Silica glass tube and method of manufacturing the tube

Abstract

To provide a method of molding a starting hollow cylinder which has a wall thickness of 20 mm or larger and is made of silica glass and large in wall thickness into a pipe which has a larger diameter, but has substantially the same or larger glass cross sectional area.SOLUTION: A hollow cylinder 2 which has a wall thickness of 20 mm or larger is supplied to a heating region 12 continuously at a relative feed speed VC while rotating on an axis 10 of rotation, and regions softened here, one body one, are drawn in a radial direction under operation of gas pressure applied into a hollow cylinder bore 16 to form a rod-like pipe continuously. In a method of manufacturing a pipe 22 drawn at a drawing rate VT, the gas pressure is used as an adjustment quantity of diameter closed-loop control and / or diameter open-loop control with respect to a pipe external diameter or a shape parameter correlative to the pipe external diameter, and rises gradually from a low initial value to a higher final value in a pressure rising stage of the gas pressure, and VT=VC±0.2 VC holds for the ratio of VC and VT.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a tube made of silica glass. In particular, the present invention relates to a tube having a hollow cylinder bore, an outer diameter C a , inner diameter C i and a wall thickness of at least 20 mm, wherein the hollow cylinder is formed by rotating the hollow cylinder about its axis of rotation at a relative feed rate V C The gas is continuously supplied to the heating zone, where it is softened zone by zone, and the softened zone is stretched radially under the action of gas pressure applied to the hollow cylinder bore, and the tube outer diameter T a , tube inner diameter T i , and a rod-shaped tube having a wall thickness is formed, and the drawing speed is V T This is done by drawing out the

[0002] Furthermore, the present invention is a , tube inner diameter T i The present invention relates to a tube made of silica glass having a wall thickness, an outer coating surface, and an inner coating surface.

[0003] With such a method and apparatus, a hollow cylinder of glass, in particular silica glass, is formed into a tube in one or more thermoforming steps, with the radial tube dimensions being changed relative to the radial dimensions of the hollow cylinder. The starting hollow cylinder, rotating about its longitudinal axis, is softened in regions and stretched under the action of a radially outwardly directed force towards a forming die positioned at a predetermined radial distance relative to the tube's longitudinal axis, or formed without a forming die. The radially outwardly directed force is due to centrifugal force and / or an internal overpressure (also referred to as "gas pressure") in the hollow cylinder's internal bore.

[0004] The term "internal bore" or "bore" in reference to a tube or hollow cylinder does not imply that the internal bore or bore is formed by a drilling operation.

[0005] Regarding the heating of the starting hollow cylinder, a distinction can be made between a flame heating zone and an electric furnace. Because silica glass is a good thermal insulator, the flame heating zone, in which the surface heating is mainly carried out by recombination reactions with a small thermal penetration depth, is limited to forming starting hollow cylinders with thin walls, for example, less than 10 mm. In contrast, in an electric furnace, the infrared beam can penetrate deep into the glass, allowing more heat to be introduced uniformly over a longer distance.

[0006] Essentially, high dimensional accuracy and surface quality of the drawn rod-shaped tube are achieved, and the high surface quality is achieved by forming without using a mold.

[0007] On the other hand, maintaining dimensional accuracy is usually easier to achieve with the use of a forming tool. In either case, continuous detection and continuous closed-loop control of the radial dimensions of the rod-shaped tube, such as the outer diameter, inner diameter, or wall thickness, are essential. The gas pressure, the relative feed rate between the hollow cylinder and the heating zone, and the temperature in the heating zone are commonly used as adjustment variables for such closed-loop control. [Background technology]

[0008] A method for producing the aforementioned silica glass tube is known from JP 2010-111557 A. In this method, thick-walled hollow silica glass cylinders with an outer diameter of 200 mm are continuously fed into a furnace with their longitudinal axis oriented horizontally and rotated at a feed rate of 4 cm / min, where they are softened in zones to a temperature of around 2100 °C. A water-cooled graphite plate, serving as a mold, is located inside the furnace and its radial distance from the tube's longitudinal axis is adjustable. Due to the overpressure inside the hollow cylinder, the softened silica glass is blown onto the graphite plate, forming a rod-shaped tube that is then withdrawn from the furnace at a withdrawal rate of 12 cm / min. During this process, the rod-shaped tube may separate from the graphite plate, which may result in a change in diameter depending on the process conditions. To achieve closed-loop control of the rod diameter to the target value of 340 mm independently, two cameras are provided, one on the left side and one on the right side of the rod, that generate images of the left and right peripheries of the rod. From these images, the rod diameter is continuously determined by image processing as the closed-loop control variable. The adjustment variable for the closed-loop control is the radial distance of the water-cooled graphite plate relative to the longitudinal axis of the tube, which can be adjusted by an electrically operated mold block tracking control.

[0009] Publications US2017 / 327403A1 and JP2007-001811A describe alternative methods for producing large diameter silica glass tubes using a forming tool such as a graphite plate. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP2010-111557A [Patent Document 2] US2017 / 327403A1 [Patent Document 3] JP2007-001811A Summary of the Invention [Problem to be solved by the invention]

[0011] Due to the rotation of the hollow cylinder about its longitudinal axis, the graphite plate spaced apart from the hollow cylinder defines a circular "pseudo-opening" into which the softened silica glass is forced during the forming process, causing defects, particularly spiral stripes, on the outer surface of the drawn silica glass tube.

