Continuous production of hollow ingots

1. An apparatus for the continuous production of hollow quartz-glass ingots, comprising:

(a) a crucible or refractory tank for providing a softened quartz-glass mass having a die orifice in the bottom of the crucible or refractory tank;

(b) a mandrel vertically drawing off the softened quartz-glass mass through a die to provide a hollow quartz-glass ingot; and

(c) a cutting station for on-line cutting of the hollow quartz-glass ingot to a specific length;

characterized in that the apparatus comprises active means for cooling the internal surface of the hollow quartz-glass ingot prior to the cutting station.

2. An apparatus according to claim 1 , wherein the mandrel is mounted on a mandrel support column, and the apparatus comprises at least one means to cool the internal surface of the hollow quartz-glass ingot selected from the group consisting of

1) a coil arranged around the mandrel support column, whereby said coil comprises a cooling fluid,

2) the mandrel support column itself which is cooled with a cooling fluid arranged inside the mandrel support column,

3) by providing a gas flow of an inert gas in the inner cavity of the hollow quartz-glass ingot, and/or

4) by providing a water spray issuing as multiple jets or as an atomized mist offine droplets.

3. The apparatus according to claim 1 , wherein the mandrel is supported from below on a mandrel support column, and the mandrel is held in place temporarily while said column is detached from the bottom carriage and each cut ingot section is removed, and said column is subsequently reattached to the bottom carriage.

4. The apparatus according to claim 1 , wherein the crucible or refractory tank for providing a softened quartz-glass mass is provided in a furnace allowing heating and containment of the quartz-glass mass.

5. The apparatus according to claim 1 , characterized in that the cutting station is beneath the die for drawing off the hollow quartz-glass ingot out of the crucible or refractory tank and is spaced away from the die along the hollow quartz-glass ingot of at most 4000 mm.

6. The apparatus according to claim 5 , wherein the hollow quartz-glass ingot being extruded is supported by one or more clamps mounted on carriages, whereby the one or more clamps mounted on the carriage are arranged to move downward from the die orifice at a speed appropriate to follow the fused silica extruded from the die orifice.

7. The apparatus according to claim 1 , wherein the cutting station is configured by a saw selected from a water-cooled saw, a heavy-duty water-cooled chain or a wire saw.

8. A hollow quartz-glass ingot, obtainable by a process characterized by the following process steps:

a. providing a softened quartz-glass mass in a crucible or refractory tank;

b. vertically drawing off the softened quartz-glass mass through a die in which is mounted a mandrel, to provide a hollow quartz-glass ingot; and

c. on-line cutting of the hollow quartz-glass ingot to a specific length, wherein

the internal surface of the hollow quartz-glass ingot is cooled by active means after vertically drawing off the softened quartz-glass mass in step b. and before on-line cutting in process step c.

9. The hollow quartz-glass ingot according to claim 8 , characterized by at least one of the following features:

1) an outside diameter of the ingot being more than 325 mm;

2) an inside diameter of the ingot being more than 175 mm;

3) a cross-sectional area of the ingot being greater than 62500 mm 2 ; and/or

4) a total stress in the ingot at the time of cutting being less than 5 MPa.

10. The hollow quartz-glass ingot according to claim 8 , wherein step b. is carried out with a mandrel mounted on a mandrel support column, and the internal surface of the hollow quartz-glass ingot is cooled by at least one of the following means:

1) a coil arranged around the mandrel support column, whereby said coil comprises a cooling fluid,

2) the mandrel support column itself which is cooled with a cooling fluid arranged inside the mandrel support column;

3) a gas flow of an inert gas in the inner cavity of the hollow quartz-glass ingot; and/or

4) a water spray issuing as multiple jets or as an atomized mist of fine droplets.

11. The hollow quartz-glass ingot according to claim 8 , wherein the mandrel is supported from below on a mandrel support column, and means are provided to hold the mandrel in place temporarily while said column is detached from the bottom carriage and each cut ingot section is removed, and said column is subsequently reattached tothe bottom carriage.

12. The hollow quartz-glass ingot according to claim 8 , wherein the external surface of the hollow quartz-glass ingot is cooled by enhancing active means of a cooling gas, a cooling fluid, active convection and/or radiation.

13. The hollow quartz-glass ingot according to claim 8 , wherein the hollow quartz-glass ingot is cooled to a temperature of the internal surface of the hollow quartz-glass ingot of less than 400° C. prior to the on-line cutting in process step c.

14. The hollow quartz-glass ingot according to claim 8 , wherein the hollow quartz-glass ingot is cooled to a temperature of the external surface of the hollow quartz-glass ingot of less than 400° C. prior to cutting in process step c.

15. The hollow quartz-glass ingot according to claim 8 , wherein a temperature difference between the internal surface and the external surface of the hollow quartz-glass ingot at the cutting zone is less than 300° C.

16. The hollow quartz-glass ingot according to claim 8 , wherein a total stress of the hollow quartz-glass ingot prior to the cutting in step c. is reduced to less than 5 MPa.

17. The hollow-quartz-glass ingot according to claim 8 , wherein the process step c. is carried out in a cutting station being spaced 1800 mm to 4000 mm from the die orifice along the hollow quartz-glass ingot extruded from the die orifice.