GLASS PANE AND ASSEMBLY OF GLASS PANES WITH LOW DEGREE OF FINE WAIVENESS, AND METHODS FOR PRODUCING AND USING SAME

Abstract

The invention relates to a glass pane, in particular a glass pane which is obtained by individualizing a floated glass strip formed by a hot forming process, in particular comprising a borosilicate glass, with a thickness (D) ranging from at least 1.75 mm to maximally 7 mm or a thickness (D) ranging from at least 0.7 mm to 7 mm, in particular from 1.1 mm to maximally 7 mm, and comprising an upper face and a lower face. The glass pane is characterized by a fine waviness of 10 nm to 26 nm, preferably between 10 nm and 15 nm, in at least one direction parallel to the surface of the glass pane on at least one surface of the upper face or the lower face of the glass pane. The invention also relates to an assembly of said glass panes and to methods for producing and using same.

Claims

1-15. (canceled)

16. A glass sheet, comprising: a borosilicate glass having a thickness of between at least 0.7 mm and at most 7 mm, comprising a top side and a bottom side, characterized by a fine waviness on at least one surface of the top side or bottom side of the glass sheet of 10 nm to 26 nm in at least one direction parallel to a surface of the glass sheet.

17. The glass sheet of claim 16, wherein the thickness is between 1.1 mm and at most 7 mm.

18. The glass sheet of claim 17, wherein the thickness is at least 1.75 mm.

19. The glass sheet of claim 16, wherein the fine waviness on the at least one surface of the top side or bottom side of the glass sheet is measured along a line having a length of 260 mm.

20. The glass sheet of claim 19, wherein the fine waviness is measured within a square area of 260 mm times 260 mm.

21. The glass sheet of claim 16, wherein the glass sheet is formed by hot forming and the at least one direction corresponds to a direction perpendicular to a drawing direction used in hot forming of the glass sheet.

22. The glass sheet of claim 16, wherein the borosilicate glass comprises the following components in % by weight: TABLE-US-00005 SiO.sub.2 70 to 87; preferably 75 to 85 B.sub.2O.sub.3 5 to 25; preferably 7 to 14 Al.sub.2O.sub.3 0 to 5; preferably 1 to 4 Na.sub.2O 0.5 to 9; preferably 0.5 to 6.5 K.sub.2O 0 to 3; preferably 0.3 to 2.5, more preferably to 2 CaO 0 to 3; and MgO 0 to 2.

23. The glass sheet of claim 22, wherein the borosilicate glass comprises the following components in % by weight: TABLE-US-00006 SiO.sub.2 75 to 85; B.sub.2O.sub.3 7 to 14; Al.sub.2O.sub.3 1 to 4; Na.sub.2O 0.5 to 6.5; K.sub.2O 0.3 to 2.5; CaO 0 to 3; and MgO 0 to 2.

24. A set of glass sheets, comprising: a plurality of glass sheets, each of the glass sheets comprising a borosilicate glass having a thickness of between at least 0.7 mm and at most 7 mm, comprising a top side and a bottom side, characterized by a fine waviness on at least one surface of the top side or bottom side of the glass sheet of 10 nm to 26 nm in at least one direction parallel to a surface of the glass sheet, wherein a median of the fine wavinesses of the set of glass sheets has a value which is less than 20 nm.

25. The set of glass sheets of claim 24, wherein the plurality of glass sheets comprises at least eight glass sheets.

26. A method for producing a glass sheet, the glass sheet comprising a borosilicate glass having a thickness of between at least 0.7 mm and at most 7 mm and comprising a top side and a bottom side, characterized by a fine waviness on at least one surface of the top side or bottom side of the glass sheet of 10 nm to 26 nm in at least one direction parallel to a surface of the glass sheet, the method comprising: providing a batch comprising glass raw materials; melting the batch to give a glass melt; adjusting a viscosity of the glass melt; transferring the glass melt to an apparatus for hot forming by floating glass on a float bath to form a glass ribbon; and singulating the hot-formed glass ribbon to give a glass sheet, wherein the viscosity in the apparatus for hot forming is adjusted such that a sum total of common logarithms of a viscosity at a distance from a component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width and of a viscosity at an end of hot forming is between at least 11.4 and at most 11.8.

27. The method of claim 26, wherein the viscosity in the apparatus for hot forming is adjusted such that the sum total of the common logarithms of the viscosity at the distance from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width and of the viscosity at the end of hot forming is between at least 11.4 and at most 11.6.

28. The method of claim 26, wherein the common logarithm of the viscosity at the distance from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width is at least 5.0 and the common logarithm at the end of hot forming is at least 6.2.

29. The method of claim 28, wherein the common logarithm of the viscosity at the distance from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width is less than 5.25 and the common logarithm at the end of hot forming is at most 6.5.

30. The method of claim 28, wherein the common logarithm of the viscosity at the distance from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width is at least 5.1 and the common logarithm at the end of hot forming is at least 6.35.

31. The method of claim 26, wherein a difference between the common logarithms of the viscosity at the distance from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width and of the viscosity at the end of hot forming is between at least 1.25 and at most 1.5.

32. The method of claim 31, wherein the difference is between at least 1.25 and at most 1.45.

33. The method of claim 26, wherein a throughput of less than 400 t of glass per day is obtained.

34. The method of claim 26, wherein the at least one direction is indicated on the glass sheet or on packaging of the glass sheet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The invention is described in greater detail below, by means of the appended drawings and with reference to preferred and particularly preferred exemplary embodiments.

