction 61 is shown for clarity. The reduction of the heat flow in the transverse direction 61 is achieved, for example, by a sequence of metallic and ceramic sections 59. The metallic sections 59 can also be connected to each other in a meandering pattern, so that they can be supplied with an electric current to heat the heating element 5. The laser irradiation then serves as supplementary heating to achieve a specific heating power profile in the transverse direction 61. To reduce unwanted heat flow, the heating element can generally comprise a material with a thermal conductivity of less than 50 W / m·K. In the example shown, this can apply to both the metallic and ceramic sections 59 and 60. The composite material can then reduce the heat flow in the transverse direction 61 even if another material in the composite has a high thermal conductivity. Reference symbol list 1 Material strip 2 Material 3 Glass melt 5 Heating element 6 Preform 7 Laser 8 Sensor 9 Beam deflection device 10 Device for producing a material strip 12 Drawing device 15 Control device 17 Thermal insulation 18 Opening in 17 19 Vessel 20 Rim of 19 21 Drawing bulb 22 Heating device 25 Nozzle 26 Nozzle body 29 Float tank 30 Melting tank 31 Drawing roller 32 Cooling furnace 33 Outlet 34 Tin bath 35 Drawing shaft 36 Opening in 36 37 Window 50 Surface of 5 51 Heating tube 52 Shell surface of 51 53 Opening in 52 55 Plate 56, 57 Sides of 55 59 Metallic section 60 Ceramic section 61 Transverse direction 70 Laser beam 71 Point of impact of 70 on 5

Claims

Method for producing a material strip (1) in which a material (2) is drawn into a material strip (1) in a heated and softened state and then cools, wherein the material (2) is heated during the forming process in which the material strip (1) is formed and drawn, wherein thermal energy is supplied to the material (2) during the forming process at least partially in the form of thermal radiation, which is emitted from the surface (50) of a heated heating element (5) arranged opposite the material (2), wherein the heating of the heating element (5) is partially effected by the energy of a laser beam (70) which is directed at the heating element (5) and locally heats the heating element (5), wherein the laser beam (70) is moved in a direction transverse to the longitudinal direction of the material strip (1).so that the energy of the laser beam is distributed on the heating element (5) in a direction transverse to the longitudinal direction of the material strip (1), and wherein the heating element (5) is additionally heated electrically or with a burner. Method according to claim 1, wherein the distance of the surface (50) of the heating element facing the material strip (1) in the direction transverse to the material strip (1) is smaller than its dimension perpendicular to the longitudinal direction of the material strip. Method according to one of the preceding claims, characterized in that the material strip (1) is a glass strip which is drawn from a glass melt (3) or from a preform (6). Method according to one of the preceding claims, characterized in that a surface of the heating element (5) facing away from the material strip (1) is irradiated with the laser beam (70). Method according to the preceding claim, characterized in that the material strip (1) is produced in the downdraw process, the overflow fusion process, the updraw process or the float process. Method according to claim 1, characterized in that the material strip (1) is drawn from a heated preform (6). Method according to one of the preceding claims, characterized in that the temperature distribution along the heating element (5) is determined and the point of impact (71) of the laser beam (70) is moved depending on this temperature distribution. Device (10) for producing a material strip (1) for carrying out a method according to one of the preceding claims, comprising: a drawing device (12) for drawing a material (2) in a heated and softened state into a strip; and a heating element (5) for heating the material (2) during the forming process in which the strip is formed and drawn, wherein the heating element (5) is designed for heating by thermal radiation, which is emitted from the surface (50) of the heating element (5) arranged opposite the heated material (2); and wherein the device (10) comprises a laser (7) arranged such that the laser beam (70) of the laser (7) is directed at the heating element (5) and locally heats the heating element (5); and wherein the heating element (5) is additionally heated electrically or with a burner, wherein a beam deflection device (9) is provided.to move the point of impact of the laser beam (70) on the heating element (5) in a direction transverse to the longitudinal direction of the material strip (1), so that the energy of the laser beam on the heating element is distributed in a direction transverse to the longitudinal direction of the material strip (1). Device according to claim 8, characterized in that the heating element (5) comprises an electrically heated heating tube (51). Device according to the preceding claim, characterized in that the longitudinal direction of the heating tube (51) runs transversely to the longitudinal direction of the material strip (1). Device according to one of the two preceding claims, characterized in that the side of the outer surface (52) of the heating tube facing away from the material strip (1) has an opening (53) so that the laser beam (70) can pass through the opening (53) and the heating tube (51) and strike the inner side of the outer surface (52) facing the material strip (1). Device according to one of claims 8 to 11, characterized in that the heating element (5) comprises a plate (55) whose one side (56) faces the material (2) of the material strip (1) and whose opposite side (57) can be heated by the laser beam (70). Device according to one of claims 8 to 12, characterized by a beam deflection device (9) to move the point of impact (71) of the laser beam (70) over the heating element (5). Device according to one of claims 8 to 13, characterized by a control device (15) which is configured to change the position of the point of impact (71) of the laser beam (70) of the laser (7) to influence the temperature distribution on the heating element (5). Device according to the preceding claim, characterized by at least one sensor (8) which detects measured values ​​and is connected to the control device (15), wherein the control device (15) is configured to control at least one of the parameters position, irradiation duration and radiant power of the laser beam (70) on the basis of the detected measured values. Device according to one of claims 8 to 15, characterized in that the heating element (5) has a thermal conductivity in the direction transverse to the longitudinal direction of the material strip (1) which is lower than the thermal conductivity in the direction from the side away from the material strip (1) to the side facing the material strip (1). Device according to one of claims 8 to 16, characterized in that the heating element (5) has a material with a thermal conductivity of less than 50 W / m·K.

Images