Process for manufacturing quartz glass pipes having a high content of silica with only minor diameter deviations

A process is described for manufacturing vertically withdrawn pipes from quartz glass or glasses having a high content of silica with only minor diameter deviation. In an electrically heated furnace, the initial material is heated up to the softening point. A pipe is formed in a deformation zone and is downwardly withdrawn. In this deformation zone, a constant pressure difference is maintained between the pressure in the pipe interior and the pressure in a chamber and/or heating chamber. The pressure is continuously measured and controlled so as to be at a constant difference. In order to maintain a higher pressure in the pipe interior as opposed to the pressure in the chamber and/or the heating chamber, the withdrawn pipe end of the pipe to be manufactured is sealed.

The invention relates to a process for manufacturing vertically withdrawn 
pipes made of quartz glass or glasses having a high silicic acid content 
wherein the initial material is heated up to the softening point in an 
electrically heated furnace and a pipe is formed in a deformation zone and 
is downwardly withdrawn. 
Processes of the aforesaid characterizing kind are known from German PS 67 
16 26. In this process, quartz granules are heated up to softening 
temperature in an electrically heated furnace which includes a melting 
vessel enclosed by an electrically heated casing. The so treated granules 
are then downwardly withdrawn into a pipe mold. In the deformation zone 
the pipe, at its inside as well as its outside, is exposed to a protective 
gas. After drawing the pipe to the desired length, it is cut off. The so 
manufactured quartz glass pipes exhibit a uniform waIl thickness, they are 
free of bubbles and are transparent. Deviations in the diameter of these 
pipes, however, cannot be avoided. 
Today's manufacturing processes which are automated because of the numerous 
applications of quartz glass pipes require absolute precision and 
maintenance of narrow tolerances, particularly with regard to the pipe 
diameter. 
Accordingly, it is an object of the invention to provide a process for 
manufacturing pipes of any desired length from quartz glass or glass 
having a high content of silicic acid, i.e. glasses with a SiO.sub.2 
-content of at least 95 wt. -%. The pipes must have a prescribed diameter 
and wall thickness, particularly a wall thickness and a diameter constant 
over a prescribed pipe length, i.e. diameter deviations must be reduced to 
a minimum. 
The object is accomplished for the above characterized process in 
accordance with the invention in that a constant pressure difference 
between the pressure in the pipe interior and the pressure in a chamber 
and/or heating chamber which encloses the pipe prevails in the deformation 
zone, in that the pressures in the pipe interior and the chamber and/or 
heating chamber are continuously measured and controlled so as to match 
this constant difference and in that the withdrawn pipe end is sealed in 
order to generate and maintain a pressure in the pipe interior which is 
higher than the pressure in the chamber and/or the heating chamber. 
The constant pressure difference is advantageously maintained in that gas 
is supplied to the chamber and/or heating chamber and/or to the pipe 
interior. Using refractory metals like molybdenum or tungsten as a 
material for the heating elements or the melting crucible, it proved to be 
particularly good supplying a hydrogen-containing gas to the chamber 
and/or the heating chamber. In order to increase the pressure in the pipe 
interior as opposed to the pressure in the chamber and/or the heating 
chamber, the withdrawn pipe end is sealed with a solid, liquid or gaseous 
plug. 
The process in accordance with the invention is suited not only for the 
manufacture of pipes from a granular basic material but also for the use 
of a hollow cylindrical body as an initial material. In the first case, it 
proved to be advantageous--as known from German PS 67 16 26, cited as 
prior art--to supply gas to the deformation zone in the pipe interior. In 
case a hollow-cylindrical initial body is used, a gas is also supplied in 
order to maintain a pressure in the pipe interior. Here, it is 
advantageous to gas-tight seal the one end of the hollow-cylindrical body 
which faces away from the deformation zone. The gas supplied must not 
necessarily contain hydrogen, it is also possible to use nitrogen or air. 
When using graphite heating elements, it is not absolutely necessary to 
rinse the heating chamber with a hydrogen-containing gas. Particularly 
good results with respect to maintaining narrow diameter tolerances can be 
achieved in accordance with the invention in that the deviations of the 
pressure difference between the pipe interior and the pressure in the 
chamber and/or the heating chamber are controlled to be at .+-.1 Pa. 
The process in accordance with the invention permits manufacturing quartz 
glass pipes having a diameter of 25 mm and a wall thickness of 3 mm. The 
diameter thereof can be maintained constant up to 0.05 mm over a length of 
2 m. The pressure difference in the deformation zone was kept constant at 
190 .+-.1 Pa. Without the pressure control in accordance with the 
invention, there are significant pressure difference deviations in the 
deformation zone which can lead to diameter deviations of up to 0.2 mm for 
a pipe length of 2 m. The process in accordance with the invention proved 
to be particularly advantageous for the manufacture of quartz glass pipes 
of larger diameters, for example, 100 mm and more. For pipes of this kind, 
the diameter deviations could be reduced to 0.3 mm, at a maximum, for a 
pipe length of 2 m. Without the process in accordance with the invention, 
the diameter deviations amount to 2 mm and more for the same pipe length. 