[0070] In the drawings,

[0071] FIG. 1 shows a schematic sectional view of an apparatus for producing a glass sheet and for implementing the presently disclosed method, in which the sectional plane runs vertically approximately through the middle of the apparatus,

[0072] FIG. 2 shows the schematic sectional view of FIG. 1 in greatly simplified form, in which the detail represented in FIG. 4 is marked with the sectional planes A and B,

[0073] FIG. 3 shows a schematic plan view of a part of the apparatus for producing a glass sheet that is shown in FIGS. 1 and 2, more particularly on a glass ribbon for hot forming on a float bath, in which illustratively, to simplify the representation, only some of the top rollers used overall are represented,

[0074] FIG. 4 shows a plan view, seen obliquely from above, of a part of the apparatus for producing a glass sheet, represented in FIGS. 1 and 2, in the form of a detail which extends between the sectional planes A and B,

[0075] FIG. 5 shows an illustrative representation of presently disclosed viscosity profiles, also indicating in particular the viscosity values .sub.A at a distance 56 from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width, and the viscosity values .sub.E at the end of the hot-forming section, and thus at the location of the perpendicular 53,

[0076] FIG. 6 shows the apparatus for producing a glass sheet, represented in FIG. 4, having measurement areas M1 to M8 indicated on the upper surface of the hot-formed glass ribbon and also having a measurement line ML for determining the fine waviness of the upper surface of the hot-shaped glass ribbon,

[0077] FIG. 7a shows a plan view of the upper surface of the glass ribbon after hot forming thereof, within the sectional planes C and D represented in FIG. 6, with the measurement areas M1 to M8, in each of which a measurement line ML is arranged,

[0078] FIG. 7b shows a plan view of the lower surface 49 of the glass ribbon 13 after hot forming thereof, within the sectional planes C and D represented in FIG. 6, with the measurement areas M1 to M8, in each of which a measurement line ML is arranged,

[0079] FIG. 8 shows a boxplot representation of the fine waviness values obtained for different viscosity values, as a function of the viscosities lg (.sub.A/dPa*s) and lg (.sub.E/dPa*s), where these fine wavinesses are each indicated for fixed values of the sum total of the viscosities lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s) and where, in addition to these sum totals of the viscosities, the difference between them lg (.sub.A/dPa*s)lg (.sub.E/dPa*s) is also indicated, with the respective boxplot representation including in each case the value of all the individual measurements resulting in this boxplot representation, and

[0080] FIG. 9 shows a boxplot representation of fine waviness values obtained for different viscosity values, as a function of the viscosities lg (.sub.A/dPa*s) and lg (.sub.E/dPa*s), where these fine wavinesses are each indicated for an interval of the sum total of the viscosities lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s) and where, in addition to these sum totals of the viscosities, an interval of difference lg (.sub.A/dPa*s)lg (.sub.E/dPa*s) is also indicated, with the boxplot representation including in each case the value of an individual measurement resulting in the boxplot representation shown in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

[0081] In the description of preferred and particularly preferred embodiments that follows, reference signs that are the same in the various figures denote identical constituents, or constituents that have the same effect, of the apparatus respectively disclosed here.

[0082] The data for the thickness D of the glass sheet 33, 33, 33 correspond to the distance between the two principal surfaces, namely the top side 34 and the bottom side 35, of the glass sheet 33, 33, 33 after its hot forming, and should each be measured perpendicularly to these principal surfaces, as represented illustratively in FIG. 4.

[0083] The float plant represented in FIGS. 1, 2 and 3 for implementing the presently disclosed method has a melting furnace 2, also referred to as a melting vessel, which is supplied conventionally with a batch for melting, more particularly a glass batch 3, and is heated by means of burners 4 until a glass melt 5 having the desired composition is formed. Further facilities for homogenizing the glass melt are known to the skilled person and are consequently not described in more detail.

[0084] Through a channel 6, the molten glass of the glass melt 5, generally under the effect of gravity, reaches a float bath 7 which contains liquid tin and on which the glass 8 for hot forming is able to spread laterally with a reduction in its height, as part of its hot forming, under the effect of gravity.

[0085] For adjusting the temperature of the glass for hot forming, the tin bath 7 may be arranged in a float bath furnace 9 which possesses electrical overhead heaters 10 by means of which the temperature of the glass for hot forming can be adjusted. Additionally, the temperature of the tin bath 7 may be adjusted in a defined manner in the drawing direction and in this way the temperature of the glass for hot forming and hence its viscosity may be influenced in a defined manner.

[0086] On exiting the melting vessel 2, the molten glass 8 for hot forming is conveyed over a downward-slanting introduction lip 11, also referred to as a spout, on which the glass already begins to widen, onto the tin bath 7. At a distance of 1.5 m from the component for throughput regulation, and hence at a distance of 1.5 m in the Y-direction in the midpoint Mi of the glass ribbon 13 with respect to the X-direction, the glass ribbon 13 has its greatest width, meaning its greatest extent in the X-direction. In the case of the embodiments disclosed, this distance is about 1.5 m and is indicated with the reference sign 56 in FIG. 4, for example. With roll-shaped top rollers 12 as a tensioning facility, the glass ribbon 13 which forms on the tin bath 7, in its spreading motion from the side, is subjected to defined influencing in its further motion. Illustratively, only three top rollers are represented in each case in FIG. 1, although, as and when required, it is also possible for more than two of these top rollers to be present and used, as is also apparent, for example, from FIGS. 3 and 4.

