Overflow downdraw glass forming method and 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.
1. An improved apparatus for forming sheet glass, wherein the apparatus includes 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 wedge, and wherein the improvement comprises:
an enclosure disposed about the trough and the wedge shaped sheet forming structure comprising:
a muffle zone, wherein said muffle zone comprises;
a muffle zone forming chamber adjacent to the wedge shaped sheet forming structure, and
a muffle heating chamber separated from the muffle zone forming chamber by a wall of the muffle zone;
a muffle door zone, wherein said muffle door zone comprises;
a muffle door zone forming chamber adjacent the formed glass sheet;
a muffle door zone chamber separated from the muffle door zone forming chamber by a wall of a muffle door;
cooling tubes that extend into the muffle door zone chamber;
a transition zone wherein the transition zone comprises;
a transition zone forming chamber located adjacent the formed glass sheet;
a transition zone cooling chamber separated from the transition zone forming chamber by a wall of a transition cooler; and
a source of pressurized cooling air in fluid communication with the transition zone cooling chamber;
an annealing and pulling machine zone, wherein the annealing and pulling machine zone comprises;
an annealing chamber located adjacent the formed glass sheet; and
one or more pressure control structures selected from;
a gas inlet in fluid communication with the muffle zone heating chamber to allow equalization of an atmospheric pressure in the muffle heating chamber and an atmospheric pressure in the muffle zone forming chamber;
a exit restriction configured to be variable in size in fluid communication with an ambient atmosphere and the muffle door zone chamber to allow equalization of an atmospheric pressure in the muffle door zone forming chamber and an atmospheric pressure in the muffle door zone chamber;
a regulated vacuum source in fluid communication with the transition zone cooling chamber and an atmospheric pressure in the transition zone cooling chamber; and
a pressure balancing chamber located at an end of the annealing chamber, or encasing the annealing chamber, and defining a pressure regulated volume separated from the annealing chamber by an outer wall of the annealing chamber.
2. The apparatus of claim 1 , wherein there are at least two pressure balancing chambers.