METHOD AND APPARATUS FOR PRODUCING A THIN GLASS RIBBON, AND THIN GLASS RIBBON PRODUCED ACCORDING TO SUCH METHOD

Abstract

An improved method and an improved apparatus are provided for producing a thin glass ribbon, which provide borders at the edges of the ribbon. The edges formed are of high mechanical quality and a formation of new secondary borders after the severing or at least the thickness of such secondary borders is reduced compared to the original borders. The method includes drawing the thin glass ribbon from a molten glass or from a preform, severing the borders, and cooling the resulting glass ribbon. The severing is effected at a location along the moving direction of the thin glass ribbon and at a time at which during the cooling of the thin glass ribbon the viscosity of the glass is in a range from 10.sup.7 dPa.Math.s to 10.sup.11 dPa.Math.s, so that the edges of the thin glass ribbon newly produced by the severing of the borders are rounded off.

Claims

1. A thin glass ribbon, comprising: a thickness of not more than 300 m; a thickness (D.sub.use) in a center of the glass ribbon; and edges that are rounded off and have a fire-polished surface, wherein the edges have a maximum thickness (D.sub.B), wherein $D_{B}2.Math.\sqrt{\frac{D_{u.Math.s.Math.e}.Math.200.Math..Math.m}{}},$ and wherein is the mathematical constant number .

2. The thin glass ribbon of claim 1, wherein the thin glass ribbon comprises a lithium aluminosilicate glass having the following composition, in wt %: TABLE-US-00019 SiO.sub.2 55-69, Al.sub.2O.sub.3 18-25, Li.sub.2O 3-5, Na.sub.2O + K.sub.2O 0-30, MgO + CaO + SrO + BaO 0-5, ZnO 0-4, TiO.sub.2 0-5, ZrO.sub.2 0-5, TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 2-6, P.sub.2O.sub.5 0-8, F .sup.0-1, and B.sub.2O.sub.3 0-2.

3. The thin glass ribbon of claim 1, wherein the thin glass ribbon comprises a soda-lime glass having the following composition, in wt %: TABLE-US-00020 SiO.sub.2 40-81, Al.sub.2O.sub.3 0-6, B.sub.2O.sub.3 0-5, Li.sub.2O + Na.sub.2O + K.sub.2O 5-30, MgO + CaO + SrO + BaO + ZnO 5-30, TiO.sub.2 + ZrO.sub.2 .sup.0-7, and P.sub.2O.sub.5 0-2.

4. The thin glass ribbon of claim 1, wherein the thin glass ribbon comprises a borosilicate glass having the following composition, in wt %: TABLE-US-00021 SiO.sub.2 60-85, Al.sub.2O.sub.3 0-10, B.sub.2O.sub.3 5-20, Li.sub.2O + Na.sub.2O + K.sub.2O 2-16, MgO + CaO + SrO + BaO + ZnO 0-15, TiO.sub.2 + ZrO.sub.2 .sup.0-5, and P.sub.2O.sub.5 0-2.

5. The thin glass ribbon of claim 1, wherein the thin glass ribbon comprises an alkali metal aluminosilicate glass having the following composition, in wt %: TABLE-US-00022 SiO.sub.2 40-75, Al.sub.2O.sub.3 10-30, B.sub.2O.sub.3 0-20, Li.sub.2O + Na.sub.2O + K.sub.2O 4-30, MgO + CaO + SrO + BaO + ZnO 0-15, TiO.sub.2 + ZrO.sub.2 .sup.0-15, and P.sub.2O.sub.5 0-10.

6. The thin glass ribbon of claim 1, wherein the thin glass ribbon comprises an alkali metal aluminosilicate glass having the following composition, in wt %: TABLE-US-00023 SiO.sub.2 50-75, Al.sub.2O.sub.3 7-25, B.sub.2O.sub.3 0-20, Li.sub.2O + Na.sub.2O + K.sub.2O 0-4, MgO + CaO + SrO + BaO + ZnO 5-25, TiO.sub.2 + ZrO.sub.2 .sup.0-10, and P.sub.2O.sub.5 0-5.

