METHOD AND APPARATUS FOR DEVELOPING AN EDGE STRESS IN A WINDSHIELD IN A WINDSHIELD BENDING FURNACE

Abstract: The invention relates to a method and apparatus for developing an edge stress in a windshield in a windshield bending furnace. Adjacent to a ring mould (2) is located a metallic, flat extra frame (3) in a manner that the lower surface of a bent windshield (1) remains at a small distance from extra frame (3). The compression stress of glass edge can be adjusted by varying the distance of extra frame (3) from glass and/or by varying its glass-oriented dimension.

Description: Method and apparatus for developing an edge stress in a windshield in a windshield bending furnace. The present invention relates to a method for develop¬ ing an edge stress in a windshield in a windshield bend¬ ing furnace, wherein a glass sheet is supported on a ring mould and heated close to a softening temperature and, following a bending operation, said glass sheet is cooled at such a cooling rate that a compression stress is developed at least in the edge portion of a glass sheet. The invention relates also to an apparatus for develop¬ ing an edge stress in a windshield in a windshield bend¬ ing furnace , said apparatus comprising a ring mould, supporting a glass sheet which is heated, bent and cooled. A compression stress is desired to develop in the edge area of a* windshield. This will make the glass more resistant to mechanical stresses e.g. in robot-operated installations and to thermal stresses, applied to glass as a result of fluctuations of cold and hot air. When producing long series of the same type of glass, the edge stress is provided by blasting cold air to the edge area after a bending operation. A problem in such mechanisms is that they have been designed for a certain type of glass and, when changing the glass type, they require the installation of a new blasting system, which takes a lot of time and halts production. Thus, i this method cannot be applied to flexible production, . wherein a glass design is changed often. One possible. "" * solution to the problem is set forth e.g. in US Patent publication 4 871 385, which could also be applied to flexible production. However, this prior known mecha¬ nism is complicated and expensive. An object of the invention is to provide a simple meth¬ od and apparatus for developing an edge stress also in miscellaneous production. This object is achieved by means of a method as set forth in the annexed claim 1 and an apparatus as set forth in claim 4. The invention will now be described in more detail with reference made to the accompanying drawings, in which: fig. 1 shows a cross-section of the edge portion of an apparatus of the invention and fig. 2 shows an apparatus of the invention in a plan view. The invention can be applied e.g. in a furnace described in Patent publication FI 68390, which is cited as re¬ ference in terms of the structure and operating princi¬ ple of the furnace. Of course, the invention can be applied in all types of furnaces intended for bending a windshield upon a ring mould. Inside or outside the supporting frame of a ring mould 2 is placed a metallic extra frame 3, which can be fitted in position mouldwise in a manner that the lower surface of a bent windshield 1 does not come to contact with extra frame 3. By virtue of extra frame 3 the edge of glass remains hemothermal; regardless of the shape of glass at appr. 600 - 620°C during a bend¬ ing opera ion,which temperature is required for the development of an edge stress without affecting, how- ever, the quality of final bending. Rapid cooling is effected e.g. in a furnace described in Patent publica¬ tion FI 68390 by using a hoist for carrying the glass along with its mould from a bending station to a lower pre-cooling station, the glass edge developing during this transfer a cooling flow which produces an edge stress. Other prior known and available cooling meth¬ ods can of course be used just as well. The distance of an edge stress from the glass edge, i.e. the width of an edge-stress zone, can be adjusted by varying the width and distance of extra frame 3. The variation of width can be effected either by re¬ placing extra frame 3 with another or by adapting said extra frame 3 or the components thereof to be pivot- able in a suitable manner. The extra frame 3 can be made of frame components, linked to each other by means of hinges 4, whereby one and the same extra frame can be used for bending windshields 1 having somewhat dif¬ ferent degrees of curvature. Adjustment of the dis¬ tance of extra frame 3 from the lower surface of wind¬ shield 1 can be readily effected by means of linking elements, e.g. levers (not shown) , between frame com¬ ponents 2 and 3. The extra frame 3 can be located inside or outside, or if necessary, on both sides of the supporting frame 2 of a ring mould. The degree of edge stress can also be adjusted by the regulation of cooling rate, e.g. in the case of FI Patent 68390, by the speed of a ring-mould shifting hoist as well as by the temperature of a station lo¬ cated below the hoist. The station located below the hoist can be supplied with cooling air e.g. by means of small fans or blowers, located outside the furnace and possibly controlled e.g. by means of thermoelements. An extra frame 3 fastened to bending mould 2 can be used to create an edge stress for each particular grade of glass. By virtue of the invention, there is no need for complicated external control equipment for each particular mould. The invention is applicable both to miscellaneous production and to long series, nor does it require extra time for replacing a mould. The apparatus is simple and inexpensive in its construc¬ tion. By means of the invention it is possible to create a uniform pattern of edge stress over the entire peripheral area. The extra frame 3 is preferably dimensioned in a manner that the major dimension of its cross-section extends in the direction of a glass sheet and the minor dimen¬ sion in the direction perpendicular thereto. In the present case, the extra frame 3 is cross-sectionally in the shape of a flat rectangle. The glass-oriented dimension of an extra frame is typically with the range of 1 - 5 cm, e.g. appr. 3 cm. The location of extra frame 3 is selected in a manner that its distance from the lower surface of windshield 1 is within the range of 1 - 8 mm, typically appr. 3 mm. It is obvious that the invention is not limited to the described design or dimensions, which are only intended to illustrate the invention. Many other embodiments are conceivable with¬ in the scope of the annexed claims.

