Method for producing a tube of glass
A method for forming a hollow cylinder, in a single step or in as small a number of steps as possible, into a quartz glass tube with a large outer diameter and high dimensional stability is provided. The cylinder, while rotating about a rotation axis, is softened in portions in a heating zone which is moved at a relative feed rate Va, and the softened portion is radially expanded by a centrifugal force and/or an internal overpressure applied in the hollow cylinder bore so as to form a deformation zone. The tube is continuously shaped with an outer diameter D 2 which is greater than that of the hollow cylinder D 1 . The radial expansion of the softened portion is carried out at a location-dependent radial expansion rate Vr, the profile of which along the deformation zone has a maximum value Vr,max which is smaller than two times the feed rate Va.
1. A method for producing a tube ( 22 ) of glass by forming a hollow cylinder ( 2 ) from the glass with a first outer diameter D 1 in that said cylinder, while rotating about a rotation axis ( 6 ), is softened in portions in a heating zone ( 20 ) which is moved at a relative feed rate Va, and the softened portion is radially expanded under an action of a centrifugal force and/or of an internal overpressure applied in a bore ( 7 ) of the hollow cylinder ( 2 ) so as to form a deformation zone ( 30 ), and the tube ( 22 ) is continuously shaped with a second outer diameter D 2 which is greater than the first outer diameter D 1 ,
wherein the radial expansion of the softened portion is carried out at a location-dependent radial expansion rate Vr, a profile of which along the deformation zone has a maximum value Vr,max which is smaller than 2 times the relative feed rate Va.
2. The method according to claim 1 , wherein the maximum value Vr,max of the radial expansion rate is smaller than 1.5 times and particularly preferably smaller than 0.7 time, the relative feed rate Va.
3. The method according to claim 1 , wherein, in the heating zone ( 20 ), a temperature profile is generated that along at least a sub-length of the deformation zone ( 30 ) has a course opposite to a profile of a tangential tension along the same sub-length.
4. The method according to claim 1 , wherein a temperature at an end of the deformation zone ( 30 ) is lower than a temperature at a beginning of the deformation zone.
5. The method according to claim 3 , wherein the heating zone ( 20 ) has a front heating portion assigned to the beginning of the deformation zone ( 30 ) and a rear heating portion assigned to the end of the deformation zone ( 30 ), wherein a higher temperature is produced by the front heating portion on a surface of the deformation zone ( 30 ) than by the rear heating portion.
6. The method according to claim 1 , further comprising at least one compression phase during which a ratio of a speed at which the hollow cylinder ( 2 ) moves into the heating zone ( 20 ) and of a speed at which the tube ( 22 ) is removed from the heating zone ( 20 ) is smaller than 1.
7. The method according to claim 1 , wherein the maximum radial expansion rate Vr,max is smaller than 20 cm/min, preferably smaller than 10 cm/min.
8. The method according to claim 1 , wherein a blow pressure is set to less than 20 mbar, preferably to less than 10 mbar.
9. The method according to claim 1 , wherein the tube ( 22 ) is produced with the second outer diameter D 2 which is greater than the first outer diameter D 1 by at least 40 mm.