7. The thin glass ribbon of claim 1, wherein the thickness is at least 5 rn.

8. An apparatus for producing a thin glass ribbon, comprising: a device configured to guide a molten glass or heat a preform during forming of the thin glass ribbon; and a severing device configured to severe borders from the thin glass ribbon, the severing device being arranged at a distance in a range from 80 mm to 400 mm from the device.

9. The apparatus of claim 8, wherein the device is a drawing orifice.

10. The apparatus of claim 8, wherein the device is a heater.

11. The apparatus of claim 8, comprising a setting device configured to adjust at least one parameter selected from the group consisting of drawing rate, mass flow rate, and the severing distance so that the severing device severs the borders at a viscosity of the glass in a range from 10.sup.7 dPa.Math.s to 10.sup.11 dPa.Math.s.

Description

DESCRIPTION OF THE DRAWINGS

[0077] The invention will now be explained in more detail with reference to the accompanying figures, wherein:

[0078] FIG. 1 shows a diagram of isotherms of the glass ribbon as a function of the height below the orifice and the width of the glass ribbon;

[0079] FIG. 2 shows a diagram of vertical temperature profiles of the glass ribbon and the environment for two cases (case 1 and case 2) of different muffle temperature profiles as a function of the distance from the orifice;

[0080] FIG. 3 shows a diagram of temperature profiles at different heights below the orifice as a function of the width of the glass ribbon for case 1, with a thickness of the glass ribbon of 100 m;

[0081] FIG. 4 shows a diagram of thickness profiles at different heights below the orifice as a function of the width of the glass ribbon for case 1, with a thickness of the glass ribbon of 100 m;

[0082] FIG. 5 shows a diagram of rate profiles at different heights below the orifice as a function of the width of the glass ribbon for case 1, with a thickness of the glass ribbon of 100 m;

[0083] FIG. 6 shows a diagram of temperature profiles at different heights below the orifice as a function of the width of the glass ribbon for case 2, with a thickness of the glass ribbon of 100 m;

[0084] FIG. 7 shows a diagram of thickness profiles at different heights below the orifice as a function of the width of the glass ribbon for case 2, with a thickness of the glass ribbon of 100 m;

[0085] FIG. 8 shows a diagram of rate profiles at different heights below the orifice as a function of the width of the glass ribbon for case 2, with a thickness of the glass ribbon of 100 m;

[0086] FIG. 9 shows a diagram of temperature profiles at different heights below the orifice as a function of the width of the glass ribbon for case 1, with a thickness of the glass ribbon of 50 m;

[0087] FIG. 10 shows a diagram of thickness profiles at different heights below the orifice as a function of the width of the glass ribbon for case 1 and with a thickness of the glass ribbon of 50 m;

[0088] FIG. 11 shows a diagram of rate profiles at different heights below the orifice as a function of the width of the glass ribbon for case 1, with a thickness of the glass ribbon of 50 m;

[0089] FIG. 12 shows a diagram of temperature profiles at different heights below the orifice as a function of the width of the glass ribbon for case 2, with a thickness of the glass ribbon of 50 m;

[0090] FIG. 13 shows a diagram of thickness profiles at different heights below the orifice as a function of the width of the glass ribbon for case 2, with a thickness of the glass ribbon of 50 m;

[0091] FIG. 14 shows a diagram of rate profiles at different heights below the orifice as a function of the width of the glass ribbon for case 2, with a thickness of the glass ribbon of 50 m;

[0092] FIG. 15 is a schematic view of the apparatus according to the invention, with a laser as a severing device;

[0093] FIG. 16 shows a diagram of temperature profiles as a function of the distance from the cutting line;

[0094] FIG. 17 is a schematic view of the apparatus according to the invention, with wheels as a severing device; and

[0095] FIG. 18 shows an apparatus according to the invention for producing a thin glass ribbon by drawing from a preform.

DETAILED DESCRIPTION

[0096] By way of example, an alkali-free glass is employed which is marketed by Schott AG, Mainz, under the name AF32, and which has the following composition, in wt %:

TABLE-US-00018 SiO.sub.2 61 Al.sub.2O.sub.3 18 B.sub.2O.sub.3 10 CaO 5 BaO 3 MgO 3

[0097] Glass AF32 has a density Q of 2430 kg/m.sup.3 and a surface tension y of 0.3 N/m, a thermal conductivity of 2 W/mK, and a specific heat capacity c.sub.p of 1360 J/kgK.

