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. A method for forming sheet glass of constant thickness in a downdraw apparatus, wherein the apparatus includes a forming block and an inflow pipe for delivering molten glass to the forming block, the forming block comprising a trough for receiving the molten glass and downwardly sloping sides converging at the bottom of the forming block, such that a glass sheet is formed when the molten glass overflows the trough and flows down the downwardly sloping sides of the forming block and meets at the bottom of the forming block, the method comprising the steps of;
a) determining a first relationship between forming block shape deformation and sheet glass thickness distribution;
b) determining a second relationship between applied forces and the shape of the forming block;
c) measuring a thickness profile of a glass sheet manufactured using the downdraw apparatus at a time during a manufacturing campaign;
d) calculating a deformed shape of the forming block from the glass sheet measured thickness profile and the first relationship;
e) based on the calculated deformed shape of the forming block and the second relationship, determining at least one change in at least one force applied to the forming block required to change a shape of the forming block to produce a more uniform thickness distribution; and
f) applying a corrective force to the forming block based upon the calculated the at least one change in at least one force applied to the forming block from step (e).
2. The method of claim 1 , wherein the apparatus further comprises:
a) a first inflow end compression block inserted into a first cavity at an inflow end of the forming block;
b) a second far end compression block inserted into a second cavity at a far end of the forming block;
c) a third inflow end compression block at a surface of the first cavity at the inflow end of the forming block;
d) a fourth far end compression block at a surface of the second cavity at the far end of the forming block; and
wherein the method further comprises the step of applying a longitudinal compressive force to the first inflow end compression block, the second far end compression block, the third inflow end compression block, and the fourth far end compression block to counteract a thermal creep deformation effect of a gravitational force from a weight of the forming block.
3. The method of claim 1 , wherein step a) uses at least one test result from oil modeling to calculate the first relationship.
4. The method of claim 1 , wherein the apparatus further comprises:
a) a first inflow end compression block at an inflow end of the forming block; and
b) a second far end compression block at a far end of the forming block; and
wherein the method further comprises the step of applying a longitudinal compressive force to the first inflow end compression block and the second far end compression block to counteract a thermal creep deformation effect of a gravitational force from a weight of the forming block.
5. The method of claim 1 , wherein the apparatus further comprises:
a) a first inflow end compression block inserted into a first cavity at an inflow end of the forming block;
b) a second far end compression block at a far end of the forming block; and
c) a third inflow end compression block at a surface of the first cavity at the inflow end of the forming block; and
wherein the method further comprises the step of applying a longitudinal compressive force to the first inflow end compression block, the second far end compression block, and the third inflow end compression block to counteract a thermal creep deformation effect of a gravitational force from a weight of the forming block.
6. A method for forming sheet glass of constant thickness in a downdraw apparatus, wherein the apparatus includes a forming block and an inflow pipe for delivering molten glass to the forming block, the block comprising a trough for receiving the molten glass and downwardly sloping sides converging at the bottom of the forming block such that a glass sheet is formed when molten glass overflows the trough; and flows down the downwardly sloping sides of the forming block and meets at the bottom of the wedge, the method comprising the steps of;
a) determining a first relationship between forming block shape deformation and sheet glass thickness distribution;
b) determining a second relationship between applied forces and the shape of the forming block;
c) calculating a combined regression of the first relationship and the second relationship versus sheet thickness deviations;
d) measuring a thickness profile of a glass sheet manufactured using the downdraw apparatus at a time during a manufacturing campaign;
e) based on the glass sheet measured thickness profile and the calculated combined regression, determining at least one change in at least one force applied to the forming block required to a shape of the forming block to produce a more uniform thickness distribution; and
f) applying a corrective force to the forming block based upon the calculated at least one change in at least one force applied to the forming block from step (e).
7. The method of claim 6 , wherein step a) uses at least one test result from oil modeling to calculate the first relationship.
8. The method of claim 6 , wherein the apparatus further comprises:
a) a first inflow end compression block at an inflow end of the forming block;
b) a second far end compression block at a far end of the forming block; and
wherein the method further comprises the step of applying a longitudinal compressive force to the first inflow end compression block and the second far end compression block to counteract a thermal creep deformation effect of a gravitational force from a weight of the forming block.
9. The method of claim 6 , wherein the apparatus further comprises:
a) a first inflow end compression block inserted into a first cavity at an inflow end of the forming block;
b) a second far end compression block at a far end of the forming block;
c) a third inflow end compression block at a surface of the first cavity at the inflow end of the forming block; and
wherein the method further comprises the step of applying a longitudinal compressive force to the first inflow end compression block, the second far end compression block, and the third inflow end compression block to counteract a thermal creep deformation effect of a gravitational force from a weight of the forming block.
10. The method of claim 6 , wherein the apparatus further comprises:
a) a first inflow end compression block inserted into a first cavity at an inflow end of the forming block;
b) a second far end compression block inserted into a second cavity at a far end of the forming block;
c) a third inflow end compression block at a surface of the first cavity at the inflow end of the forming block;
d) a fourth far end compression block at a surface of the second cavity at the far end of the forming block; and
wherein the method further comprises the step of applying a longitudinal compressive force to the first inflow end compression block, the second far end compression block, the third inflow end compression block, and the fourth far end compression block to counteract a thermal creep deformation effect of a gravitational force from a weight of the forming block.