Claims: Claims 1. A method for developing an edge stress in a wind¬ shield in a windshield bending furnace, wherein a glass sheet is supported on a ring mould (2) and heated close to a softening temperature and, following a bending operation, said glass sheet is cooled at such a cool¬ ing rate that a compression stress is developed at least in the edge area of said glass sheet, c h a r - a c t e r i z e d in that, at least during a heating operation, the temperature of glass edge is affected by means of a metallic frame (3), which is located adjacent to ring mould (2) and is not in contact with glass. 2. A method as set forth in claim 1 , c h a r a c t e r ¬ i z e d in that the glass-edge compression stress is adjusted by varying the distance of metallic frame (3) from glass (1) and/or by varying the glass-oriented dimension of metallic frame (3) . 3. A method as set forth in claim 1 or 2, c h a r a c ¬ t e r i z e d in that the distance of metallic frame (3) from glass is set to be substantially less than the glass-oriented dimension of said metallic frame. 4. An apparatus for developing an edge stress in a windshield in a windshield bending furnace, said appa¬ ratus comprising a ring mould (2) , supporting a glass sheet (1) which is heated, bent and cooled, c h a r - a c t e r i z e d in that adjacent to the supporting frame of ring mould (2) is mounted a metallic extra frame (3) , located at a small distance from the lower surface of a bent windshield (1). « 5. An apparatus as set forth in claim 4, c h a r a c ¬ t e r i z e d in that the major dimension of the cross- section of extra frame (3) extends in the direction of glass sheet (1) and the minor dimension extends in the direction perpendicular thereto. 6. An apparatus as set forth in claim 5, c h a r a c ¬ t e r i z e d in that said extra frame (3) is cross- sectionally in the shape of a flat rectangle. 7. An apparatus as set forth in any of claims 4 - 6, c h a r a c t e r i z e d in that said extra frame (3) is replaceably fastened to ring mould (2) . 8. An apparatus as set forth in any of claims 4 - 7, c h a r a c t e r i z e d in that said extra frame (3) consists of frame components, linked to each other by means of hinges (4) . 9. An apparatus as set forth in claim 5 or 6, c h a r ¬ a c t e r i z e d in that the glass-oriented dimension of extra frame (3) is in the order of 1 - 5 cm and its distance from the lower surface of bent glass sheet (1) is in the order of 1 - 8 mm, preferably appr. 3 mm.

Inventors: SALONEN TAUNO TAPIO, SALONEN, TAUNO, TAPIO
Applicants: TAMGLASS OY
Application year: 1991
Main classification: C03B23
IPC codes: C03B23, C03B27