[0098] The transformation temperature T.sub.g of the glass AF32 is 713 C.

[0099] For manufacturing the thin glass ribbon with a target thickness of 100 m or 50 m, the glass is heated in a furnace and is drawn through an orifice having a lateral width of 800 mm, for example. A suitable opening width of the orifice is between 8 mm and 18 mm. The temperature of the orifice is preferably above 1100 C. In this manner, the thin glass ribbon may be drawn at a flow rate of more than 1.5 kg per minute, at a drawing rate of 6 m/min and more.

[0100] FIG. 1 shows a diagram of the isotherms at 1000 C., 900 C., and 800 C. of the thin glass ribbon 1 as a function of the height below the orifice and the width of the glass ribbon.

[0101] While being drawn out of the orifice the glass ribbon is cooling down, and at the periphery or the edges of the thin glass ribbon 1 that is being drawn increasing thickenings, called borders, are forming on the narrowing glass ribbon, due to the process and due to material properties of the highly viscous molten glass. Thereby, temperature inhomogeneities arise within the glass ribbon, which may lead to stresses and unevennesses of the glass or warp of the glass.

[0102] At the borders, the glass can be drawn out of the orifice and can be guided.

[0103] FIG. 2 shows a diagram of vertical temperature profiles of the glass ribbon and of the environment for two cases of different muffle temperature profiles as a function of the distance from the orifice.

[0104] Two cases (Case 1 and Case 2) are considered, with the vertical muffle temperature profiles of:

Case 1 (with a temperature gradient of 4000 K/m): T(y)=max (700 C.; 1055 C.)+4000* y; and

Case 2 (with a temperature gradient of 2000 K/m): T(y)=max (700 C.; 1055 C.)+2000* y.

[0105] Here, y denotes the distance to the drawing orifice in meters. The resulting glass temperatures for the thin glass ribbon having a thickness of 50 m and for the thin glass ribbon having a thickness of 100 m are virtually identical.

[0106] FIGS. 3 to 8 each show diagrams with a thickness of the glass ribbon of 100 m, with FIG. 3 to FIG. 5 given for case 1, and FIG. 6 to FIG. 8 given for case 2.

[0107] FIG. 3 and FIG. 6 each show a diagram of temperature profiles at different heights of 50 mm, 100 mm, 200 mm, 300 mm, and 400 mm below the orifice as a function of the width of the glass ribbon, for case 1 and case 2, respectively, with a thickness of the glass ribbon of 100 m in each case.

[0108] As can be seen therefrom, up to approximately 400 mm below the orifice the temperature is still above the transformation temperature of the glass AF32 of 713 C. Therefore, the range with a distance of less than and up to 400 mm below the orifice is preferred for severing the borders.

[0109] FIG. 4 and FIG. 7 each show a diagram of thickness profiles at different heights of 20 mm, 40 mm, 60 mm, and 80 mm below the orifice as a function of the width of the glass ribbon, for case 1 and case 2, respectively, with a thickness of the glass ribbon of 100 m in each case.

[0110] As can be seen from FIG. 4 and FIG. 7, respectively, the final thickness profile (for 100 m) of the glass ribbon, as shown in dashed lines, is already almost obtained at approximately 80 mm below the orifice. Therefore, the range with a distance of more than 80 mm below the orifice is preferred for severing the borders.

[0111] FIG. 5 and FIG. 8 each show a diagram of rate profiles at different heights of 50 mm, 100 mm, and 150 mm below the orifice as a function of the width of the glass ribbon, for case 1 and case 2, respectively, with a thickness of the glass ribbon of 100 m in each case.

[0112] As can be seen from FIG. 5 and FIG. 8, respectively, at approximately 150 mm the thin glass ribbon advances almost like a solid body, with the drawing rate of 7.2 m/min, represented in dashed lines. Therefore, if the borders are severed in this range, the central region of the thin glass ribbon and in particular the useful glass ribbon will not again experience constriction caused by drawing forces.

[0113] Thus, it becomes clear from the diagrams of FIGS. 3 to 5 and FIGS. 6 to 8, respectively, that the process of severing the borders from the thin glass ribbon should be performed at a distance in a range from 80 mm to 400 mm, preferably from 150 mm to 400 mm, more preferably in a range from 150 mm to 300 mm from the orifice.