[0012] Due to the internal overpressure, a circumferential ridge of soft silica glass may form in front of the graphite plate (seen in the direction of the hollow cylinder's feed into the furnace) due to the gas pressure. Since the drawing speed of the rod is much faster than the feed speed of the hollow cylinder, the volume of the circumferential ridge is subject to a renewal mechanism, in which new glass is constantly deposited and withdrawn in equal amounts along the entire length of the rod. This speed ratio results in the "stretching" of the hollow cylinder, and the resulting glass cross-sectional area is necessarily smaller than that of the starting hollow cylinder.

[0013] All attempts to achieve silica glass tubes with the same or larger glass cross-sectional area using known methods have failed. The failure is believed to be due to the need to reverse the aforementioned speed ratio, i.e., to require a slower drawing speed for the rod than for the hollow cylinder, a phenomenon commonly referred to as "upset." During upset, the softened glass volume is permanently deposited in front of the "pseudo-opening" in an amount greater than the volume of glass being drawn, so the aforementioned mechanism for renewing the ridge volume no longer works. During the formation of the hollow cylinder, this "push" of the softened glass volume against the mold results in either the need to briefly open the mold, or the ridges being pressed inward at irregular intervals or remaining adhered to the mold. In the first case, fluctuations in the inner and outer diameters occur; in the second, fluctuations in the inner diameter occur; and in the third, interruptions to the forming process occur. In either case, the process becomes unstable.

[0014] Due to the short heating zone, in the flame heating zone, upsetting of the starting hollow cylinder is possible due to its small circumference, but as mentioned above, this is only achieved if the starting hollow cylinder to be formed has a thin wall thickness of 10 mm or less.

[0015] Therefore, a silica glass tube that is large, thick-walled, and has good surface quality is required.

[0016] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a tube made of silica glass that corresponds to the above.

[0017] A further object of the present invention is to provide a method by which a starting hollow cylinder having a wall thickness of 20 mm or more, made of silica glass and having a thick wall, can be formed into a tube having a larger diameter but having approximately the same or a larger glass cross-sectional area. [Means for solving the problem]

[0018] This problem is solved according to the invention by starting from the method as described in the introduction, in which the gas pressure is used as a regulating variable for the diameter closed-loop control and / or the diameter open-loop control of the outer diameter of the tube or of a geometric parameter that is correlated with the outer diameter of the tube, and in which the gas pressure is gradually increased from a lower initial value to a higher final value during the pressure increase phase, and V C and V T This is solved by holding that for the ratio of:V T =V C ±0.2V C .

[0019] The starting hollow cylinder, or hollow cylinder for short, is made of silica glass. The silica glass may be melted from a naturally occurring raw material or formed from synthetic SiO2. The silica glass may be undoped or may contain one or more dopants. The hollow cylinder has a thick wall, e.g., greater than 20 mm, greater than 40 mm, greater than 60 mm, or greater than 80 mm.

[0020] In the forming process, the starting hollow cylinder is substantially oriented so that its longitudinal axis extends, inter alia, horizontally. The result of the forming method is a rod-shaped tube from which one or more silica glass tubes of desired length are cut. To this extent, the terms "tube" and "rod-shaped tube" are used interchangeably herein. The silica glass tube has larger outer and inner diameters than the starting hollow cylinder. Unless an explicit distinction between inner and outer diameters is required, the collective terms "diameter" or "radial shape" are used below for both dimensions. The "radial shape" of the starting hollow cylinder is also referred to as the "starting shape," and the "radial shape" of the rod-shaped tube is also referred to as the "target shape."

[0021] The goal of the forming process is a silica glass tube with a glass cross-sectional area approximately equal to or greater than that of the starting hollow cylinder, where glass cross-sectional area refers to the area occupied by the glass in a cross section perpendicular to the longitudinal axis of the tube.

[0022] The target shape, in particular the rod outer diameter, is a target quantity for closed-loop or open-loop control. Closed-loop control is performed by computer control to target the target quantity based on measured values, while open-loop control can also be performed, at least in part, by manual adjustment based on read-out parameter values. Combinations of closed-loop and open-loop control also exist, for example, combinations in which pilot open-loop control is used in closed-loop control. Unless explicitly excluded, the terms "closed-loop control" or "closed-loop controlling" hereinafter also refer to the generic terms "open-loop control" or "open-loop controlling" and combinations of these measures.

[0023] The molding process according to the present invention differs from conventional methods in at least three aspects:

[0024] (i) For the reasons mentioned above, no mold is used, because "pushing" the softened glass volume through the "pseudo opening" of the mold can lead to an unstable process in which the softened glass volume continues to accumulate in front of the mold, causing undulations in the rod. Here, variations in cross-sectional area are observed that cannot be determined by calculations based on the upset rate (ratio of feed rate to withdrawal rate) and the cross-section of the starting hollow cylinder.

[0025] The absence of a mold offers advantages: the surface of the tube is smooth and fire polished; no chemical contamination of the glass by mold materials occurs.

