Method and casting mold for manufacturing a quartz glass body
The method provides an improved quartz glass body, especially an improved quartz glass crucible for melting non-metals, non-ferrous metals, or silicon. The quartz glass body is made by a method in which a quartz glass-water mixture is supplied to a gypsum casting mold comprising an outer part and an inner part, dried in the gypsum mold, and put under an overpressure via a system of ducts in the form of textile tubes embedded in the mold during the drying. Later a resulting green glass body is removed from the mold. At least a portion of the shaping surfaces of the inner part of the gypsum mold are surfaces of a water-impermeable substance. The method provides a cast body with a reduced tendency to crack and with a smaller open porosity.
1. A method of making a quartz glass body, said method comprising the steps of:
a) feeding a quartz glass-water mixture into a gypsum casting mold, said gypsum casting mold comprising an outer part and an inner part having respective shaping surfaces for contacting the quartz glass-water mixture and at least one of said inner part and said outer part having a corresponding duct system under at least one of said respective shaping surfaces, said duct system comprising a plurality of ducts in the form of textile tubes and at least one connector for connection of a compressed medium;
b) drying the quartz glass-water mixture in the gypsum casting mold;
c) during said drying of step b), putting an overpressure on the quartz glass-water mixture by applying the compressed medium to said corresponding duct system in said at least one of the inner part and the outer part; and
d) transmitting or communicating said overpressure to the quartz glass-water mixture through said plurality of said ducts of said corresponding duct system in said at least one of the inner part and the outer part;
e) during said drying of step b), applying a hydrostatic pressure on said quartz glass-water mixture in addition to said overpressure applied during said drying through said duct system; and
f) after said drying of step b, removing a resulting green glass body from the gypsum casting mold, whereby during said removing a pressure difference is applied across the gypsum casting mold, or a portion thereof;
wherein at least a portion of the shaping surfaces of the inner part are surfaces of a water-impermeable substance.
2. The method as defined in claim 1 , further comprising providing a pressure difference across the gypsum casting mold, or a portion of the gypsum casting mold, during said feeding of the quartz glass-water mixture into the gypsum casting mold.
3. The method as defined in claim 1 , wherein said outer part is an outer shell, said inner part is a core;
said inner part and said outer part are shaped so that said quartz glass body is a quartz glass crucible; and said quartz glass-water mixture is fed into the gypsum casting mold under the force of gravity from a level above an upper crucible edge and through an inlet opening in the gypsum casting mold.
4. The method as defined in claim 1 , wherein said inner part is at least partially clad with a prefabricated mold made from a water-impermeable material.
5. The method as defined in claim 4 , wherein said water-impermeable material is polyurethane foam or silicone rubber.
6. The method as defined in claim 1 , wherein said inner part is at least partially coated with a water-impermeable coating.
7. The method as defined in claim 6 , wherein said water-impermeable coating consists of a polyurethane foam layer or a silicone rubber layer.
8. The method as defined in claim 1 , wherein said inner part comprises a combustible material and is burned after said drying.
9. The method as defined in claim 8 , wherein said combustible material is polyurethane foam.
10. The method as defined in claim 1 , further comprising sintering the resulting green glass body after removing the green glass body from the gypsum casting mold.
11. The method as defined in claim 1 , further comprising putting said hydrostatic pressure on said quartz glass-water mixture during said feeding of said quartz glass-water mixture into said gypsum casting mold.
12. The method as defined in claim 11 , wherein said hydrostatic pressure is adjusted by selection of a filling height above an upper crucible edge of a quartz glass crucible, said quartz glass body consisting of said quartz glass crucible.
13. The method as defined in claim 1 , wherein said overpressure is between 0.002 MPa and 0.14 MPa above standard pressure.
14. The method as defined in claim 1 , wherein said overpressure is between 0.01 MPa and 0.02 MPa above standard pressure.
15. The method as defined in claim 1 , further comprising putting said quartz glass-water mixture under said overpressure during said feeding as well as during said drying, and wherein said gypsum casting mold is completely filled with said quartz glass-water mixture by said feeding over a time interval of 5 minutes to one hour.
16. The method as defined in claim 1 , said wherein said overpressure acts on said quartz glass-water mixture during a first stage of said drying and said inner part is lifted from the green glass body during a second stage of said drying.
17. The method as defined in claim 16 , wherein said first stage of said drying is from 2 to 24 hours in duration and said second stage of said drying is from 10 hours to 12 days in duration.
18. The method as defined in claim 1 , wherein said quartz glass-water mixture has a water content of at most 15% by weight during said feeding into said gypsum casting mold.
19. The method as defined in claim 1 , wherein said compressed medium is compressed air.