Method for producing a glass tube with a cross section of a noncircular form by reshaping

Abstract

The invention relates to a method for producing glass tubes with a cross section of a noncircular form by reshaping. This method includes at least providing a glass tube, heating the glass tube, providing at least one reshaping tool, which has a forming body with a forming area for reshaping the heated glass tube, the forming body including at least one open-porous material, setting a gas pressure in the interior of the reshaping tool lower than 90 kPa, so that a negative pressure is produced on the forming area, and reshaping the heated glass tube by applying a compressive force perpendicularly to the longitudinal axis of the glass tube, the compressive force being generated by the reshaping tool and being applied to the outer surface of the glass tube, and the outer surface of the glass tube being fixed by the negative pressure.

Claims

1. Method for producing a glass tube with a cross section of a noncircular form by reshaping, comprising at least the steps of: providing a glass tube having a longitudinal axis and an outer surface, heating the glass tube, providing at least one reshaping tool having an interior, which has a forming body with a forming area for reshaping the heated glass tube, the forming body comprising at least one open-porous material, setting a gas pressure in the interior of the at least one reshaping tool that is lower than 90 kPa, so that a negative pressure is produced on the forming area of the forming body, and reshaping the heated glass tube by applying a compressive force perpendicularly to the longitudinal axis of the glass tube, the compressive force being generated by the at least one reshaping tool and being applied to the outer surface of the glass tube, characterized in that the glass tube has an aspect ratio of a cross section, and the aspect ratio after the reshaping is greater than the aspect ratio before the reshaping.

2. Method according to claim 1, characterized in that the forming area is mirror-polished.

3. Method according to claim 1, characterized in that the forming body has a curved surface.

4. Method according to claim 1, characterized in that the outer surface of the glass tube being made to lie against at least part of the forming area by the negative pressure on the forming area of the forming body.

5. Method according to claim 1, characterized in that the at least one reshaping tool has in its interior at least one cavity.

6. Method according to claim 1, characterized in that the at least one open-porous material comprises graphite, ceramic and/or metal.

7. Method according to claim 1, characterized in that the at least one open-porous material has an open porosity of between 1% and 50%.

8. Method according to claim 1, characterized in that, before the heating, the glass tube provided has either a circular cross section or an elliptical cross section.

9. Method according to claim 1, characterized in that the glass tube is heated to a temperature between the annealing point and the softening point of the glass tube.

10. Method according to claim 1, characterized in that, before the reshaping, the pressure at least in a partial volume in the interior of the at least one reshaping tool is set to a value between 0.1 kPa and 30 kPa.

11. Method according to claim 1, characterized in that the outer surface of the glass tube is fixed by the negative pressure on the forming area of the forming body.

12. Method for producing a glass tube with a cross section of a noncircular form by reshaping, comprising at least the steps of: providing a glass tube having a longitudinal axis and an outer surface, heating the glass tube, providing at least one reshaping tool having an interior, which has a forming body with a forming area for reshaping the heated glass tube, the forming body comprising at least one open-porous material, setting a gas pressure in the interior of the at least one reshaping tool that is lower than 90 kPa, so that a negative pressure is produced on the forming area of the forming body, and reshaping the heated glass tube by applying a compressive force perpendicularly to the longitudinal axis of the glass tube, the compressive force being generated by the at least one reshaping tool and being applied to the outer surface of the glass tube, characterized in that the outer surface of the glass tube is fixed by the negative pressure on the forming area of the forming body.

13. Method according to claim 12, characterized in that, before the reshaping, the pressure at least in a partial volume in the interior of the at least one reshaping tool is set to a value between 0.1 kPa and 30 kPa.

14. Method for producing a glass tube with a cross section of a noncircular form by reshaping, comprising at least the steps of: providing a glass tube having a longitudinal axis and an outer surface, heating the glass tube, providing at least one reshaping tool having an interior, which has a forming body with a forming area for reshaping the heated glass tube, the forming body comprising at least one open-porous material, setting a gas pressure in the interior of the at least one reshaping tool that is lower than 90 kPa, so that a negative pressure is produced on the forming area of the forming body, and reshaping the heated glass tube by applying a compressive force perpendicularly to the longitudinal axis of the glass tube, the compressive force being generated by the at least one reshaping tool and being applied to the outer surface of the glass tube, characterized in that the outer surface of the glass tube being made to lie against at least part of the forming area by the negative pressure on the forming area of the forming body.