[0026] However, to ensure that the desired target shape can be reliably and accurately produced by free forming, a separate closed-loop control of the rod-shaped tube diameter is required. Therefore, in the method according to the present invention, the gas pressure is used directly or indirectly as an adjustment variable for the diameter closed-loop control and / or diameter open-loop control of the rod-shaped tube outer diameter or a shape parameter correlated with the tube outer diameter. In this sense, for example, the adjustment variable is used indirectly when it is not the gas pressure but a parameter correlated with the gas pressure, such as the flow rate of the compressed gas. In this case, the inner and / or outer diameter of the drawn rod-shaped tube is measured or read.

[0027] Geometric parameters correlated with the tube outer diameter include the tube inner diameter and wall thickness. The tube outer diameter can be detected optically. The inner diameter is obtained from the cross-sectional area and the upset rate (the ratio of the feed rate to the withdrawal rate). The assumption is a stable process where the cross-sectional area does not change.

[0028] The forming of a hollow cylinder is based on the action of gas pressure, centrifugal force during rotation (spinning), or both. The ratio of gas pressure to the tangential stress σT in the tube wall is expressed by the so-called Barlow equation: σT=pressure*radius / wall thickness (1), However: Pressure = Gas pressure Radius = tube radius in the deformation region Wall thickness = Wall thickness in the deformation region

[0029] In the "deformation zone," the wall thickness, diameter, and temperature (viscosity) change continuously. For example, the diameter of the softened glass strand increases continuously from the hollow cylinder to the tube, depending on the glass's resistance to deformation, which is determined, inter alia, by the wall thickness and viscosity. In longitudinal section, the deformation zone as a whole exhibits a funnel shape with a pronounced or inconspicuous S-shaped transition between the hollow cylinder and the tube, which will hereinafter also be referred to as the "shoulder." In practice, the deformation zone typically has a length of several hundred millimeters, for example, 200 to 800 mm, often about 500 mm.

[0030] Again, the tangential stress causes an adjustment of the tube radius (r) in the deformation zone according to the following differential equation: d radius / dt = stress * radius / viscosity (2) However, d radius / dt = strain rate Stress = Tangential stress Radius = tube radius in the deformation region Viscosity = average value across the wall thickness for local viscosity

[0031] (ii) For any given set of parameters (radial shape of the starting hollow cylinder and rod, inlet and outlet speeds, gas pressure, and process parameters such as temperature distribution in the deformation zone), there is only one stable, steady-state shoulder shape, which is obtained as a solution to differential equation (2). Ideally, starting from the starting shape and depending on the process parameters, the target shape is obtained, provided that the value for the gas pressure, which is the basis for this calculation, is known.

[0032] This value for the gas pressure can be used as a nominal initial value to base the closed-loop or open-loop control of the radial shape of the rod-shaped tube (as long as the gas pressure is used at least as an auxiliary adjustment variable for the closed-loop / open-loop control). However, it has been found that any pressure change does not act on only one point in the deformation zone, but on all points in the deformation zone, and with different strengths depending on the current local conditions (as is also evident from equation (2) above). Therefore, any pressure change leads to a deviation of the target shape, which becomes visible only after a long time, i.e., after a very long dead time, and which can no longer be corrected. In this case, the closed-loop control easily becomes unstable and fluctuates, which leads to a rod-shaped tube containing a wavy structure.

[0033] The long dead time cannot be significantly reduced in the manufacturing process. Therefore, it is proposed to avoid sudden pressure changes, such as those that may occur at the beginning of the forming process to adjust the gas pressure to a nominal initial value. Instead, it is proposed to gradually increase the gas pressure from a lower initial value to a higher final value during the pressure adjustment phase. Gradual means that the gas pressure always increases (in small steps, if possible, or ideally continuously). The initial value is zero or a pressure value much lower than the final gas pressure value, and the final value is a pressure value within the range of adjustment values ​​for closed-loop / open-loop control of the target shape. During the pressure increase phase, the forming process is already taking place. That is, the silica glass material of the starting hollow cylinder is continuously formed into the silica glass material of the rod-shaped tube, even though the rod-shaped tube has not yet reached the desired radial shape.

[0034] To avoid unstable and fluctuating conditions, there is a maximum possible slope for each pair of starting and target shapes. This slope can be calculated using differential equation (2). This maximum possible slope is an ideal value. A higher pressure rise slope may result in a deterioration in the consistency of the outer diameter after drawing and / or a greater loss of material. On the other hand, a pressure rise slope lower than the ideal value does not affect the consistency of the outer diameter. The opposite is true, and no lower limit could be established for this. However, a lower pressure rise slope results in a longer period until the desired final gas pressure value and thus the desired diameter value are achieved, with the attendant loss of time and material. This is because, by this time, the desired radial shape of the tube has not yet been achieved, and the tube material produced up to this point represents only material loss. Therefore, there is an economic incentive to adjust the pressure rise slope to the largest possible value. While time loss cannot be reduced, material loss can be reduced by using a starting hollow cylinder with a front end made of relatively low-quality dummy material.