[0114] FIGS. 9 to 14 show the corresponding diagrams as in FIGS. 3 to 8, but with a thickness of the glass ribbon of 50 m, with FIG. 9 to FIG. 11 given for case 1, and FIG. 12 to FIG. 14 given for case 2.

[0115] Thus, the Following Applies:

[0116] FIG. 9 and FIG. 12 each show a diagram of temperature profiles at different heights of 50 mm, 100 mm, 200 mm, 300 mm, and 400 mm below the orifice as a function of the width of the glass ribbon, for case 1 and case 2, respectively, with a thickness of the glass ribbon of 50 m in each case.

[0117] As can be seen therefrom, up to approximately 400 mm below the orifice the temperature is still above the transformation temperature of the glass AF32 of 713 C. Therefore, without limitation to the specific exemplary embodiment, the range with a distance of less than and up to 400 mm below the orifice is preferred for severing the borders.

[0118] FIG. 10 and FIG. 13 each show a diagram of thickness profiles at different heights of 20 mm, 40 mm, 60 mm, and 80 mm below the orifice as a function of the width of the glass ribbon, for case 1 and case 2, respectively, with a thickness of the glass ribbon of 50 m in each case.

[0119] As can be seen from FIG. 10 and FIG. 13, respectively, the final thickness profile (for 50 m) of the glass ribbon, as shown in dashed lines, is already almost obtained at approximately 80 mm below the orifice.

[0120] Therefore, without limitation to the specific exemplary embodiment, the range with a distance of more than 80 mm below the orifice is preferred for severing the borders.

[0121] FIG. 11 and FIG. 14 each show a diagram of rate profiles at different heights of 50 mm, 100 mm, and 150 mm below the orifice as a function of the width of the glass ribbon, for case 1 and case 2, respectively, with a thickness of the glass ribbon of 50 m in each case.

[0122] As can be seen from FIG. 11 and FIG. 14, respectively, at approximately 150 mm the thin glass ribbon advances almost like a solid body, with the drawing rate of 7.2 m/min, as represented in dashed lines. Therefore, if the borders are severed in this range, the central region of the thin glass ribbon and in particular the useful glass ribbon will not again experience constriction caused by drawing forces.

[0123] Thus, it becomes clear from the diagrams of FIGS. 3 to 14 that the process of severing the borders from the thin glass ribbon should be performed at a distance in a range from 80 mm to 400 mm, preferably from 150 mm to 400 mm, more preferably in a range from 150 mm to 300 mm from the orifice.

[0124] Accordingly, an apparatus 2 is preferably employed which comprises a device for guiding the molten glass 3, preferably a drawing orifice 4, and a device for severing the borders 7, 8 from the thin glass ribbon 1, which severing device is arranged at a distance in a range from 80 mm (millimeters) to 400 mm, preferably from 150 mm to 400 mm, more preferably from 150 mm to 300 mm from the nearest melt contact surface of the device guiding the molten glass 3, in particular the drawing orifice 4. Examples of such an apparatus 2 will be described below with reference to FIG. 15 and FIG. 17.

[0125] In addition, it can be seen from the diagrams that the thickenings in the peripheral regions of the thin glass ribbon 1, i.e. the borders 7, 8, have a width in a range from 30 mm (millimeter) to 150 mm, in particular in a range from 50 mm to 100 mm of the entire thin glass ribbon 1. Consequently, it is preferred to sever precisely this peripheral width region that exhibits the temperature inhomogeneities, in order to avoid tensions and unevenness or warp of the glass. With respect to the width of a thin glass ribbon 1, according to one embodiment of the invention without limitation to the illustrated exemplary embodiments, borders 7, 8 are separated which have an added width of at least 1/10, preferably at least of the width of the thin glass ribbon 1 after the borders 7, 8 have been severed. Generally, the width of borders 7, 8 is less dependent on the absolute width of the thin glass ribbon 1 that is being drawn. Typically, without being limited to the exemplary embodiments, good homogenization of the temperature profile during the cooling of the thin glass ribbon 1 can be achieved and so permanent mechanical stresses may be effectively suppressed by severing a strip of more than 30 millimeters. Therefore, according to one embodiment of the invention it is suggested that borders 7, 8 are severed which each have a width of at least 30 millimeters, preferably at least 40 millimeters.

