Method and apparatus for making a glass sheet with controlled heating

Abstract

A method and apparatus for forming a glass sheet using a fusion down-draw process, wherein the heating powers of the heating elements are managed such that in case of a failure of one heating element, the heating power of the adjacent heating element is immediately increased. The method decreases the thermal stress the forming body is exposed to due to the failure of the heating element.

What is claimed is: 
     
         1 . An apparatus for making a glass sheet in an overflow process, the apparatus comprising:
 a forming body having a top, a first side surface, and a second side surface joining the first side surface at a root, wherein the forming body is configured such that molten glass flows over at least part of the first side surface to form a first glass ribbon, molten glass flows over at least part of the second side surface to form a second glass ribbon, and the first glass ribbon and the second glass ribbon fuse at the root to form a third glass ribbon;   a first side heat plate proximate to the first side surface, wherein the first side heat plate is configured to radiatively heat the first side surface;   a first array of heating elements located behind the first side heat plate opposite of the first side surface, wherein the first array of heating elements are configured to heat the first side heat plate;   a second side heat plate proximate to the second side surface, wherein the second side heat plate is configured to radiatively heat the second side surface;   a second array of heating elements located behind the second heat plate opposite of the second side surface, wherein the second array of heating elements are configured to heat the second side heat plate; and   a power supply unit configured to separately monitor, control and adjust a heating power of individual heating elements in the first and second arrays of the heating elements; and   the power supply unit further configured to control the heating power of the individual heating elements of the first and second arrays such that if the heating power of a malfunctioning heating element in one of the first and second arrays decreases to a threshold level, then the heating power of the adjacent, functioning heating element is increased, such that an undesirable thermal gradient does not form between a part of the first side surface or the second side surface across from the malfunctioning heating element and a bulk of the forming body under the part of the first side surface or the second side across from the malfunctioning heating element.   
     
     
         2 . The apparatus according to  claim 1 , further comprising a top plate positioned over the top of the forming body connecting the first and second side heat plates, and the first and second side heat plates and the top plate define an enclosure housing the forming body. 
     
     
         3 . The apparatus according to  claim 2 , wherein the top plate is made of a material selected from platinum, platinum-containing alloys, SiC, Si 3 N 4 , SiO 2 , MgO, and BeO. 
     
     
         4 . The apparatus according to  claim 1 , wherein the first and second side heat plates are made of a material selected from platinum, platinum-containing alloys, SiC, Si 3 N 4 , SiO 2 , MgO, and BeO. 
     
     
         5 . The apparatus according to  claim 1 , wherein the first and second side heat plates are made of SiC or Si 3 N 4  and have a thickness of at most 50 mm. 
     
     
         6 . The apparatus according to  claim 1 , wherein the individual heating elements are essentially parallel to the root. 
     
     
         7 . The apparatus according to  claim 1 , wherein each heating element in the first and second arrays of heating elements is separated from an adjacent heating element by a separation plate. 
     
     
         8 . The apparatus according to  claim 1 , wherein at any given time, a temperature gradient in a top layer of the first and second side surfaces having a thickness of 5 cm is maintained by the power supply unit at a level of at most 100° C. 
     
     
         9 . The apparatus according to  claim 1 , wherein the forming body is symmetrical with respect to a plane parallel to a gravity vector and passing through the root. 
     
     
         10 . The apparatus according to  claim 1 , wherein the first and second side heat plates are symmetrical with respect to a plane parallel to a gravity vector and passing through the root. 
     
     
         11 . The apparatus according to  claim 1 , wherein the first and second arrays of heating elements are symmetrical with respect to a plane parallel to a gravity vector and passing through the root. 
     
     
         12 . The apparatus according to  claim 1 , wherein the top plate, the first side heat plate, and the second side heat plate are all made of same material. 
     
     
         13 . The apparatus according to  claim 1 , wherein a temperature difference between the top and the root of the forming body is maintained by the power supply unit from DTTB−25° C. to DTTB+25° C., where DTTB is the average of a temperature difference between the top and root of the forming body. 
     
     
         14 . The apparatus according to  claim 1 , further comprising:
 a third side heat plate proximate to the top, wherein the third side heat plate is configured to radiatively heat the top;   a third array of heating elements located behind the third side heat plate opposite of the top, wherein the third array of heating elements are configured to heat the third side heat plate; and   
     
     
         15 . The apparatus according to  claim 14 , wherein there is no separation wall between individual heating elements in the third array of heating elements.