[0035] (iii) One of the goals of the forming process is a tube with a larger diameter than the starting hollow cylinder, but with approximately the same or a larger glass cross-sectional area. To achieve this, the drawing speed V T and the feed rate V of the starting hollow cylinder C The ratio of and must be adjusted so that the following equation holds: V T =V C ±0.2V C

[0036] Therefore, in the forming process of the present invention, the withdrawal speed V T is the feed rate V C or the feed rate V C In contrast, when drawing the starting cylinder into a rod-shaped tube, the feed rate to the heating zone is much lower than the withdrawal rate of the rod-shaped tube from the heating zone. This provides a certain degree of inherent stability to the drawing process during drawing, which facilitates adjustment and closed-loop control of the radial shape of the rod-shaped tube. However, during drawing, the glass cross-sectional area of ​​the starting cylinder and the glass cross-sectional area of ​​the rod-shaped tube are usually significantly different. The inherent stability provided to the drawing process cannot be achieved in the forming process of the present invention. The method of the present invention can compensate for this drawback based on the above-mentioned measures (i) and (ii).

[0037] In this way, for the first time, it is possible to form from a thick-walled silica glass hollow cylinder by free forming a tube having a wall thickness of 20 mm or more and a glass cross-sectional area equal to or greater than the glass cross-sectional area of ​​the starting hollow cylinder.

[0038] Normally, when closed-loop control is performed to the target shape of the rod, the gas pressure is adjusted as quickly as possible starting from a low initial value. However, for the reasons mentioned above, this can lead to process instability and fluctuations. Therefore, the gradual increase in gas pressure during the pressure increase phase is preferably performed independently of the diameter closed-loop or diameter open-loop control.

[0039] In the simplest case, the gas pressure is manually increased in small steps until the target gas pressure is reached. Alternatively, the gas pressure is increased linearly or by open-loop control based on a non-linear function until the target gas pressure is reached. Advantageously, however, the gradual increase in gas pressure during the pressure increase phase is likewise performed by closed-loop control, independent of the usual diameter closed-loop control.

[0040] In a typical diameter closed-loop or diameter open-loop control, a diameter target value for the outer and / or inner pipe diameter is assigned, where the diameter target value is assigned to a target gas pressure. It is advantageous if the lower initial value is in the range of 0-50% of the target gas pressure, in particular in the range of 10-30% of the target gas pressure, and the higher final value is in the range of 70-110% of the target gas pressure, in particular in the range of 90-100% of the target gas pressure.

[0041] The pressure buildup phase precedes the actual forming process. At the start of the pressure buildup phase, the hollow cylinder has reached its softening temperature, and process parameters such as the temperature of the heating zone, the feed rate, and the withdrawal rate are adjusted to the extent expected in the actual forming process. During the pressure buildup phase, the gas pressure in the hollow cylinder's internal bore (and thus in the rod-shaped tube's internal bore) slowly increases from a lower initial pressure value to a final value. Only when the target gas pressure is reached is a rod-shaped tube formed with the desired shape. The rod-shaped tube material formed up to this point is lost. Material loss can be reduced if the initial portion of the starting hollow cylinder is made of low-quality silica glass. Regardless of this, the pressure buildup phase should be as short as possible and limited to the required length, on the one hand to keep material loss small and, on the other hand, to achieve a steady state in the forming process through a flat pressure buildup gradient. Even if the pressure rise slope is flat, it is advantageous to have some initial value of the gas pressure above 0 up to about 50% of the target gas pressure, in particular up to a maximum of 30% of the target gas pressure, in order to keep the pressure rise phase as short as possible.

[0042] The target gas pressure is, for example, in the range of 2 to 20 mbar, in particular in the range of 3 to 15 mbar, particularly preferably in the range of 4 to 10 mbar. Particularly high gas pressures above 20 mbar can impair the stability of the process.

[0043] On the other hand, in the pressure increase phase, it is not necessary to increase the pressure exactly to the target gas pressure, since if at least 70%, in particular at least 90%, of the target gas pressure has been reached or if the pressure is already slightly higher than the target gas pressure, the usual diameter closed-loop or diameter open-loop control can take over the pressure regulation.

[0044] The choice must be made between, on the one hand, making the pressure rise phase as short as possible and thus minimizing material losses, and, on the other hand, making the pressure rise slope as flat as possible and thus minimizing diameter fluctuations and accurately adjusting the target shape. A particularly good compromise has been found when the pressure rise phase has a duration of 1 to 120 minutes, in particular 5 to 100 minutes, particularly preferably 10 to 80 minutes, and in particular 15 to 60 minutes.

[0045] From these considerations, the time course of the pressure increase during the pressure increase phase can also be determined. The pressure increase can be effected, for example, with a constant slope (linear), or the pressure increase can have a non-linear change with a decreasing or increasing slope at the end. It has been found to be advantageous if the gas pressure during the pressure increase phase increases, at least temporarily, with a time gradient Δp, for which the following equation holds: 0.01mbar / min<Δp<0.8mbar / min, In particular: Δp<0.5 mbar / min, In particular, Δp<0.2 mbar / min.

[0046] Regardless of the time course of the pressure rise in the pressure rise phase, the pressure rise slope can also be assigned an average value over the entire pressure rise phase, whereby the gas pressure is preferably determined over the entire pressure rise phase by an average time gradient Δpm This average temporal gradient Δp m For , the following holds: 0.01mbar / min<Δp m <0.5mbar / min, In particular: Δp m <0.1mbar / min, In particular, Δp m <0.06mbar / min.