[0126] According to one exemplary embodiment, borders 7, 8 each having a width of 50 millimeters are severed from a thin glass ribbon 1 which has a width of 600 millimeters.

[0127] As can be seen in particular from FIGS. 1, 3, 6, 9, and 12, the thin glass ribbon 1 has a particularly homogeneous temperature profile in the center of the ribbon, and therefore the useful glass ribbon after separation of borders 7, 8 as well. For example, if in the example shown in FIG. 6 the border is separated at a distance of 300 or 400 millimeters below the orifice and so that a glass ribbon of a width of 0.4 meters is obtained, the difference in temperature from the edge to the center will be less than 20 C. in both cases. In case of a smaller distance of the severing location from the orifice the temperature difference will even be smaller. Generally, without being limited to the exemplary embodiments, it is therefore provided according to one variation of the invention that when the borders have been severed, the thin glass ribbon 1 exhibits a temperature difference between the edge and the center of the ribbon of less than 20 C., measured perpendicular to the drawing direction.

[0128] Upstream of the severing location 10 or cut, borders 7, 8 preferably serve to span the thin glass ribbon 1 perpendicularly to the moving direction. Because of their greater thickness as compared to the center of thin glass ribbon 1 or the useful glass, borders 7, 8 are more rigid.

[0129] In order to provide an improved method which allows to sever the borders 7, 8 so as to form edges 11, 12 of high mechanical quality while preventing a formation of new secondary borders after the severing from the ribbon-shaped molten glass 3 or at least reducing the thickness of the secondary borders as compared to the original borders 7, 8, and so as to avoid stresses in the glass after the severing process, which stresses could otherwise cause unevenness or warp of the glass, the invention provides the following method described below, wherein apparatuses 2 as shown in FIGS. 15 and 17 are preferred for performing the method.

[0130] For manufacturing a thin glass ribbon 1 of particularly high quality the glass mentioned above is used, for example.

[0131] For performing the method for producing a thin glass ribbon 1, a preferred apparatus 2 according to FIG. 15 comprises a device for guiding the molten glass 3, preferably a drawing orifice 4, which is disposed inside heating means 5 and a heating muffle 6. The thin glass ribbon 1 is drawn from the molten glass 3, whereby borders 7, 8 are forming at both edges of the thin glass ribbon 1, which borders have a greater thickness than the center of the thin glass ribbon 1. After having been drawn from the molten glass 3, the thin glass ribbon 1 cools down, and the borders 7, 8 are severed from the thin glass ribbon 1 by means of a severing device 9, in particular a laser 9a, at a separation location 10, that means a location along the moving direction of the thin glass ribbon 1 and at a time at which during the cooling of the thin glass ribbon 1 the viscosity of the glass is in a range from 10.sup.7 dPa.Math.s to 10.sup.11 dPa.Math.s, so that the edges 11, 12 of the thin glass ribbon newly formed by severing the borders 7, 8 are rounding off and in particular have a fire-polished surface.

[0132] According to the embodiment of FIG. 15, the thin glass ribbon 1 is withdrawn by rollers 13, 14 which only engage the already severed borders 7, 8. Therefore, tensile forces are applied to the thin glass ribbon 1 only in the region upstream the severing location 10, i.e. where the glass is still above the glass transformation temperature T.sub.g and therefore soft. This embodiment is preferred, since rollers 13, 14 do not act on the actual thin glass ribbon 1. Alternatively or cumulatively, rollers 15, 16 as shown in dashed lines may be used, by which the thin glass ribbon 1 is withdrawn in the central region of the ribbon.

[0133] Apparatus 2 preferably comprises setting means, not shown in the figures, for adjusting at least one of parameters drawing rate, mass flow rate, and/or severing location so that the severing of the borders 7, 8 while the thin glass ribbon 1 cools down is effected at a viscosity of the glass in a range from 10.sup.7 dPa.Math.s to 10.sup.11 dPa.Math.s, preferably in a range from 10.sup.8 dPa.Math.s to 10.sup.11 dPa.Math.s, more preferably in a range from 10.sup.9 dPa.Math.s to 10.sup.1 dPa.Math.s.