In accordance with the invention, a process for manufacturing vertically 
withdrawn pipes from at least one of quartz glass and glasses having a 
high content of silicic acid comprises heating initial material up to a 
softening point in an electrically heated furnace and forming a pipe in a 
deformation zone and downwardly withdrawing the pipe. The process also 
includes continuously measuring the pressures in an interior of the pipe 
and in at least one of a chamber and heating chamber enclosing the pipe. 
The process also includes controlling the aforesaid pressures so as to 
maintain, in the deformation zone, a constant pressure difference between 
the pressure in the interior of the pipe and the pressure in at least one 
of the chamber and heating chamber enclosing the pipe and including 
sealing an end of the withdrawn pipe to generate and maintain a pressure 
in the pipe interior which is higher than the pressure in the at least one 
of the chamber and the heating chamber. 
For a better understanding of the invention, together with other and 
further objects thereof, reference is made to the following description, 
taken in connection with the accompanying drawings, and its scope will be 
pointed out in the appended claims.

The initial material for the quartz glass pipe in FIGS. 1 and 2 is quartz 
granules and in the embodiment of FIG. 3 a quartz hollow cylinder. 
All of the embodiments for the working of this process which are 
represented in FIGS. 1 to 3 are based on the same teaching which states 
that in the deformation zone a constant pressure difference is maintained 
between the pipe interior and the pressure in chamber (FIGS. 1 and 2) 
and/or heating chamber (FIG. 3) which encloses the pipe and to which a 
protective gas is supplied. The pressures in the pipe interior and in the 
chamber and/or the heating chamber are continuously measured and 
controlled so as to maintain a constant difference. The withdrawn pipe end 
is sealed in order to generate and maintain a pressure in the pipe 
interior which is higher than the pressure in the chamber and/or in the 
heating chamber. 
From FIG. 1 it can be seen that in the melting furnace 1, quartz granules 
2a are used as an initial material which are supplied to the melting 
crucible 4 via an inlet funnel 3. The melting crucible 4, as known, is 
made of a refractory metal, for example molybdenum. By means of electric 
heating elements 5, the melting crucible 4 is heated up to a temperature 
such that the quartz granules are melting. The electric heating elements 5 
are insulated toward the exterior by means of a thermal insulation 6. At 
the lower end of the melting crucible 4, there is a nozzle-like outlet 7 
for the discharge of the quartz glass which has reached its softening 
point. The shape of the nozzle-like outlet 7 determines the geometry of 
the quartz glass pipe 33. The outlet extends into the interior of the 
chamber 8. At the bottom, i.e. in the direction in which the quartz glass 
pipe 33 is withdrawn, there is a discharge sluicegate 9 which is 
configured as system of diaphragms in this embodiment. Through this 
sluicegate, the quartz glass pipe which is withdrawn by means of the 
drawing device 11 is discharged the interior of the chamber 8. Between the 
drawing device 11 and the sluicegate 9, there is a measuring device 10 
which detects the diameter of the withdrawn quartz glass pipe 33 as well 
as the wall thickness thereof. A commercially available laser-scanner, for 
example, can serve as such a measuring device. In its interior, the 
withdrawn quartz glass 33 has a sealing plug which, for example, can be an 
inflated, heat-resistant plastic body. This sealing plug 12 is supported 
by the support pipes 13 and 13a which are coupled to one another by an 
automatic snap closure 15. The sealing plug can be inflated via the 
compressed-air-terminal 25 and the support pipes 13 and 14a to correspond 
to the internal diameter of the withdrawn quartz glass pipe 33. Together 
with the terminal 25 for compressed air, the support pipes 13 and 14a are 
disposed on a rotary table 16 for conveying cut off quartz glass pipes. A 
cutting blade 23, which is disposed underneath the sealing plug, separates 
segments of quartz glass pipe from the withdrawn quartz glass pipe. After 
the pipe is lowered, the gripper 24 disposed on the rotary table picks up 
the coupling snap closure 15 such that the support pipe 14a can be 
telescopically introduced. A support pipe 14b is also disposed on the 
rotary table 16. After turning the table by 180.degree., as the arrow 44 
indicates, the cut-off quartz glass pipe segment is, on the one hand, 
conveyed out of the area of the drawing device and, on the other side, the 
support device 14b takes over the place of the support device 14a below 
the drawing device. Subsequently, the snap closure couples it, via pipe 
13, to the sealing plug 12 such that the sealing plug is secured again in 
its position. Subsequently, the arm of the gripper 24 is retracted and the 
quartz glass pipe segment which was cut off can be withdrawn from the 
rotary table. 
