Overflow downdraw glass forming method and apparatus

1. An improved apparatus for forming sheet glass, the apparatus comprising a trough for receiving molten glass that has sides attached to a wedged shaped sheet forming structure that has downwardly sloping sides converging at the bottom of the wedge such that a glass sheet is formed when molten glass flows over the sides of the trough, down the downwardly sloping sides of the wedged shaped sheet forming structure, and meets at the bottom of the wedge, wherein the improvement comprises:

a) at least one support web which is positioned at a centerline of the forming structure, attached to the bottom of the trough and supported above the weir, wherein the support web, which has greater resistance to thermal creep than the forming structure, supports the forming structure in a manner such that there is negligible thermal creep longitudinal curvature of the forming structure; and

b) at least one downcomer glass feed pipe at an inlet end of the forming structure;

such that any deformation of the forming structure that results from thermal creep has a minimal effect on a thickness variation of the glass sheet.

2. The apparatus of claim 1 , further comprising edge directors at each end of the forming structure.

3. The apparatus of claim 1 , wherein ends of the forming structure are curved such that glass flows over each end section completely enveloping the forming structure.

4. The apparatus of claim 3 , wherein the forming structure is made of refractory metal.

5. The apparatus of claim 1 , wherein the support web is made of refractory metal.

6. The apparatus of claim 5 , wherein the refractory metal is protected from oxidation by platinum cladding.

7. The apparatus of claim 5 , wherein the refractory metal is protected from oxidation by operating the process in an inert atmosphere.

8. An improved apparatus for forming sheet glass, the apparatus comprising a trough for receiving molten glass that has sides attached to a wedged shaped sheet forming structure that has downwardly sloping sides converging at the bottom of the wedge such that a glass sheet is formed when molten glass flows over the sides of the trough, down the downwardly sloping sides of the wedged shaped sheet forming structure, and meets at the bottom of the wedge, wherein the improvement comprises:

a) at least two support webs which are positioned at a centerline of the forming structure, attached to the bottom of the trough and supported above the weir, wherein the support webs, which have greater resistance to thermal creep than the forming structure, support the forming structure in a manner that there is negligible thermal creep longitudinal curvature of the forming structure; and

b) at least one downcomer glass feed pipe at a center of the forming structure;

such that any deformation of the forming structure that results from thermal creep has a minimal effect on a thickness variation of the glass sheet.

9. The apparatus of claim 8 , further comprising edge directors at each end of the forming structure.

10. The apparatus of claim 8 , wherein ends of the forming structure are curved such that glass flows over each end section completely enveloping the forming structure.

11. The apparatus of claim 10 , wherein the forming structure is made of refractory metal.

12. The apparatus of claim 8 , wherein the support web is made of refractory metal.

13. The apparatus of claim 12 , wherein the refractory metal is protected from oxidation by platinum cladding.

14. The apparatus of claim 12 , wherein the refractory metal is protected from oxidation by operating the process in an inert atmosphere.