Sheet glass forming apparatus
One embodiment of the present invention is an improved sheet glass forming apparatus. In another embodiment, a precise thermal control system redistributes the flow of molten glass at the weirs to counteract the degradation of the sheet forming apparatus which inevitably occurs during manufacturing. In yet another embodiment, the invention introduces a counteracting force to the stresses on the trough in a manner such that the thermal creep which inevitably occurs has a minimum impact on the glass flow characteristics of the forming trough. Another embodiment creates a variable external cross-section which provides hydraulic stresses that are in opposition to the surface tension and body force stresses and thus, reduces the influence of surface tension and body forces. In an alternative embodiment, the glass is preferentially cooled across its width to create forming stresses during solidification, which ensures that the glass sheet drawn is inherently flat. In an additional embodiment, the internal pressure is adjusted in each of the major components of the forming apparatus such that the pressure difference across any leakage path to the forming zone is essentially zero. Therefore, air leakage in the apparatus is minimized even though the cracks and openings exist during initial operation and develop during manufacturing.
1. A method for reducing air leakage when forming sheet glass using an apparatus that includes a forming 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, and the apparatus including at least two adjacent zones including a forming zone wherein the sheet glass is formed and another zone positioned adjacent the forming zone, wherein at least the forming zone is enclosed by a forming zone chamber of the apparatus, and the another zone adjacent to the forming zone is enclosed by at least one adjacent chamber of the apparatus, wherein the method comprises the step of:
equalizing an internal pressure between at least the forming zone chamber and the at least one adjacent chamber of the apparatus, during a glass cooling process such that a pressure difference across a gas leakage path between the forming zone chamber and the at least one adjacent chamber of the apparatus is substantially zero, thereby eliminating gas leakage.
2. The method of claim 1 , wherein the forming zone is selected from the group consisting of:
a muffle zone enclosing the forming trough;
a muffle door zone, located at a bottom of the wedge of the trough;
a transition zone, which transitions the molten glass between the muffle door zone and an annealer and pulling machine zone; and
the annealer and pulling machine zone.
3. The method of claim 1 , wherein equalizing the internal pressure is accomplished by adding air flow to the at least one adjacent chamber.
4. The method of claim 1 , wherein equalizing the internal pressure is accomplished by restricting an outflow of air from the at least one adjacent chamber to factory ambient pressure.
5. The method of claim 1 , further comprising a vacuum source, wherein the vacuum source lowers a pressure in the at least one adjacent chamber.
6. The method of claim 1 , further comprising at least one supplementary pressure chamber that encases the at least one adjacent chamber.
7. The method of claim 6 , wherein there are at least two supplementary pressure chambers.
8. The method of claim 1 , wherein the gas is air.
9. The method of claim 1 , wherein the gas is nitrogen.
10. The method of claim 1 , wherein the gas is a gas other than air.