[0087] Top roller refers to an essentially roll-shaped body that is well known to the person skilled in this field of art, which is in contact by its outer annular shoulder with the principal surface remote from the tin bath, or upper surface 48, of the glass 8 to be hot-formed and which exerts a force on the glass 8 to be hot-formed in each case by a rotating movement in each case about its longitudinal axis or axis of symmetry 50, 51. This axis of symmetry 50, 51 is shown merely illustratively for the top rollers 42 and 44. In the context of the present disclosure, the term top roller may also be regarded as an essentially roll-shaped transport apparatus for the glass to be hot-formed. In this context, the first top roller 12, 42 constitutes an essentially roll-shaped transport apparatus for the glass to be hot-formed at the start of the section Hs, especially a defined, thickness-based hot-forming zone, and the last top roller 40, 44 constitutes an essentially roll-shaped transport apparatus for the glass to be hot-formed at the end of the section Hs of the hot-forming zone. Over the course of this thickness-based hot-forming zone Hs, the thickness of the glass ribbon 13 is adjusted in a defined manner, but this hot-forming zone Hs does not include all hot-forming measures, since, even after the distance 56 from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width up to the start of the section Hs, there is already forming of the glass 8 to be hot-formed in the glass ribbon 13.

[0088] The portion of the glass 8 to be hot-formed which is in contact with the outer annular shoulder of the respective top roller causes it to move in a defined manner. The top roller is in each case driven in a defined manner, being controllable by motor with an essentially rod-shaped axle.

[0089] The location or position of the top roller, especially in flow direction Y of the glass 8, is understood in the context of the present disclosure in each case to be the perpendicular 52, 53 in negative z-direction proceeding from the respective axis of symmetry 50, 51 of the corresponding top roller 42, 44 from the surface, especially from the principal surface 48, of the glass 8 to be hot-formed.

[0090] The location or position of the respective first top roller 12, 42 defines the entry of the glass 8 into the section Hs for hot forming thereof with regard to its thickness.

[0091] The location or position of the respective last top roller 40, 44 defines the exit of the glass 8 from the section Hs for thickness-based hot forming thereof and hence for overall hot forming thereof.

[0092] By way of simplification, in the context of the present disclosure, the mention of the first top roller in each case refers to the pair of top rollers, for example the top rollers 42, 12, that are at the same site in flow direction, and the mention of the last top roller in each case refers to the pair of top rollers, for example the top rollers 44, 40, that are each at the same site in flow or y-direction.

[0093] The site of entry of the glass 8 into the section Hs for thickness-based hot forming is consequently apparent by virtue of the dashed line 54, whereas the site of exit of the glass 8 from the section Hs for hot forming is indicated by the dashed line 55.

[0094] A further dashed line indicates the site or distance 56 from the component for throughput regulation at which the glass 8 to be hot-formed has reached its maximum width after impinging on the float bath 7.

[0095] The length Hsl of the section Hs for thickness-based hot forming in the context of the present disclosure is understood to mean the distance in flow or y-direction between the perpendicular 52 of the first top roller 42 and the perpendicular 53 of the last top roller 44.

[0096] After hot forming thereof, the glass ribbon 13 can optionally be transferred to a lehr 14, which may likewise have electrical overhead and floor heaters 15, in order to subject the glass ribbon 13 to a defined lowering of temperature, although only overhead heaters are shown by way of illustration in FIG. 1.

[0097] After leaving the lehr 14, the glass ribbon 13 is then available for further processing, especially singulation into glass sheets 33, 33, 33.

[0098] In order, in the description of preferred embodiments that follows, to be able to more clearly illustrate spatial arrangements of different assemblies or of properties, for example of glasses to be hot-formed or glass sheets 33, 33, 33 singulated after hot forming, reference is firstly made to the Cartesian coordinate system shown in FIGS. 1, 2, 3 and 4, which defines an orthogonal X-, Y- and Z-direction, to which all statements continue to relate hereinafter in the various figures.

[0099] The X- and Y-directions form a plane that extends horizontally and hence also runs essentially parallel to the surface of the tin bath 7. Running perpendicular to this plane, the Z-direction extends upward and thereby also defines the normal direction in relation to the glass ribbon 13.

[0100] Reference is made hereinafter to FIG. 1, which, as an apparatus for production of a glass ribbon 13 from which the presently disclosed glass sheets 33, 33, 33 can be singulated, comprises the float apparatus that has been given the reference sign 1 as a whole, which has all the facilities or apparatuses described with reference to FIGS. 2, 3 and 4.

[0101] Facilities for melting 16 that are included here are the melting vessel or melting furnace 2, a feed device for the glass batch 3, and the burners 4. In addition, the melting vessel 2 has a channel 6 for transfer of the molten glass 8 to be hot-formed to the tin bath 7.

[0102] By way of illustration, the control slider 17, i.e. the component for throughput regulation of the glass flow, which is also referred to as a tweel, is disposed behind the channel 6. By movement of the control slider or tweel 17, which forms the component 17 for throughput regulation, in the direction of the double-headed arrow shown alongside reference sign 17, it is possible to constrict or enlarge the cross section of the channel 6, which regulates, and especially adjusts in a defined manner, the amount of molten glass 8 to be hot-formed that exits from the melting vessel 2 per unit time. In addition, a feeder may be disposed between the melting vessel 2 and the float bath furnace 9, especially upstream of the tweel 17, which in this case forms the channel 6, especially also over a longer distance than that shown in FIG. 1. A more detailed description of throughput regulation can be found in this applicant's DE 10 2013 203 624 A1, which is also incorporated into the subject-matter of the present application by reference.

[0103] Viewed in flow direction of the molten glass 8 to be hot-formed, a facility 18 for defined adjustment of the viscosity of the molten glass 8 to be hot-formed is disposed upstream of the component for throughput regulation 17 and upstream of the spout 11.

[0104] This facility 18 for defined adjustment of viscosity comprises a chamber 19 that is divided from the melting vessel 2 or else may form part thereof, and accommodates the molten glass 8 to be formed to a glass substrate for defined adjustment of the viscosity thereof.

[0105] In addition, the facility 18 for defined adjustment of viscosity comprises regions 20, 21 through which fluid flows, especially regions through which water flows, which absorb heat from the glass 8 to be hot-formed and may take the form of a metallic pipe system. This metallic pipe system may also be colored for better absorption of heat or provided with a heat-resistant paint on the surface thereof.