[0134] According to the invention the severing of the borders 7, 8 is preferably performed still in the hot-forming section, wherein the severing device 9 for separating the borders 7, 8 from the thin glass ribbon 1 is preferably arranged at a distance in a range from 80 mm to 400 mm, more preferably from 150 mm to 400 mm, most preferably in a range from 150 mm to 300 mm from the drawing orifice 4. This corresponds to glass temperatures of about 750 C. to 900 C. and respective associated glass viscosities in the range disclosed. Approximately 150 mm below the orifice 4 the thin glass ribbon moves almost like a solid body, so that when the borders 7, 8 are severed the useful region will not again be constricted by drawing forces.

[0135] To obtain a particularly homogeneous temperature profile of the thin glass ribbon 1 as a useful glass ribbon and to thereby avoid the creation of stresses and associated therewith a formation of warp, the borders 7, 8 are preferably severed with a width ranging from 30 mm to preferably at most 150 mm, particularly preferably from 10 to preferably at most 100 millimeters.

[0136] In the apparatus 2 shown in FIG. 15, the severing of the borders 7, 8 from the thin glass ribbon 1 is performed in a particularly advantageous embodiment of the method in the region of homogeneous temperature by means of a laser 9a, wherein the laser 9a or the laser beam 90 generated by the laser 9a melts the glass.

[0137] The heating of the cutting edge by laser 9a may (theoretically) lead to a capillarity-driven formation of secondary borders.

[0138] This can be Approximated as Follows:

[0139] It is assumed that the glass AF32 is heated in the focus of the laser to a temperature T.sub.0: The time t of exposure to the laser is

[00002]$\begin{array}{cc}t=\frac{D_L}{v_{\mathrm{draw}}}& \left(1\right)\end{array}$

wherein D.sub.L is the diameter of the laser focus and v.sub.draw is the drawing rate of the glass ribbon.

[0140] When the method is employed in an online process directly in conjunction with the shaping of the thin glass, the drawing rate depends on the speed of the glass ribbon during the creation thereof and on the glass thickness. In correlation with the glass volume, a thinner glass will be drawn more quickly than a thicker one. In the present example the drawing rate v.sub.draw for a thin glass of 100 m thickness is 7.2 m/min (120 mm/s), and is 15 m/min (250 mm/s) for a thin glass having a thickness of 50 m.

[0141] In the example where the laser focus has a diameter D.sub.L of 1 mm (0.001 m) and the drawing rate v.sub.draw is 7.2 m/min (120 mm/s), the resulting time of exposure to the laser is

[00003]$\begin{array}{cc}t=\frac{D_L}{V_{\mathrm{draw}}}=\frac{0.0.Math.0.Math.1}{0.1.Math.2}.Math.s=0.0.Math.08.Math.s.& \left(2\right)\end{array}$

[0142] During this time, the increase in temperature spreads within the glass ribbon 1 approximately according to the formula (*)

[00004]$\begin{array}{cc}\frac{T\left(t,x\right)-T_{}}{T_0-T_{}}=1-\mathrm{erf}\left(\frac{x}{\sqrt[2]{\frac{.Math.t}{.Math.c_p}}}\right)& \left(3\right)\end{array}$ [0143] wherein [0144] t is the time of exposure to the laser; [0145] x is the distance from the cutting line; [0146] T.sub.0 is the heating temperature of the glass in the focus of the laser; [0147] T.sub. is the glass temperature at the level of the laser focus; [0148] D.sub.L is the diameter of the laser focus i; [0149] Q is the density of the glass (for AF32=2430 kg/m.sup.3); [0150] is the surface tension of the glass (for AF32=0.3 N/m); [0151] is the thermal conductivity of the glass (for AF32=2 W/mK); and [0152] c.sub.p is the specific heat capacity of the glass (for AF32=1360 J/kgK).

[0153] FIG. 16 shows a diagram of the temperature profiles according to formula (3) as a function of the distance x from the cutting line. As can be seen therefrom, even with a very large laser focus (D.sub.L1 mm) the temperature elevation spreads only by about 200 m into the region of useful glass.