15. Method according to claim 13, characterized in that, before the reshaping, the pressure at least in a partial volume in the interior of the at least one reshaping tool is set to a value between 0.1 kPa and 30 kPa.

Description

DESCRIPTION OF THE DRAWINGS

(1) The invention is described below by way of example and with reference to the accompanying drawings, from which further features and advantages emerge. In the schematic drawings:

(2) FIG. 1a shows a reshaping tool, a glass tube and a base before the beginning of the reshaping of the glass tube by the reshaping tool in a section along the longitudinal axis of the glass tube.

(3) FIG. 1b shows the reshaping tool, the glass tube and the base as shown in FIG. 1a at the end of the reshaping of the glass tube by the reshaping tool in a section along the longitudinal axis of the glass tube.

(4) FIG. 2a shows two reshaping tools with planar forming bodies and a glass tube before the beginning of the reshaping of the glass tube by the reshaping tools in cross section perpendicularly to the longitudinal axis of the glass tube.

(5) FIG. 2b shows the reshaping tools and the glass tube as shown in FIG. 2a at the end of the reshaping of the glass tube by the reshaping tools in cross section perpendicularly to the longitudinal axis of the glass tube.

(6) FIG. 3a shows a reshaping tool, a glass tube and a base before the beginning of the reshaping of the glass tube by the reshaping tool in cross section perpendicularly to the longitudinal axis of the glass tube.

(7) FIG. 3b shows the reshaping tool, the glass tube and the base as shown in FIG. 3a at the end of the reshaping of the glass tube by the reshaping tool in cross section perpendicularly to the longitudinal axis of the glass tube.

(8) FIGS. 4a to d show four examples of glass tubes in plan view of the cross section that can be formed by reshaping by means of the method according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(9) FIG. 1a shows in a schematic representation, in a section along the longitudinal axis of the heated glass tube 50, a reshaping tool 10, which has a cavity 20, a forming body 30 with a forming area 31 and a wall 40 and also a base 60, on which the glass tube 50 rests. The cavity 20 is in this case enclosed by the forming body 30 and the wall 40. The glass tube 50, the reshaping tool 10 and the base 60 are shown after the heating of the glass tube and before the reshaping by the reshaping tool.

(10) The forming body 30 is made of an open-porous material, for example of isostatically pressed graphite, and is configured with a planar forming area 31. Furthermore, the forming area 31 is mirror-polished.

(11) The glass tube 50 is shorter in the direction of its longitudinal axis than the reshaping tool 10 and the forming body 30, so that the tube 50 can be reshaped in its entire length without leaving unreshaped peripheral regions of the tube 50.

(12) The glass tube 50 rests on the base 60. The base 60 is for example likewise produced from isostatically pressed graphite, but may also be produced from other sufficiently temperature-resistant materials, such as ceramics or metals. The surface of the base 60 that is in contact with the glass tube 50 is mirror-polished. The base 60 may also be configured in such a way that it can be used for transporting the glass tube, for example from a device for heating the glass tube to the reshaping tool 10, and consequently serves as a transporting support. The use of the base 60 as a transporting support inside an oven is also possible, in particular if the space inside the oven has an inert or reducing atmosphere.

(13) In the region of the cavity 20, a gas pressure of less than 90 kPa prevails. This negative pressure may be generated for example by a means for generating a negative pressure, for example a diaphragm pump or rotary vane pump, being connected to an opening provided for it of the reshaping body 10 and operated. Since the forming body 30 is formed with an open porosity, and consequently so as to be permeable to gases, a gas flow through the forming body and the reshaping tool is produced by the negative pressure, which is schematically represented in FIG. 1a by arrows. Since the material thickness of the forming body 30 represented in FIG. 1a is constant along its entire extent and the cavity 20 likewise extends over the entire length of the forming body 30, the gas flow through the forming body 30 is substantially constant along its length. This results in a substantially homogeneous negative pressure on the forming area 31 when there is contact with the glass tube 50.