[0106] Alternatively or additionally, the walls 22, 23, 24 and 25 of the chamber 19 may also absorb heat from the glass 8 to be hot-formed in that the temperature thereof is adjusted in a defined manner, for example by further cooling facilities.

[0107] The chamber 19, with its walls 22, 23, 24 and 25, may also be formed spatially separately from the melting vessel 2 and have high-temperature-resistant metallic walls, in order to provide improved dissipation of heat.

[0108] As described above, the facility 18 for defined adjustment of viscosity comprises at least one cooling facility by means of which the temperature and hence also the viscosity of the glass 8 to be hot-formed is adjustable in a defined manner.

[0109] Contactless and, alternatively or additionally, direct temperature measurements in contact with the glass to be measured are known to the person skilled in the art. Corresponding sensors are described, for example, by the sensor device or unit 26 in the context of this disclosure.

[0110] The sensor device or unit 26 may be in direct contact with the glass and hence undertake a direct temperature measurement, or else may comprise a radiative measurement device that detects the temperature by detection of the spectrum emitted by the glass 8 to be hot-formed with reference to the spectrum itself and/or the intensity of the radiation emitted.

[0111] The apparatus 1 comprises a facility or apparatus 47 for hot forming, which will be described in more detail hereinafter, which is present beyond the facility 18 for defined adjustment of viscosity in flow direction or drawing direction and receives the glass 8 to be hot-formed via the spout 11.

[0112] The spout 11 directs the glass 8 to be hot-formed onto a tin bath 7 accommodated in the float bath furnace 9.

[0113] A further cooling facility 57 is disposed above the glass 8 to be hot-formed at a distance from the component for throughput regulation 17 of about 2 m based on the middle thereof in Y-direction. This cooling facility 57 projects above the melt and may have a width in Y-direction of 300 mm, a height in Z-direction of 80 mm and a length in X-direction of 2.5 meters, and may be in two-part form. In this case, a portion of the cooling facility 57 projects over the glass to be hot-formed from respective opposite sides in X direction, and hence provides an essentially complete cover of the glass 8 to be hot-formed in X-direction and regionally in Y-direction.

[0114] The cooling facility 57 shadows the glass 8 to be hot-formed not just with respect to the overhead heaters 10, but also brings about a cooling air stream that comes from above the glass 8, with which it is possible to cool the glass 8 present beneath the cooling facility 57 down by about 20 to 25 K. In this way, given the already initially high viscosity of the glass 8, it is possible to create a flatter progression of the viscosity curve overall in the continued progression in drawing direction, as also shown by way of example in FIG. 5.

[0115] Above the glass ribbon 13 that forms on the tin bath 7, as also readily apparent from FIG. 3, further top rollers 38 to 44 are disposed alongside the top roller 12 for mechanical movement of the glass ribbon 13.

[0116] In this context, the number of top rollers shown in FIG. 3 is merely illustrative since, in preferred embodiments of the invention, preferably 10 to 12 pairs of top rollers are used.

[0117] The top rollers 41 and 38 serve merely for adjustment of the width of the glass ribbon Bg 13 that results from the hot forming, and are optional since the width Bg is also adjustable in other ways, for example by regulating the volume of glass 8 which is provided for hot forming.

[0118] FIG. 3 also shows an alternative or additional configuration of the facility 18 for defined adjustment of viscosity. The molten glass 8 is present in a channel 6 of the melt vessel 2, not shown in FIG. 3, to the float bath furnace 9. The walls 45, 46 of the channel 6 have been formed from a metal of high thermal stability, for example platinum, which may also be disposed as a metallic layer on a mineral refractory material. The defined adjustment of the temperature of these walls allows heat to be withdrawn from the glass 8, and also the temperature and viscosity thereof to be adjusted in a defined manner. In this embodiment too, the above-described sensor unit 26 may preferably be disposed close to the tweel 17.

[0119] A drawing facility has been described above for the apparatus 47 for hot forming, which comprises a float facility, especially a float bath furnace 9 with a tin bath 7.

[0120] The method disclosed here is described by way of illustration hereinafter with reference to a float method.

[0121] FIG. 4 shows a detail extending between the sectional planes A and B of the apparatus 1 for production of a glass ribbon 13 for a glass sheet 33, 33, 33 to be singulated therefrom, in which, for better clarity, only the glass 8 to be hot-formed, and also the float bath 7 in the form of a tin bath, are shown.

[0122] The glass 8 moves from the left-hand side of FIG. 4 at an entry speed onto the first top roller 42, 12, at which the thickness-based hot forming disclosed here to give a glass ribbon 13 for a glass sheet 33, 33, 33 to be singulated therefrom commences. This speed corresponds to the speed of the glass 8 at the first top roller 42, 12. The glass 8, after the last top roller 40, 44, and hence after it has been hot-formed as described here, moves onward in flow direction to a glass ribbon 13 for a glass sheet 33, 33, 33 with an exit thickness D to be singulated therefrom.

[0123] Where reference is made for short merely to hot forming in the context of the present disclosure, this refers, for linguistic simplicity, to the hot forming described in more detail hereinafter to give a glass ribbon 13 for a glass sheet 33, 33, 33 to be singulated therefrom, especially after cooling of the glass ribbon 13, both along the section Hs of the thickness-based hot-forming zone and further hot-forming steps that may have already taken place before attainment of the first top roller, as, for example, in the pouring of the glass 8 onto the float bath 7, where the glass can spread out two-dimensionally and assume its equilibrium thickness Dg of about 7 mm+/1 mm.

[0124] After the hot forming, the glass 8 has an exit thickness of D that it assumed after the last top roller 40, 44.