[0154] Due to the preferred short time of exposure to the laser 9a, a small portion of the useful glass is heated during severing. Due to capillary forces a secondary border may be caused thereby, with a maximum thickness D.sub.B at the edges of the thin glass ribbon for which applies:

[00005]$\begin{array}{cc}{D}_{B}2.Math.\sqrt{\frac{D_{u.Math.s.Math.e}.Math.200.Math..Math.m}{}}& \left(4\right)\end{array}$

wherein D.sub.use is the thickness of the thin glass ribbon in the center of the ribbon and =3.1415 is the mathematical constant number .

[0155] With a thickness of the useful glass D.sub.use in the example of 100 m, the maximum thickness D.sub.B at the edges of the thin glass ribbon 1 is therefore

[00006]$\begin{array}{cc}{D}_{B}2.Math.\sqrt{\frac{100.Math..Math.m.Math.200.Math..Math.m}{}}160.Math..Math.m,& \left(5\right)\end{array}$

[0156] and with a thickness of the useful glass D.sub.use in the example of 50 m, the maximum thickness D.sub.B at the edges of the thin glass ribbon 1 is therefore

[00007]$\begin{array}{cc}{D}_{B}2.Math.\sqrt{\frac{50.Math..Math.m.Math.200.Math..Math.m}{}}110.Math..Math.m& \left(6\right)\end{array}$

[0157] For a thin glass ribbon 1 that is coiled into a roll with a winding core of 500 mm this means that the permanent stresses in borders 7, 8 are reduced from 50 MPa to 24 MPa and 16 MPa, respectively.

[0158] As an alternative to the apparatus 2 comprising a laser 9a as the severing device 9 according to FIG. 15, in which the glass is melted by the laser 9a, the severing of the borders 7, 8 of the thin glass ribbon 1 may be performed according to the apparatus 2 of FIG. 17 by squeezing using wheels 9b without breaking the glass which is still viscoelastic after the drawing.

[0159] For performing the method for producing a thin glass ribbon 1, a further preferred apparatus 2 according to FIG. 17 comprises a device for guiding the molten glass 3, preferably a drawing orifice 4, which is disposed inside heating means 5 and a heating muffle 6. The thin glass ribbon 1 is drawn from the molten glass 3, whereby borders 7, 8 are forming at both edges of the thin glass ribbon 1, which borders have a greater thickness than the center of the thin glass ribbon 1. After having been drawn from the molten glass 3, the thin glass ribbon 1 cools down, and the borders 7, 8 are severed from the thin glass ribbon 1 using a severing device 9, in particular using wheels 9b, at a separation location 10, that means at a location along the moving direction of the thin glass ribbon 1 and at a time at which during the cooling of the thin glass ribbon 1 the viscosity of the glass is in a range from 10.sup.7 dPa.Math.s to 10.sup.11 dPa.Math.s, so that the edges 11, 12 of the thin glass ribbon newly formed by severing borders 7, 8 are rounding off and in particular have a fire-polished surface.

[0160] According to the embodiment of FIG. 17, the thin glass ribbon 1 is withdrawn by rollers 13, 14 which only engage the already severed borders 7, 8. Therefore, tensile forces are applied to the thin glass ribbon 1 only in the region upstream the severing location 10, that means where the glass is still above the glass transformation temperature T.sub.g and therefore soft. This embodiment is preferred, since rollers 13, 14 do not act on the actual thin glass ribbon 1. Alternatively or cumulatively, rollers 15, 16 may be used (as in FIG. 15), which are not shown in FIG. 17, by means of which the thin glass ribbon 1 is withdrawn in the central region of the ribbon.

[0161] In the exemplary embodiments of the invention described above the glass ribbon was drawn from a molten glass 3, in which case the dimensions of the glass ribbon are essentially determined by the shape of an orifice 4. As mentioned above, the invention may also be applied in similar manner to the drawing of glass ribbons from preforms. In such method typically a plate-shaped preform is provided, and a longitudinal section of the preform is heated by heating means to such an extent that the glass of the preform softens. By applying a tensile force the softened glass can then be drawn into a glass ribbon. FIG. 18 shows an example of such an apparatus 2 for producing the thin glass ribbon 1. Here, the plate-shaped glass preform 18 is shown in a side view, looking to an edge face or to the border of the thin glass ribbon 1 which is forming during drawing.