[0047] The forming method is particularly suitable for the industrial production of large-volume, thick-walled silica glass tubes. In this connection, hollow cylinders (cylinders; C) and rod-shaped tubes (tubes; T) have, inter alia, the following radial shapes: 150mm <C a < 300mm, 30mm <C i < 180mm, 40mm <(C a -C i ) / 2) < 100mm, 300mm <T a < 500mm, 250mm <T i < 450mm, 20mm <(T a -T i ) / 2) < 60mm.

[0048] The forming process results in a silica glass tube with a larger outer diameter and a larger inner diameter than the starting hollow cylinder. In this case, the following relationships arise, inter alia, for the hollow cylinder and the rod-shaped tube: 4·C i <T i <8 C i , C a +100mm <T a <C a +300mm.

[0049] Particularly preferably, the outer diameter of the tube T a is the outer diameter of the hollow cylinder C a The range is 1.5 to 2.2 times.

[0050] Furthermore, the forming process is particularly suitable for producing silica glass tubes with large diameters and glass cross-sectional areas that are approximately the same as or larger than the starting hollow cylinder, whereby it is advantageous to have a glass cross-sectional area A C and the glass cross-sectional area A of the rod-shaped tube T The following relationship exists between: A T =A C ±0.15 A C , Specifically: 0.90 A C ≦A T ≦1.15 A C , Especially: A C ≦A T ≦1.10 A C .

[0051] In this case, the glass cross-sectional area A C and A T Especially 250-1000cm 2 The range is.

[0052] When drawing the starting cylinder into a rod-shaped tube, the feed rate into the heating zone is much lower than the withdrawal rate of the rod-shaped tube from the heating zone. This provides a certain degree of inherent stability to the drawing process during drawing, which facilitates adjustment and closed-loop control of the radial shape of the rod-shaped tube. However, the glass cross-sectional area of ​​the starting cylinder and the glass cross-sectional area of ​​the rod-shaped tube are significantly different. In contrast, in the forming process of the present invention, the withdrawal rate of the rod-shaped tube may be in the range of the feed rate or even lower than the feed rate. This does not provide the inherent stability provided in the drawing process. The method of the present invention avoids this drawback and provides a V C and V T For the ratio to , in particular, the following holds: 0.8 V C ≦V T ≦1.05 V C , In particular: 0.9 V C ≦V T ≦0.99 VC

[0053] The wall thickness of the rod-shaped tube is C and V T If this speed ratio is less than 1, the rod-shaped tube will not be stretched and deposition will occur.

[0054] It has been found that process instabilities can be more easily avoided if the throughput of molten glass is relatively low. In this regard, V is set so that the throughput of fused silica glass is at least 30 kg / h and less than 60 kg / h, in particular less than 45 kg / h. C This was demonstrated when the

[0055] In the context of a tube made of silica glass, the above object is achieved in accordance with the invention by providing a tube having a flame-polished outer coating surface and an outer diameter T a , tube inner diameter T i and the pipe wall thickness by: 300mm <T a < 550mm, 250mm <T i < 450mm, 20mm <(T a -T i ) / 2) < 60mm.

[0056] The silica glass tube has a large outer diameter, but is still thick-walled, meaning that its wall thickness is greater than 20 mm. The silica glass tube has an outer coating surface that is produced without the action of a forming die, i.e., is flame-polished, as a result of the melt flow forming process, and this outer coating surface is smooth and streak-free. The silica glass tube can be produced according to, and in particular is produced based on, the method of the present invention. In particular, the tube consists of synthetic silica glass.

[0057] Advantageously, the outer diameter of the tube T a , tube inner diameter T i and pipe wall thickness (T a -T i ) / 2), the following holds: 330mm <Ta < 500mm, 275mm <T i < 400mm, 25mm <(T a -T i ) / 2) < 50mm.

[0058] Example The present invention will be described in detail below based on examples and with reference to the accompanying drawings. [Brief explanation of the drawings]

[0059] [Figure 1] 1 shows a schematic side view of an apparatus for forming a hollow cylinder of silica glass into a silica glass tube. [Figure 2] 1 shows a photograph of a portion of a silica glass tube as it exits a furnace during the pressure adjustment stage of the forming process according to a first comparative example. [Figure 3] 3 shows a graph depicting the outer diameter and wall thickness profile of a portion of the formed silica glass tube of FIG. 2. [Figure 4] 1 shows a graph illustrating the gas pressure over time during and after the initial stage of the molding process in a first comparative example. [Figure 5] 10 shows a photograph of a portion of a silica glass tube as it exits a furnace during a forming process according to a second comparative example. [Figure 6] 1 shows a photograph of a portion of a silica glass tube as it exits a furnace during a forming process according to a first embodiment of the present invention. [Figure 7] 7 shows a graph depicting the outer diameter and wall thickness profile of a portion of the molded silica glass tube of FIG. 6. [Figure 8] 1 shows a graph illustrating the gas pressure over time during and after the initial stage of the molding process in a first comparative example. [Figure 9] 1 shows a graph depicting the outer diameter and wall thickness profile for a reference example. [Figure 10] In the reference example, a graph showing the gas pressure over time during the molding process is shown.