[0125] The glass 8, throughout its thickness-based hot forming to give a glass ribbon 13 for a glass sheet 33, 33, 33 to be singulated therefrom, between the first top roller 42, 12 and the last top roller 40, 44, and hence in the section Hs, has a width Bg, i.e., an extent in x-direction of Bg, which is altered preferably by less than 3% in this thickness-based hot forming in x-direction. This can be ensured by adjusting the speed and angle of rotation along the axis of symmetry (axis of rotation) of the respective top rollers. In this case, it is especially also possible to alter the angle of the respective axis of symmetry of the corresponding top roller such that this results in greater or lesser contributions of the movement of the glass 8 to be hot-formed or of parts of the glass ribbon 13 in x-direction in the course of transport of glass 8 to be hot-formed, especially along the thickness-based hot-forming zone Hs.

[0126] At a distance 56 from a component for throughput regulation 17 at which the glass after impinging on the float bath has acquired its maximum width, the viscosity .sub.A, especially by adjustment of the temperature of the glass ribbon 13 at this site, is adjusted such that this has a value of lg (.sub.A/dPa*s) of at least 5.0, more preferably at least 5.1, and preferably less than 5.25.

[0127] At the end of the hot forming zone Hs, the viscosity .sub.E, especially by adjustment of the temperature of the glass ribbon 13 at this site, is adjusted such that this has a value of lg (.sub.E/dPa*s) of at least 6.2, preferably at least 6.3, more preferably at least 6.35, where a preferred upper limit assumes the value of 6.5 at most.

[0128] According to the invention, the viscosity in the apparatus for hot forming is adjusted such that the sum of the common logarithms of the viscosity lg (.sub.A/dPa*s) and lg (.sub.E/dPa*s) at the distance 56 from a component for throughput regulation 17 at which the glass after impinging on the float bath has acquired its maximum width, and at the end of hot forming, is between at least 11.4 and at most 11.8 dPa*s.

[0129] An illustrative representation of corresponding viscosity profiles can be seen in FIG. 5, in which, in particular, the viscosity values .sub.A at the distance 56 from a component for throughput regulation 17 at which the glass after impinging on the float bath has acquired its maximum width, and the viscosity values .sub.E at the end of the hot forming zone, and hence of the perpendicular 53, can also be inferred.

[0130] Reference is made below to FIG. 6, which shows the apparatus represented in FIG. 4 for producing a glass sheet, having measurement areas M1 to M8 indicated on the upper surface 48 of the hot-formed glass ribbon 13, and having an illustrative measurement line ML for determining the fine waviness of the upper surface 48 of the hot-formed glass ribbon 13. Although the measurement line ML represented in FIG. 6 is represented, for the sake of simplicity, initially as a continuous line, it consists of respective measurement lines ML of the measurement areas M1 to M8, as is elucidated further in more detail with reference to FIGS. 7a and 7b.

[0131] Glass sheets 33, 33, 33 singulated from the glass ribbon 13 may comprise one or more of these measurement areas or else may comprise fractions of these measurement areas. From the data elucidated in more detail below and represented in FIGS. 8 and 9, it is also evident that the fine waviness values of the invention are reliably achieved as soon as a singulated glass sheet 33, 33, 33 with its dimensions in the X-direction reaches at least the length of a measurement zone ML, since even mutually bordering measurement zones ML, which do not each have to have been covered over their full length, lead essentially to the fine wavinesses of the invention as soon as the length of one measurement zone ML is reached overall. The same applies to the measurement areas M1, M2, M3, M4, M5, M6, M7 and M8 that are located at the bottom side 49. Merely by way of illustration, the dimensions of glass sheets 33 and 33 subsequently singulated from the glass ribbon are represented in FIGS. 7a and 7b in this respect.

[0132] Also represented in dashed form in FIG. 6 are sectional planes C and D which extend in the Z- and X-directions and which the glass ribbon 13 after hot forming thereof passes through in the drawing direction Y. The measurement areas M1 to M8 indicated on the upper surface of the hot-formed glass ribbon are represented illustratively for a defined timepoint t after the hot forming of the glass ribbon 13, and move with the glass ribbon 13 in drawing direction Y, and form a part of the upper surface of the hot-formed glass ribbon, more particularly for the subsequent singulation thereof into glass sheets 33, 33, 33. Since the glass ribbon after hot forming thereof undergoes no further change in size, the measurements were performed subsequently on singulated glass sheets 33, for which a measured glass sheet 33 comprised a respective one of the measurement areas M1, M2, M3, M4, M5, M6, M7 or M8 and one of the measurement areas M1, M2, M3, M4, M5, M6, M7 or M8.

[0133] The measurement areas M1, M2, M3, M4, M5, M6, M7 and M8 are shown in FIG. 6 arranged on the upper surface 48 and they each have a square shape with an extent in both the X-direction and the Y-direction of 260 mm. This dimension of 260 mm is also shared by the respective measurement zone ML.

[0134] For the sake of clarity, the dimensions shown in the figures are not represented to scale; instead, the thickness D, in particular, is initially represented in a greatly enlarged form for the sake of greater ease of perceptibility.

[0135] Fine waviness measurements were performed at the upper surface 48 and also at the lower surface 49 along a measurement line ML represented in each of FIGS. 7a and 7b.

[0136] Because the measurement areas M1, M2, M3, M4, M5, M6, M7 and M8 and also the measurement areas M1, M2, M3, M4, M5, M6, M7 and M8 extend transverse to the extent of the glass ribbon 13 in the X-direction, the recording of the entire glass ribbon 13 used for the singulation of the glass sheets after the hot forming is made possible in the X-direction in this way.

