Overflow downdraw glass forming method and apparatus
The present invention significantly modifies “The Overflow Process”. It includes a method and apparatus for measuring glass flow rate and maintaining a constant glass flow rate. It also embodies design features and methods that support and stress the forming apparatus in a manner such that the deformation that results from thermal creep is corrected, thus minimizing the effect of the thermal creep on the thickness variation of the glass sheet. The present invention also embodies design features that change the process from a single step (combined flow distribution and cooling) to a two step process; step one being flow distribution and step two being cooling.
1. An improved apparatus for forming sheet glass, wherein the apparatus comprises an inflow pipe for delivering molten glass, a trough for receiving molten glass that has sides attached to a wedged shaped sheet forming block that has downwardly sloping sides converging at a 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 block and meets at the bottom of the wedge, wherein the improvement comprises:
a) a first inflow end compression insert inserted into a first cavity at an inflow end of the forming block, shaped to apply at least a portion of an inflow end compression force at an inflow end compression surface at zero cavity depth, wherein the first inflow end compression insert has a higher resistance to thermal creep than the forming block and a cross-sectional area that maintains a force at a bottom of the first cavity; and
b) a second far end compression insert inserted into a second cavity at a far end of the forming block;
such that a longitudinal compressive force is applied to a bottom of the first cavity and a bottom of the second cavity to counteract a thermal creep deformation effect of a gravitational force from a weight of the forming block such that a thickness profile of a glass sheet remains substantially constant.
2. The improved apparatus of claim 1 , wherein a depth of the first cavity and a depth of the second cavity are independent and range from 0 to 40 percent of a length of the bottom of the wedge.
3. The improved apparatus of claim 2 , wherein the depth of the first cavity ranges from 20 to 35 percent of a length of the bottom of the wedge.
4. The improved apparatus of claim 3 , wherein the depth of the second cavity ranges from 10 to 25 percent of a length of the bottom of the wedge.
5. The improved apparatus of claim 1 , wherein the first inflow end compression insert and the first cavity have a stepped design in order to apply compression forces at multiple depths of the first cavity.
6. The improved apparatus of claim 1 , wherein the second far end compression insert is shaped to apply at least a portion of a far end compression force at a far end compression surface at zero cavity depth.
7. The improved apparatus of claim 6 , wherein the second far end compression insert has a higher resistance to thermal creep than the forming block and a cross-sectional area that maintains a force at a bottom of the second cavity.
8. The improved apparatus of claim 6 , wherein the second far end compression insert applies all of the far end compression force at the far end compression surface at zero cavity depth.