A measuring device 17, the measuring pipe 37 of which extends into the 
deformation zone of the quartz glass pipe, is provided for determining the 
internal pressure. Via the gas supply 18, the gas supplied through the 
feed pipe 38 serves to change the pressure in the pipe interior in the 
deformation zone. The pressure in the chamber interior 8 can be measured 
by means of the chamber pressure measuring device 19. The gas feed line 20 
which ends into the chamber 8 serves to adjust the pressure in the chamber 
interior 8. The gas supply 18 and the gas feed line 20, respectively, are 
connected to gas cylinders 21 which are filled with hydrogen or a 
hydrogen-containing gas. Corresponding to the desired diameter for the 
withdrawn quartz glass pipe, the control device 22 controls the pressure 
difference between the pressure P2 in the pipe interior and the pressure 
Pl in the chamber so as to remain at a constant value. Advantageously, the 
deviations of the pressure difference amounts to a maximum of .+-.1 Pa of 
the value selected. The diameter is monitored by the measuring device 10. 
The device for the working of the process in accordance with the invention 
of FIG. 2 is essentially distinguished from the one in FIG. 1 in that the 
principle of the gaseous plug is applied rather than using a solid sealing 
plug for the withdrawn pipe. As it can be seen from FIG. 2, the withdrawn 
pipe enters the cutting chamber 26 which contains the cutting blade 23 via 
sluicegate 27. The cutting chamber 26 is connected to a ventilator 29 
which maintains a prescribed pressure P3 inside the cutting chamber. This 
pressure is measured by means of the pressure measuring device 30 which in 
turn is connected to the control device 22. The latter in turn interacts 
with the ventilator and, hence, affects the pressure P3 in the cutting 
chamber 26. The pressure P3 in the cutting chamber is controlled such that 
the pressure P2 in the pipe interior of the deformation zone is maintained 
constant at a prescribed value. In the present case the pressure P3 in the 
cutting chamber is by approximately 130 Pa lower than the pressure P2 of 
the deformation zone and with increasing pipe length, it had to be 
decreased by approximately another 11 Pa/m. This amounts to a pressure 
change of approximately 22 Pa in the cutting chamber per 2 m pipe segment. 
Via sluicegate 31 and gate 28, the cut-off quartz glass segment is then 
discharged from the pressure chamber 26. 
In the device of FIG. 3 for the working of the process in accordance with 
the invention, a hollow cylinder 2b is used as an initial material. 
Through an inlet sluicegate 35, an advance 32 continuously introduces this 
hollow cylinder 2b into the heating chamber 44 where it is heated up to 
the softening point by means of electric heating elements 5 and formed 
into the shape of a pipe which then is withdrawn via an outlet sluicegate 
9 by means of a drawing device. The diameter and the wall thickness of the 
withdrawn pipe 33 are also measured and monitored by means of a measuring 
device such as a laser-scanner. The end of the quartz glass hollow 
cylinder which faces away from the deformation zone is gastight sealed by 
means of a plug 36. Through this plug, the measuring pipe 37 of the 
pressure gauge 17 as well as the feed line 38 of the gas supply 18 extend 
into the interior of the quartz glass hollow cylinder until they reach the 
deformation zone. The end of the withdrawn quartz glass pipe is withdrawn 
into a container 39 which is filled with a liquid 40, for example, water. 
Instead of water it is also possible to use silicone fluid. The liquid 
acts as a "liquid plug" and gastight closes the interior of the withdrawn 
quartz glass pipe 33 such that via the gas supply 18, a prescribed 
internal pressure can be selected and maintained in the pipe interior. In 
the heating chamber 44, the pressure is monitored via the pressure 
measuring device 42 and via the gas supply of the heating chamber 43, gas 
is supplied from one of the cylinders 21, if necessary. A cutting blade 23 
cuts off segments of the quartz glass pipe 33 below the liquid level 41 in 
the quartz glass pipe 33. Otherwise, gas escapes from the interior of the 
withdrawn pipe and the pressure control in the pipe interior would be 
rendered ineffective. Adjustment, control and monitoring of the pressure 
difference in the deformation zone is carried out in the same way as 
described in connection with FIG. 1. 
As it can be seen from FIGS. 1 and 2, drawing device 11 for the quartz 
glass pipe 33 is also connected with the control device 22. The measured 
values for diameter and wall thickness of the quarts glass pipe, which are 
recovered by the measuring device 10, the laser-scanner, are not only used 
for controlling the pressure difference which has to be maintained 
constant but it serves also to control the diameter and the wall thickness 
of the withdrawn quartz glass 33. The drawing speed is selected at the 
drawing device according to the desired pipe diameter and the wall 
thickness. 
While there have been described what are at present considered to be the 
preferred embodiments of this invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the invention, and it is, therefore, aimed 
to cover all such changes and modifications as fall within the true spirit 
and scope of the invention.