[0137] Regions lying under the top rollers 38 to 44 were not recorded by the fine waviness measurements and are each delimited in the X-direction by the lines Mt1 and Mt2, illustratively, with respect to the region of the glass ribbon 13 lying between these lines Mt1 and Mt2, so that surface-altering effects of the top rollers were not recorded by the fine waviness measurements. Lateral borders which may rise above the upper principal surface 48 in the Z-direction at the edge of the glass ribbon 13 are each situated behind the top rollers 38 to 44, in relation to the midpoint Mi of the glass ribbon, and are therefore likewise situated outside a respective measurement region ML. They too, therefore, were not recorded by a respective fine waviness measurement.

[0138] FIG. 7a shows a plan view of the upper surface 48 of the glass ribbon 13 after hot forming thereof within the sectional planes C and D represented in FIG. 6, with the measurement areas M1 to M8, each containing a measurement line ML, and FIG. 7b shows a plan view of the lower surface 49 of the glass ribbon 13 after hot forming thereof within the sectional planes C and D represented in FIG. 6, with the measurement areas M1 to M8, each containing a measurement line ML.

[0139] The measurement areas M1 to M8 indicated in FIG. 7b at the lower surface 49 of the hot-formed glass ribbon 13 are also represented illustratively for a defined timepoint t after the hot forming of the glass ribbon 13, move with the glass ribbon 13 in the drawing direction Y, and form a part of the lower surface 49 of the hot-formed glass ribbon 13, more particularly for its subsequent singulation into glass sheets 33, 33, 33.

[0140] By way of illustration, glass sheets 33 were singulated in particular such that there was a respective measurement area of the measurement areas M1 to M8 at their upper surface and a respective measurement area of the measurement areas M1 to M8, with numerical correspondence of the respective measurement areas, at their lower surface.

[0141] The fine waviness was determined presently by measuring the surface of a glass sheet using a ZEISS Surfcom 1400-350 surface and contour measuring instrument (with the release P000083259) along a direction perpendicular to the drawing direction Y and thus running in the X-direction on the surface of the top side and the surface of the bottom side of the respective glass sheet within a measurement area M1 to M8 and also M1 to M8. The aforementioned X-direction can be seen, for example, in the Cartesian coordinate system represented in FIGS. 1 to 4. For this instrument, a lower cut-off wavelength c was chosen at 0.25 mm and an upper cut-off wavelength f of 8 mm. Filtering took place using a Gaussian filter. The fine waviness values output by this contour measuring instrument are also referred to in the manner customary in the art as Wfpd values and are also output thus referenced by said measuring instrument.

[0142] FIGS. 8 and 9 show boxplot representations, known per se to the skilled person, which are ascertained in each case for a multiplicity of measurement values, as for example eight measurement values of eight measurement lines ML.

[0143] Merely for the sake of completeness it may be noted that a boxplot representation comprises a rectangular box with a line running transversely in this box and with whiskers extending upward and downward out from this box. This line running transversely in the box corresponds in each case to the median of the measurement values, being the value for which a first half of the measurement values are located above or on this line and the second half of the measurement values are located below or on this line. The upper border of the box represents the three-quarters quartile of the measurement values, for which three quarters of the measurement values are located below or on the upper border of the box and one quarter of the measurement values are located above or on the upper border of the box. The lower border of the box represents the one-quarter quartile of the measurement values, for which one quarter of the measurement values are located below or on the lower border of the box and three quarters of the measurement values are located above or on the lower border of the box. Via their length, these whiskers extending upward or downward out from the box each indicate expected values which correspond to 1.5 times the distance from the bottom end of the box to the top, unless the values actually measured do not reach that far, in which case they extend from the bottom end of the box to the respective measured minimum value or extend from the top end of the box to the respective measured maximum value.

[0144] FIG. 8 shows a box plot representation of the fine waviness obtained for various viscosity values as a function of the viscosities lg (.sub.A/dPa*s) and lg (.sub.E/dPa*s).

[0145] As in FIG. 9, the indication Position Top denotes the measurement values of the upper surface 48 of the glass ribbon 13, and especially the measurement values of the surface of the top side 36 of glass sheets 33, 33 and 33 singulated from this glass ribbon. Just as in FIG. 9, the indication Position Bottom denotes the measurement values of the lower surface 49 of the glass ribbon 13, and especially the measurement values of the surface of the bottom side 37 of glass sheets 33, 33 and 33 singulated from this glass ribbon.

[0146] FIG. 8 indicates the values of the fine wavinesses as the ordinate in a lower caption line in each case for fixed values of the sum total of the viscosities lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s), and indicates the values for the difference between the viscosities lg (.sub.A/dPa*s)lg (.sub.E/dPa*s) in an upper caption line of the ordinate.

[0147] The sum total of the above-indicated viscosities was as follows:

[00001]$lg\left({}_A/\mathrm{dPa}*s\right)+lg\left({}_E/\mathrm{dPa}*s\right)=11\text{.15}$$\lg \left({}_A/\mathrm{dPa}*s\right)+lg\left({}_E/\mathrm{dPa}*s\right)=11\text{.17}$$\lg \left({}_A/\mathrm{dPa}*s\right)+lg\left({}_E/\mathrm{dPa}*s\right)=11.31$$\lg \left({}_A/\mathrm{dPa}*s\right)+lg\left({}_E/\mathrm{dPa}*s\right)=11.37$$\lg \left({}_A/\mathrm{dPa}*s\right)+lg\left({}_E/\mathrm{dPa}*s\right)=11.44$$\lg \left({}_A/\mathrm{dPa}*s\right)+lg\left({}_E/\mathrm{dPa}*s\right)=11.5$$\lg \left({}_A/\mathrm{dPa}*s\right)+lg\left({}_E/\mathrm{dPa}*s\right)=11.52$

[0148] It can be seen that below a value for the sum total of the viscosities of about lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)=11.4, the fine waviness values are above 26 nm, with this value of 26 nm being reproduced in FIGS. 8 and 9 as a line bearing the designation OSG.