[0060] FIG. 1 shows a schematic diagram of an apparatus for forming a silica glass hollow cylinder 2 into a tube 22. The hollow cylinder 2 is made of undoped synthetic silica glass. The hollow cylinder 2 is produced in a soot deposition process according to the so-called OVD (Outside Vapor Deposition) method. In this process, a liquid or gaseous starting material undergoes a chemical reaction (hydrolysis or pyrolysis) from the gas phase to solid SiO2, which is deposited on a rotating mandrel. The starting material is, for example, silicon tetrachloride (SiCl4) or a chlorine-free silicon compound. As an intermediate product, a porous SiO2 soot body in the shape of a hollow cylinder is obtained, the inner bore of which is formed by the mandrel, which is subsequently removed. By vitrifying the soot body, the hollow cylinder 2 made of synthetic silica glass is obtained.

[0061] The hollow silica glass cylinder 2 to be formed has holding tubes 4, 5 welded to its end faces, which are clamped to a chuck 6 of a horizontal glass lathe 8 and rotate synchronously about an axis of rotation 10. The holding tube 4 located in front in the feed direction 32 is closed by a stopcock 34. The other holding tube 5 can be filled with compressed gas.

[0062] The hollow cylinder 2 is continuously rotated by the glass lathe 8 about its axis of rotation and longitudinal axis 10 and is pushed at a predetermined feed rate into a resistance furnace 12 which annularly surrounds the hollow cylinder 2 and is heated in its interior to a temperature of around 2100° C. in each zone. The resistance furnace is connected to a computer 24 via a closed-loop controllable current source 26.

[0063] Compressed gas can be introduced into the internal bore 16 of the hollow cylinder 2 and tube 22 via a gas inlet 14, and a predetermined gas pressure can be adjusted. The gas inlet 14 is connected to a closed-loop controllable valve 18, which is connected to a compressed gas container 20 and a computer 24.

[0064] Driven by centrifugal force and gas pressure, the low-viscosity mass of the hollow silica glass cylinder 2 expands to the target diameter of the silica glass tube 22. A measurement and closed-loop control system is provided, including a high-resolution CCD camera 30 aimed at the rod-shaped tube 22, for measuring the resulting outer diameter and for closed-loop control of the outer diameter. The measurement data detected by the camera 30 is continuously transmitted to a computer 24, which has an image data processing program installed. This image data processing program determines the current outer diameter of the rod-shaped tube 22. This is supplied as the actual value (the rod-shaped tube outer diameter) of the closed-loop control unit, which adjusts the gas pressure. Gas lines are shown schematically by double lines, current lines by solid lines, and data lines by dashed lines.

[0065] An example of the molding process will be described below based on FIG.

[0066] Comparative Example 1 [Table 1]

[0067] In this comparative example, the starting hollow cylinder 2 was placed in a heating furnace 12 while rotating about its longitudinal axis 10, and the longitudinal portion of the starting hollow cylinder 2 was uniformly heated in the heating furnace 12. The gas pressure was then adjusted to a value (target gas pressure) calculated based on equation (2) to achieve a predetermined rod-shaped tube outer diameter. In this case, the target gas pressure was 5.3 mbar. At the same time, the feed rate was set to 7.5 mm / min, and the free end of the rod-shaped tube was withdrawn at a predetermined withdrawal rate of 7.4 mm / min.

[0068] This caused the outer diameter of the rod to increase rapidly after a few hours, so the gas pressure had to be reduced to 1 mbar to prevent the glass from coming into contact with the heating tube of the resistance furnace 12. This also caused the outer diameter of the rod to decrease rapidly after a few hours, so the pressure had to be increased again to 4 mbar. However, a stable state did not occur, and the target shape could not be reached by the end of the forming process.

[0069] 2 shows a photograph of the rod 22 emerging from the furnace 12. The non-uniformity of the outer diameter is readily apparent.

[0070] The graph in Figure 3 shows the corresponding changes in outer diameter and wall thickness. a and wall thickness d in millimeters, and on the x-axis the drawn rod length L in meters. Curve A represents the change in outer diameter, and curve B represents the change in wall thickness (which could not be measured continuously due to the large diameter variations).

[0071] The graph in Figure 4 shows the time course of the gas pressure mentioned above: on the y-axis, the pressure p is plotted in mbar, and on the x-axis, the time t is plotted in minutes.

[0072] Comparative Example 2 [Table 2]

[0073] In this comparative example, a mold defining the outer diameter of the drawn rod-shaped tube 22 was installed in the heating furnace 12. The starting hollow cylinder 2 was inserted into the heating furnace 12 while rotating about its longitudinal axis 10, and the longitudinal portion of the starting hollow cylinder 2 was uniformly heated in the heating furnace 12. The feed rate was set to 7.5 mm / min, and the free end of the rod-shaped tube was drawn at a predetermined drawing speed of 7.4 mm / min. The gas pressure was initially fixed at 4 mbar. An attempt was made to press the rod-shaped tube against a mold adjusted to an outer diameter of 363 mm. However, upon initial contact with the mold, the silica glass began to twist. The pressure was then reduced to 2 mbar and then increased again at a flat rate of 1 mbar / h (approximately 0.017 mbar / min). Despite multiple attempts to stabilize the forming process, the predetermined target dimensions were not reached, and the mold had to be finally opened to remove the fused silica glass.

