GB1103192A - Process for tempering glass sheets - Google Patents

Abstract

A process of tempering glass sheets wherein one or a succession of glass sheets are rapidly cooled through the tempering range comprises cooling the major surfaces of each successive sheet at respectively different rates so that one surface cools faster through the said range than the other surface, the differential rate of cooling eventually causing the glass sheet to assume a curvature, which may be compound, different from that it possessed above the tempering range. As described flat glass sheets are fed on rollers 20 and between radiant heaters 22 to a gaseous support bed 40 inclined at 5 degrees to the horizontal. The support bed passes through a furnace heated by radiant heaters 54, 55 and the glass sheets are conveyed therealong by edge rolls 26. Heated air is fed from a plenum chamber 43 into the individual modules 41 of the bed 40. The curvature of the support bed 40 is altered so that the heated glass sheets can conform to it and so enter a quenching section with a predetermined curvature. The characteristics of the bed are similar to those disclosed in Specification 1,021,842. As the sheets are conveyed over the bed 40 they operate a sensing device 148 (Fig.3) which causes the last three rollers 26 and the first few 260 of the quenching section to be speeded up to quickly transfer the sheet from the heating section to the bending section, whereupon the seeed of the rolls reverts to the original speed. The quenching section comprises a gaseous support bed 80 having the same contour as the bed 40. Each module 81 of said bed 80 comprises a prismatic body portion 160 having an upper surface containing a plurality of coplanar, arcuate grooves 162 extending outwardly from the centre of the module, which is connected to a plenum chamber below (see Fig.12). There is also an upper head assembly 92, adjustable in distance from the support bed, which conforms exactly to the shape of the lower bed. The quenching section is divided into three stages 1A, 1B and II Stage II is the same length as 1A plus 1B. Air is fed to each stage from fans, e.g. 89, which have valves 93, 94 for controlling the flow rate to the upper and lower chambers 80, 92. To maintain these at a uniform temperature cooling water is circulated through them. The glass sheet curved during its passage through the heating section is conveyed through the quenching section by edge rolls 260. It is subjected to the greatest rate of cooling in stage 1A, and thereafter the cooling rate may be diminished. In the case of a sheet convex upwards the upper <PICT:1103192/C1/1> <PICT:1103192/C1/2> <PICT:1103192/C1/3> <PICT:1103192/C1/4> surface would always be cooled faster than the concave surface. This controlled differential tempering causes the sheet to assume a final curvature different from that which it had on entering the quenching section. The glass sheets may have any desired curvature, or be flat on entering the quenching section. Also the sheets may be conveyed on porous beds, perforated plates, or rollers, or be balanced or suspended vertically.