[0149] Located above the value for the sum total of the viscosities of lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)=11.4 is a region of decreasing fine wavinesses, more particularly of fine wavinesses having values below 26 nm, more particularly decreasing down to 10 nm.

[0150] For example, the fine waviness values obtained on the bottom side for a sum total of the viscosities of about lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)=11.6 and a difference between the viscosities lg (.sub.A/dPa*s)lg (.sub.E/dPa*s)=1.42 reliably show values of between 10 nm and 15 nm.

[0151] The fine waviness values rise slightly again beyond a value for the sum total of the viscosities of about lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)=11.42.

[0152] A preferred range comes about in particular if the viscosity in the apparatus for hot forming is adjusted such that the sum total of the common logarithms of the viscosity at the distance from a component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width, lg (.sub.A/dPa*s), and of the viscosity at the end of hot forming, lg (.sub.E/dPa*s), is between at least 11.4 and at most 11.6, since in that case the values of the medians, of a multiplicity of glass sheets, for example, in particular of at least eight glass sheets 33, 33, 33, more particularly of glass sheets 33, 33, 33 having the features of the claims from 1 to 4, each formed from the fine wavinesses, measured along the line ML, of a respective glass sheet 33, 33, 33 of the set, are each reliably situated at a fine waviness of 10 nm to 26 nm. It is assumed in this context that the respective glass sheet 33, 33, 33 of the set in each case comprised at least one size extent in X-direction with the length of one complete measurement zone M1, even if this extent was provided not in each case independently by a single measurement zone ML but instead from fractions, for example, of two measurement zones ML.

[0153] Up to a value of a value of about lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)=11.8, it was likewise still possible to obtain correspondingly measured fine waviness values of less than 26 nm.

[0154] Reference is made below to FIG. 9, which shows a boxplot representation of fine waviness values obtained for various viscosity values, as a function of the viscosities lg (.sub.A/dPa*s) and lg (.sub.E/dPa*s).

[0155] The fine waviness values are indicated in each case for an interval of the sum total of the viscosities lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s).

[0156] In addition to these sum totals of the viscosities, for the first interval, being the interval on the left-hand side of FIG. 9, an interval of difference lg (.sub.A/dPa*s)lg (.sub.E/dPa*s) is also indicated.

[0157] The inventors have surprisingly determined a further advantageous criterion which may be explained illustratively with reference to the values, represented in FIG. 9, for the sum totals of the viscosities of lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)=11.6 and also for the differences between the viscosities, lg (.sub.E/dPa*s)lg (.sub.A/dPa*s).

[0158] In the interval of the middle representation in FIG. 9, for which the sum total of the viscosity values is subject to 11.4lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)11.6, values for the fine waviness of less than 26 nm were reliably achieved even for the median of the presently described set, and in particular for the upper surface 48, median fine waviness values of the set were measured which are less than 20 nm and greater than 10 nm.

[0159] Where, however, an additional criterion was introduced, namely that the difference between the common logarithms of the viscosity at the distance from a component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width, lg (.sub.A/dPa*s), and of the viscosity at the end of hot forming, lg (.sub.E/dPa*s), is between at least 1.25 and at most 1.45, and preferably is 1.42, in that case virtually all the values measured for the fine waviness, especially on an entire set described here, were below 26 nm, and the spread of these values was significantly reduced.

[0160] The fine waviness values obtained with this additional criterion are represented in the left-hand column of FIG. 9.

[0161] In the interval of the left-hand representation in FIG. 9, for which the sum total of the viscosity values is subject to 11.4lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)11.6 and the difference is subject to 1.25lg (.sub.A/dPa*s)lg (.sub.E/dPa*s)1.45, values for the fine waviness of less than 20 nm were reliably achieved even for the median of the presently described set, not only for the surface 48 but also for the surface 49, and the values of the median were generally greater than 10 nm.

[0162] Where median values are presently indicated, they were obtained in each case, for the surface 48, from values of the measurement areas M1 to M8 and, for the bottom side, from values of the measurement areas M1 to M8. However, where these values did not include those of the outer measurement areas M1 and M8 and also M1 and M8, fine waviness values smaller than are represented in FIGS. 8 and 9 were generally obtained.

[0163] The value for the difference between the viscosities lg (.sub.E/dPa*s)lg (.sub.A/dPa*s)=1.25 was attained for example through the preferred viscosities lg (.sub.A/dPa*s)=5.1 and lg (.sub.E/dPa*s)=6.35. The value for the difference between the viscosities lg (.sub.E/dPa*s)lg (.sub.A/dPa*s)=1.45 was attained for example through the viscosities lg (.sub.A/dPa*s)=5.0 and lg (.sub.E/dPa*s)=6.45, with the above viscosity value at the end of hot forming of 6.45 being only slightly below the indicated preferred maximum limit of 6.5, and being obtained as a direct result of the methodology presently described.

[0164] In the case, however, of singulation of glass sheets 30 which encompassed the measurement areas M3 to M6 or M3 to M6 situated closer to the midpoint line Mi in X-direction, then all of the fine waviness values measured in these measurement areas along the respective measurement line ML were reliably below 26 nm. This means that all of the measured fine wavinesses within a positive and also negative distance in X-direction from the midpoint line Mi of 910 mm were then below 26 nm, as is represented for example in the left-hand column of FIG. 9.

[0165] In particular, the statement in this paragraph is also valid for intervals with 11.4lg (.sub.A/dPa*s)+lg (.sub.E/dPa*s)11.6 and 1.25lg (.sub.A/dPa*s)lg (.sub.E/dPa*s)1.45, for which the value for the difference between the viscosities lg (.sub.E/dPa*s)lg (.sub.A/dPa*s)=1.5 was achieved for example through the viscosities lg (.sub.A/dPa*s)=5.0 and lg (.sub.E/dPa*s)=6.5.