(14) FIG. 1b shows the reshaping tool 10, the base 60 and the glass tube 50 as shown in FIG. 1a at the end of the reshaping by the reshaping tool. The reshaping tool 10 was thereby moved perpendicularly to the longitudinal axis of the glass tube 50, so that it comes into contact with the glass tube and presses it against the base 60 in such a way that it exerts a compressive force on the glass tube 50 perpendicularly to the longitudinal axis of the glass tube 50. The glass tube 50 is in this way fixed on the forming area 31 by the negative pressure on the forming area 31. It is thereby prevented that undesired deformations, such as for example constrictions or depressions, form in the middle region of the glass tube 50.

(15) FIG. 2a shows in a cross section perpendicularly to the longitudinal axis of the glass tube 50 two reshaping tools 10 with in each case a planar forming body 30 and a wall 40, which together enclose a cavity 20. The reshaping tools 10 and the glass tube 50, which has a circular cross section, are shown before the beginning of the reshaping of the glass tube by the reshaping tools.

(16) The reshaping tools 10 represented in FIG. 2a are configured in each case in the same way as the reshaping tool 10 as shown in FIG. 1a and are arranged in such a way that they can be moved perpendicularly to the longitudinal axis of the glass tube 50, and consequently, by moving in opposite directions, exert a compressive force on the glass tube 50 perpendicularly to the longitudinal axis of the glass tube 50.

(17) FIG. 2b shows the reshaping tools 10 and the glass tube 50 as shown in FIG. 2a at the end of the reshaping of the glass tube 50 by the reshaping tools 10. The outer wall of the glass tube 50 is fixed in the contact region to the forming areas 31 by the negative pressure and lies against the forming areas 31 on the respective sides. The fact that the two forming areas 31 are configured as planar and are arranged plane-parallel has the result that the reshaped glass tube 50 has an oval cross section with two planar side parts parallel to one another and also two rounded peripheral regions. Since the negative pressure on the forming area 31 is constant over the entire width of the forming area 31, the surface of the glass tube 50 is fixed on the forming area 31 with a constant force.

(18) The embodiment of a reshaping tool 10 that is represented in FIG. 3a differs from the embodiment of the reshaping tools 10 that is represented in FIG. 2a to the extent that the forming body 30 is not formed as planar. On the one hand, it has a curved forming area 31, on the other hand the material thickness of the forming body 30 varies over its width.

(19) In the cavity 20, a gas pressure of less than 90 kPa prevails, whereby a gas flow through the forming body 30 and the reshaping tool 10 is produced, which is schematically represented in FIG. 3a by arrows. This results in a greater gas flow in the regions of smaller material thickness, since the forming body offers a smaller resistance to the gas flow there. When the forming area 30 is in contact with the glass tube 50, this results in a stronger negative pressure on the forming area 31 in the regions of the forming body 30 of smaller material thickness, and consequently stronger fixing of the surface of the glass tube 50 on the forming area 31.

(20) The glass tube 50 represented in FIG. 3a has in its initial form an elliptical cross section. This is particularly advantageous in the embodiment represented there, since the elliptical form of the glass tube 50 is more similar to the curvature of the forming area 31 than a circular form would be, one effect of which is to allow quicker reshaping.

(21) FIG. 3b shows the reshaping tool 10, the glass tube 50 and the base 60 as shown in FIG. 3a at the end of the reshaping of the glass tube by the reshaping tool 10. The glass tube 50 was pressed against the base 60 by means of the forming area 31 by moving the reshaping tool 10, resulting in the cross section shown of the glass tube after the reshaping. In the contact region with the forming area 30, the outer wall of the glass tube 50 is fixed by the negative pressure and lies against the forming areas 31 over the full area thereof. In this case, it is more strongly fixed in the regions of smaller material thickness of the forming body 30, that is to say in the middle region of the reshaped glass tube that is particularly susceptible to undesired deformation, than in the regions of greater material thickness.

(22) FIGS. 4a to 4d show various examples of glass tubes with a cross section of a noncircular form that can be produced by means of the method according to the invention with high precision and high throughput. The respective aspect ratio of the glass tubes shown is obtained from the ratio of the width of the cross section B to the height H thereof.