[0074] The photograph in Figure 5 shows the twisting and uneven outer diameter of the drawn rod tube up to the point of trial.

[0075] Example 1 [Table 3]

[0076] In Example 1, the starting hollow cylinder 2 was inserted into the heating furnace 12 while rotating around its longitudinal axis 10. The initial gas pressure was adjusted to 1 mbar and maintained constant during heating. After the longitudinal portion of the starting hollow cylinder 2 was uniformly heated in the heating furnace 12, the feed rate was set to 7.5 mm / min, and the free end of the rod-shaped tube was withdrawn at a predetermined withdrawal rate of 7.4 mm / min. Simultaneously, the initial value of the gas pressure was increased with a closed-loop controlled ramp of 0.06 mbar / min until the gas pressure reached a final value of 5.3 mbar. This final value corresponded exactly to the target gas pressure calculated based on Equation (2). Thus, the pressure increase phase ended, and the rod-shaped tube 22 had the desired outer diameter of 363 mm. Until this point (during the pressure increase phase), the actual closed-loop diameter control by the computer 24 was not performed. Subsequently, closed-loop control of the outer diameter was initiated to maintain the shape within the desired range. In this case, the gas pressure was used as a regulating variable in the closed loop control. The target outer diameter of the tube, T, was achieved by blowing air into the initially relatively small volume of the hollow cylinder inner bore to create a larger volume of the tube inner bore. a A continuous gas supply is required to maintain the gas pressure that results.

[0077] 6 shows a photograph of the tubular rod 22 emerging from the furnace, which has a substantially constant outer diameter and a smooth wall with a fire-polished, therefore unbroken, surface as a result of the forming process described above.

[0078] The graph in Figure 7 shows the corresponding change in outer diameter and wall thickness. On the y-axis, the diameter T a and wall thickness d in millimeters, and on the x-axis the drawn rod length L in meters. Curve A represents the change in outer diameter, and curve B represents the change in wall thickness.

[0079] The graph in Figure 8 shows the time course of the above-mentioned gas pressure. On the y-axis, the pressure p in mbar is plotted against the time t in minutes. The pressure rise phase is defined by a period a with an initial pressure value of 1 mbar, which is initially constant, followed by a period b, during which the gas pressure slowly rises to 5.3 mbar. During the pressure rise phase, there is no diameter closed-loop control.

[0080] From the drawn rod-shaped tube 22, thick-walled tubes made of synthetic silica glass are cut out, each having a large average outer diameter of about 363 mm, a large average inner diameter of about 290 mm, and at the same time a large average wall thickness of about 36.5 mm. These tubes are characterized by smooth, nearly defect-free outer and inner coating surfaces resulting from the melt flow forming process.

[0081] Reference Example [Table 4]

[0082] The reference example corresponds to the prior art. The hollow cylinder used as the starting cylinder here is made of undoped silica glass, which is prepared by fusing naturally occurring silica raw materials using the Verneuil process. Here, a large, elongated block is formed by fusing crystalline silica powder using an oxyhydrogen flame. A central bore is formed using a core drill machine equipped with diamond abrasive particles, and finally, the inner and outer walls are mechanically smoothed.

[0083] In this molding process, a mold is used, and a rod-shaped tube is pulled (not upset) from the heated zone and softened fused silica glass while rotating around its longitudinal axis. After the longitudinal portion of the starting hollow cylinder 2 is uniformly heated in the heating furnace 12, the feed rate is set to 12 mm / min, and the free end of the rod-shaped tube is pulled at a predetermined pulling rate of 66 mm / min. At the same time, the gas pressure is increased very rapidly with a ramp of 1 mbar / min, here manually controlled in a closed loop. The glass is blown with the gas pressure into a mold block adjusted to an outer diameter of 343 mm.

[0084] The graph in Figure 9 (similar to Figure 7) shows the outer diameter and wall thickness over time. As can be seen from Figure 10, the outer diameter remains fairly constant as a result of using the mold, and remains very constant despite fluctuations in gas pressure. Fluctuations in gas pressure do not have a clear effect on the outer diameter and wall thickness in this molding process. This fluctuation is likely due to the fact that the gas pressure is manually and only very loosely controlled in a closed loop to maintain the outer diameter within the desired range after the target outer diameter is reached.

Claims

1. Hollow cylinder bore (16), outer diameter C ａ , inner diameter C ｉ 1. A method for producing a tube (22) made of silica glass by forming a hollow cylinder (2) having a wall thickness of at least 20 mm, the hollow cylinder being formed by rotating the hollow cylinder about its axis of rotation (10) at a relative feed rate V Ｃ The heated material is continuously supplied to a heating region (12) at a temperature of 100° C., and softened in the heating region (12) in a region-by-region manner. The softened region is stretched radially under the action of gas pressure applied to the hollow cylinder bore (16). The softened region is continuously stretched from the softened region to the tube outer diameter T ａ , tube inner diameter T ｉ , and a rod-shaped tube (22) having a tube wall thickness is formed, and the drawing speed V Ｔ the gas pressure is used as an adjustment variable for at least one diameter closed-loop control (24) or diameter open-loop control of the outer diameter of the tube or a geometric parameter correlated with the outer diameter of the tube, and the gas pressure is gradually increased in a pressure increase phase from a lower initial value to a higher final value, and the gas pressure in the pressure increase phase is increased at least temporarily with a time gradient Δp, where the following equation holds for the time gradient: 0.01 mbar / min<Δp<0.8 mbar / min, And V Ｃ and V Ｔ For the ratio of V Ｔ =V Ｃ ±0.2V Ｃ A method characterized by:

2. 2. The method of claim 1, wherein the gradual increase in gas pressure during the pressure increase stage is performed independently of the diameter closed-loop control (24) or diameter open-loop control.