[0166] It is for the skilled person to recognize that such precise viscosity values required both exact sensor detection of these viscosities, in particular through corresponding temperature measurements for the temperature of the glass ribbon 13 at the appropriate site, and also suitable measures for the removal and supply of thermal energy.

[0167] For this purpose, the temperature of the glass may be detected using sensor facilities or units 26, in which case these sensor units 26 do not only have to be disposed preferably close to the tweel 17 but may also be located at further sites, in particular along the section Hsl for the thickness-based hot forming, in order in particular to be able to detect the temperature of the glass ribbon 13 always with the necessary accuracy and to regulate it accordingly.

[0168] Appropriate supply of heat may be undertaken with overall local thermal control by means of the burners 4 and also by means of a float bath 7 with sector-specific temperature control.

[0169] Appropriate removal of heat may take place, for example, by special apparatuses for cooling such as fans, which are not represented in the figures but are known to the skilled person, or with cooling facilities 57, which may be suitably arranged along the section Hsl for the thickness-based hot forming. The sector-specific thermal control of the float bath 7 may also contribute to appropriate removal of heat.

[0170] Further, the amount of the glass 8 for hot forming, especially of the glass ribbon 13 formed from it, per unit time may be adjusted using the component for throughput regulation, more particularly the control slider or tweel 17, such that the temperature of the glass 8 for hot forming is always within a secure control range, which may be exceeded, for example, when the heat capacity of the glass with increasing throughput hinders temperature control only through the surface, owing to the increasing glass volume.

[0171] The method according to the present disclosure may for this purpose also be carried out, advantageously, such that a throughput of less than 400 t of glass per day, preferably less than 200 t of glass per day and more preferably less than 100 t of glass per day is obtained; for ascertaining the throughput, the basis used is the amount of glass which is conveyed per unit time through the component for throughput regulation, more particularly the control slider or tweel 17.

LIST OF REFERENCE SIGNS

[0172] 1 Float plant [0173] 2 Melting vessel [0174] 3 Batch for melting, especially glass batch [0175] 4 Burner [0176] 5 Glass melt [0177] 6 Channel [0178] 7 Float bath [0179] 8 Glass for hot forming [0180] 9 Float bath furnace [0181] 10 Overhead heater [0182] 11 Spout [0183] 12 Top roller [0184] 13 Glass ribbon [0185] 14 Lehr [0186] 15 Overhead and floor heater [0187] 16 Melting facility [0188] 17 Component for throughput regulation, especially control slider or tweel [0189] 18 Facility for defined adjustment of the viscosity of the molten glass 8 for hot 18 forming ahead of the component for throughput regulation 17 [0190] 19 Chamber which is separated from the melting vessel 2 or else may form a part of said vessel and accommodates the molten glass 8, intended for shaping to a glass ribbon 13, for the defined adjustment of its viscosity [0191] 20 Region traversed by fluid flow 20 [0192] 21 Region traversed by fluid flow [0193] 22 Wall of chamber 19 [0194] 23 Wall of chamber 19 [0195] 24 Wall of chamber 19 [0196] 25 Wall of chamber 19 [0197] 26 Sensor facility or unit [0198] 27 Bay or vessel section 1 [0199] 28 Bay or vessel section 2 [0200] 29 Bay or vessel section 3 [0201] 30 Bay or vessel section 4 [0202] 31 Bay or vessel section 5 [0203] 32 Bay or vessel section 6 [0204] 33 Glass sheet, also with reference signs 33 or 33 [0205] 34 Top side of glass sheet 33 [0206] 35 Bottom side of glass sheet 33 [0207] 36 Surface of top side 34 of glass sheet 33 [0208] 37 Surface of bottom side 35 of glass sheet 33 [0209] 38 Top roller [0210] 39 Top roller [0211] 40 Top roller [0212] 41 Top roller [0213] 42 Top roller [0214] 43 Top roller [0215] 44 Top roller [0216] 45 Wall of channel 6 [0217] 46 Wall of channel 6 [0218] 47 Facility or apparatus for hot forming [0219] 48 Upper surface, upper principal surface of the glass ribbon 13 or glass 8 for hot forming [0220] 49 Lower surface, lower principal surface of the glass ribbon 13 or glass 8 for hot forming [0221] 50 Axis of symmetry [0222] 51 Axis of symmetry [0223] 52 Perpendicular in negative z-direction [0224] 53 Perpendicular in negative z-direction [0225] 54 Site of entry of the glass 8 into the section Hs for thickness-based hot forming, represented with a dashed line [0226] 55 Site of emergence of the glass 8 from the hot-forming section Hs [0227] 56 Distance from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width [0228] 57 Further cooling facility [0229] M1 to M8 Area or measurement area for determining the fine waviness of the upper surface 48 of the glass ribbon 13 and the upper surface 36 of the top side 34 of the glass sheets 33, 33, 33 [0230] M1 to M8 Area or measurement area for determining the fine waviness of the lower surface 49 of the glass ribbon 13 and the surface 37 of the bottom side 35 of the glass sheets 33, 33, 33 [0231] ML Measurement line with a length of 260 mm, disposed respectively within a measurement area M1 to M8 or within a measurement area M1 to M8 [0232] Mi Midpoint of the glass ribbon in X-direction [0233] Mt1 Boundary line to the region of the respective surface of the glass ribbon 13 that is covered by the top rollers 38 to 44 [0234] Mt2 Boundary line to the region of the respective surface of the glass ribbon 13 that is covered by the top rollers 38 to 44 [0235] OSG Line at a fine waviness value of 26 nm [0236] Viscosity [0237] .sub.A Viscosity at a distance from the component for throughput regulation at which the glass after impinging on the float bath has acquired its maximum width [0238] .sub.E Viscosity at the end of hot forming