[0162] In apparatus 2 the glass preform 18 is moved from above downwards, for example. Apparatus 2 comprises heating means 20 arranged in a central section of apparatus 2. In this embodiment, heating means 20 comprise shields 23 for thermally shielding a deformation zone 25 that is forming. A portion of the glass preform 18 which is located in the deformation zone 25 is heated to such an extent that it reaches a temperature T2 at which the viscosity of the glass is below 10.sup.8 dPa.Math.s, preferably at most 10.sup.7.6 dPa.Math.s. Glass preform 18 is drawn in the drawing direction 110, for example downwards, by drawing means 26 which are implemented in form of two driven rollers 13, 14 here. Since the feeding means 27 which are likewise implemented in form of rollers here are advancing the glass preform 18 more slowly than the drawing means 26 are drawing, the glass preform 18 is deformed in deformation zone 25. Therefore the glass preform 18 becomes thinner, the thickness d of the so formed glass ribbon 1 after deformation is smaller than the thickness D before the deformation.

[0163] Generally, without limitation to the specific example of an apparatus 2 shown in FIG. 18, the glass preform 18 is preferably pre-heated prior to the heating in the deformation zone 25. For this purpose, apparatus 2 preferably comprises a preheating zone 28 in which the preform 18 can be heated to a temperature T1. Pre-heating zone 28 is preferably arranged in a section upstream of the deformation zone 25 as seen in the drawing direction 110, for example in an upper section of apparatus 2. Temperature T1 preferably corresponds to a viscosity 1 from 10.sup.10 to 10.sup.14 dPa.Math.s. Thus, the glass preform 18 is preferably pre-heated before entering the deformation zone 25. Therefore, it may be advanced faster through the deformation zone 25, because the time required to reach the temperature T2 for softening the glass is shorter. Moreover, preheating zone 28 prevents glasses having a high coefficient of thermal expansion from breaking due to excessive temperature gradients. Generally, without limitation to the exemplary embodiment, temperature T2 is chosen so that the glass softens to such an extent that the viscosity of the glass has a value of not more than 10.sup.8 dPa.Math.s, preferably at most 10.sup.7.6 dPa.Math.s.

[0164] After passing through the deformation zone 25, the so obtained thin glass ribbon 1 is fed to an annealing device 29 which is symbolized by an ice crystal in the figure. Preferably, the glass is slowly cooled down in controlled manner to relief stresses. Actually, the cooling device 29 may therefore be implemented in form of an annealing furnace or lehr, with the glass passing through the viscosity range between the annealing point and the strain point in the annealing furnace.

[0165] Like in the exemplary embodiment shown in FIG. 15, a laser 9a is provided as a severing device, with a laser beam 90 that melts the glass of the thin glass ribbon 1 which is still viscoelastic after the drawing, so that the thin glass ribbon melts through at the point of impingement of the laser beam 90. The laser beam 90 may for example be introduced through an opening in the wall of apparatus 2, as shown, so that the laser beam 90 impinges on the thin glass ribbon 1 below the deformation zone 25. Here, the point of impingement is chosen so that the viscosity of the glass at this point is still in a range from 10.sup.7 dPa.Math.s to 10.sup.11 dPa.Math.s.

[0166] It will be apparent to those skilled in the art that the invention is not limited to the embodiments described above, but rather may be modified in various ways within the scope of the appending claims. In particular, the features of individual exemplary embodiments may be combined. For example, instead of laser 9a the apparatus 2 shown in FIG. 18 may be equipped with a cutting wheel or wheels 9b as shown in the embodiment of FIG. 17, for severing the borders.

LIST OF REFERENCE NUMERALS:

[0167] 1 Thin glass ribbon [0168] 2 Apparatus for performing the method for producing the thin glass ribbon 1 [0169] 3 Molten glass [0170] 4 Drawing orifice [0171] 5 Heating means [0172] 5 Heating muffle [0173] 7, 8 Borders [0174] 9 Severing device [0175] 9a Laser as the severing device [0176] 9b Wheels as the severing device [0177] 10 Severing location [0178] 11, 12 Edges [0179] 13, 14 Rollers [0180] 15, 16 Rollers [0181] 18 Preform [0182] 20 Heating means for heating 18 [0183] 25 Deformation zone [0184] 26 Drawing means [0185] 27 Feeding means [0186] 28 Preheating zone [0187] 29 Annealing device [0188] 90 Laser beam [0189] 110 Drawing direction