3. 3. The method according to claim 1, wherein the diameter closed-loop control or the diameter open-loop control is assigned a diameter target value for at least one of the outer pipe diameter or the inner pipe diameter, the diameter target value is associated with a target gas pressure, the lower initial value is in the range of 0-50% of the target gas pressure, and the higher final value is in the range of 70-110% of the target gas pressure.

4. 4. The method according to claim 3, wherein the target gas pressure is adjusted in the range of 2 to 20 mbar.

5. 5. The method according to claim 4, wherein the target gas pressure is adjusted to a range of 3 to 15 mbar.

6. 5. The method according to claim 4, wherein the target gas pressure is adjusted to a range of 4 to 10 mbar.

7. 2. The method of claim 1, wherein the pressure increase stage has a duration of 1 to 120 minutes.

8. 8. The method of claim 7, wherein the pressure increase stage has a duration of 5 to 100 minutes.

9. 8. The method of claim 7, wherein the pressure increase stage has a duration of 10 to 80 minutes.

10. 8. The method of claim 7, wherein the pressure increase stage has a duration of 15 to 60 minutes.

11. The gas pressure during the pressure increase phase increases, at least temporarily, with a time gradient Δp such that: Δp<0.5 mbar / min, 11. The method according to any one of claims 1 to 10, characterized in that

12. The gas pressure during the pressure increase phase increases, at least temporarily, with a time gradient Δp such that: Δp<0.2 mbar / min, 11. The method according to any one of claims 1 to 10, characterized in that

13. The gas pressure increases with an average temporal gradient Δp ｍ and the average temporal gradient Δp ｍ For , the following holds: 0.01mbar / mm<<Δp ｍ <0.5mbar / men, 11. The method according to claim 1, wherein

14. The gas pressure increases with an average temporal gradient Δp ｍ and the average temporal gradient Δp ｍ For , the following holds: Δp ｍ <0.1mbar / min、 The method according to claim 13, characterized in that:

15. The gas pressure increases with an average temporal gradient Δp ｍ and the average temporal gradient Δp ｍ For , the following holds: Δp ｍ 14. The method according to claim 13, characterized in that the pressure is <0.06 mbar / min.

16. The following equations hold for the hollow cylinder (2) and the rod-shaped tube (22): 150mm <C ａ < 300mm、 30mm <C ｉ < 180mm、 40mm <(C ａ -C ｉ ) / 2) < 100mm、 300mm <T ａ < 550mm、 250mm <T ｉ < 450mm、 20mm <(T ａ -T ｉ ) / 2) < 60mm、 11. The method according to any one of claims 1 to 10, characterized in that

17. The following equations hold for the hollow cylinder (2) and the rod-shaped tube (22): 4・C ｉ <T ｉ < 8・C ｉ 、 C ａ +100mm <T ａ <C ａ +300mm、 17. The method according to any one of claims 1 to 16, characterized in that

18. Said tube outer diameter T ａ is the outer diameter C of the hollow cylinder ａ 18. The method according to claim 1, wherein the concentration is in the range of 1.5 to 2.2 times.

19. The starting hollow cylinder (2) has a glass cross-sectional area A Ｃ The rod-shaped tube (22) has a glass cross-sectional area A Ｔ and the following holds: A Ｔ =A Ｃ ±0.15・A Ｃ 、 19. The method according to any one of claims 1 to 18, characterized in that:

20. The starting hollow cylinder (2) has a glass cross-sectional area A Ｃ The rod-shaped tube (22) has a glass cross-sectional area A Ｔ and the following holds: 0.90・A Ｃ ≦A Ｔ ≦1.15・A Ｃ 、 20. The method of claim 19, wherein:

21. The starting hollow cylinder (2) has a glass cross-sectional area A Ｃ The rod-shaped tube (22) has a glass cross-sectional area A Ｔ and the following holds: A Ｃ ≦A Ｔ ≦1.10・A Ｃ 20. The method of claim 19, wherein:

22. The glass cross-sectional area A Ｃ and A Ｔ is 250 to 1000 cm ２ 20. The method of claim 19, wherein the temperature is in the range of

23. V Ｃ and V Ｔ The following equation holds for the ratio: 0.8 V Ｃ ≦V Ｔ ≦1.05 V Ｃ 11. The method according to claim 1, wherein

24. V Ｃ and V Ｔ The following equation holds for the ratio: 0.9 V Ｃ ≦V Ｔ ≦0.99 V Ｃ 24. The method of claim 23, wherein:

25. V so as to produce a throughput of fused silica glass of at least 30 kg / h, but less than 60 kg / h. Ｃ 11. The method according to claim 1, wherein:

26. V so as to produce a throughput of fused silica glass of at least 30 kg / h, but less than 45 kg / h. Ｃ 26. The method of claim 25, wherein:

Images