(23) FIG. 4a shows the cross section of a glass tube 50 as in FIG. 2b. The oval glass tube 50 has two planar longitudinal sides running parallel to one another and also two semicircularly rounded peripheral regions and resembles in its form the running track of a sports field. The wall thickness is constant over the entire circumference.

(24) FIG. 4b shows a glass tube with an elliptical cross section. The wall thickness of such an elliptical tube may be constant over the entire circumference or, as represented in FIG. 4b, vary steadily and mirror-symmetrically.

(25) FIG. 4c shows a glass tube which has a planar longitudinal side and also a convexly curved side, the bend radii in the two corner regions being very small. The wall thickness in the region of the planar longitudinal side is constant. FIG. 4d shows a glass tube with a substantially rectangular cross section and rounded corners. The wall thickness is constant over the entire circumference.

(26) In an exemplary embodiment, a glass tube 50 of DURAN with a round cross section, an outside diameter of 54 mm, a wall thickness of 1.8 mm and a length of 170 mm is provided. DURAN is a borosilicate glass, which approximately has the following composition in percent by weight on an oxide basis:

(27) TABLE-US-00001 SiO.sub.2 81% by weight B.sub.2O.sub.3 13% by weight Na.sub.2O + K.sub.2O 4% by weight Al.sub.2O.sub.3 2% by weight.

(28) The annealing point of DURAN is about 560 C., the softening point is about 825 C.

(29) The glass tube 50 is continuously heated in a tunnel oven to a temperature of 690 C. In order to avoid oxidation of the glass contact material, the oven is flushed with a reducing or inert gas, for example forming gas, nitrogen or argon. The glass tube 50 rests with its outer wall on a base 60 during the heating. The region of the base 60 that comes into contact with the glass tube comprising isostatically pressed graphite with a mirror-polished surface.

(30) Furthermore, a reshaping tool 10 with a planar forming body 30, as schematically represented in FIGS. 1 and 2, is provided in the oven. The planar forming body 30 likewise comprises isostatically pressed graphite with a mirror-polished surface. The porosity of the forming body is 15%, its thickness 0.5 mm. For mechanical stabilization, the forming body has on its rear side webs with a thickness of 9.5 mm arranged in the form of a honeycomb. Formed in the interior of the reshaping tool is a cavity, which also comprises the intermediate spaces between the webs arranged in the form of a honeycomb and in which a pressure of 10 kPa is set by means of a rotary vane pump, so that a gas flow through the porous forming body 30 results.

(31) As soon as the glass tube 50 has reached the target temperature with an accuracy of 5 K, it is positioned in the oven under the reshaping tool in such a way that the surfaces of the base 60 and of the forming body 30 are aligned plane-parallel to one another. The temperature of the forming tool in this case corresponds to the temperature in the oven. In a next step, the reshaping tool 10 is moved perpendicularly to the longitudinal axis of the glass tube 50, so that the reshaping tool 10 exerts a compressive force on the outer surface of the glass tube 50 and presses it against the base 60. In this case, when it is in contact with the forming area 31, the glass tube 50 is fixed by the negative pressure on the forming area 31 without the surface of the glass tube being damaged. The fixing has the effect of preventing undesired dipping in the middle region of the tube 50, or even the complete collapse of the glass tube 50, and consequently sticking together of the inner surfaces of the tube 50.

(32) The reshaping tool 10 is moved until a distance of 8 mm is achieved between the forming area 31 and the surface of the base 60. With a correspondingly exact design of the surfaces and the end position of the forming body 30 and of the base 60, an exact plane-parallelism of the planar regions of the outer surfaces of the reshaped tube 50 can be achieved.

(33) This process consequently results in a reshaped glass tube 50 with a width of B=70 mm and a height of H=8 mm, corresponding to an aspect ratio of 8.75:1, the cross section of which corresponds substantially to the schematic representation in FIG. 4a.

(34) The entire disclosures of all applications, patents and publications, cited above and below, and of corresponding German application DE 10 2017 202 762.2 filed Feb. 21, 2017, are hereby incorporated by reference.

(35) The present invention will be illustrated below by a series of examples. However, the present invention is not limited to the examples mentioned.

EXAMPLES

(36) The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

(37) From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.