METHOD AND APPARATUS FOR CONTROLLING BENDING OF A GLASS IN A GLASS BENDING OVEN

Abstract: A method and apparatus to control glass sheet bending in a glass sheet bending furnace, where the glass sheet (1) is supported with an edge mould (4, 8) and the bending takes place mainly due to the force of gravity. The distribution of glass (1) temperature is influenced by blasting air into a substantially flat and small-mass box (3) placed beside the glass in order to produce a cooling effect on the glass sheet and through convection at least at the said box.

Description: METHOD AND APPARATUS FOR CONTROLLING BENDING OF A G L ASS IN A GLASS BENDING OVEN The invention relates to a method and apparatus , by means of which bending produced by the force of gravity in a glass sheet bending furnace is controlled by restricting the increase of temperature on a required area in the glass sheet. On using an edge mould for glass sheet bending, the problem has been that due to gravity the glass sheet bends too much, generally in the centre of the sheet before the glass sheet edge areas have even reached the required degree of bending. That is the case especially on bending big size glass sheets with an edge mould. That kind of glass sheets are typically bent in a one-chamber or multi-chamber furnace, where heating is carried out with radiation elements above the glass sheet. The problem is reduced in connecting radiation elements above the area of strongest bending only. For reduction of the problem accessories are known, among others different kinds of plates to shade the radiation of heating elements above the glass sheet, where excessive bending could take place. Also massive metal plates under the glass sheet have been locally used for reduction of glass sheet heating. The disadvantage of these optional accessories is slowed-down heating speed because of the glass breaking risk. The massive plate and shading cause differences of temperature in the glass sheet and that is why maximum heating speed cannot be applied in temperatures of 20° — 500°C. Only at temperatures over 500°C even great differences of temperature can be main- tainted and said accessories will not be a disadvantage. They make the heating time longer and the massive plate lengthens also the cooling time. These facts show also that due to the above accessories the bending process uses more energy. With a method and apparatus according to this method the current problem is solved and the invention is characterized in what is presented in the patent claims. The object of this invention is to produce a small-mass cooling surface, which is at least in the bending section and in connection with the bending glass sheet causing differences of temperature into the glass. A box type horizontal surface, made for instance of two metal sheets and placed under the glass sheet, does not hinder the glass sheet from getting heated. The surface can be set in function advantageously only by blasting compressed air at a certain time. When fed air is steered into and out of the box type construction, it does not cause any interference of flow in the furnace. By means of the temperature and quantity of compressed air the surface temperature can be adjusted and thus the effect on the glass sheet. Cooling by the box is used only at the final stage of heating, when harmful bending would begin, at a temperature over 550°C. Thus real heating phase can be done quickly. By means of the method and especially in connection with laminated windshields, it is possible to make sure that on bending two glass sheets the centre portion of the lower sheet does not bend excessively due to the force of gravity. This can easily happen if trying to restrict bending by prior known means from the glass sheet upper side, for instance by shading resistance elements. Then the upper one of the overlapping glass sheets may even get less heated than the lower sheet and remain more straight. The most advantageous embodiment is a box underneath the glass sheet, whereat much surface is achieved with as little mass as possible. Such a box can be fixed in the edge mould, the edge mould chassis or the bending section of the furnace. It can be also built in the bending section of a series furnace, into which the edge moulds move, for instance, in turns along a track and whereby mould and glass sheet can stop at it and move across it. In the following the invention is disclosed with reference to the enclosed drawing, where Fig. 1 is a side view of a glass sheet in an edge mould. Fig. 2 is a diagonal view of a bent glass sheet in an edge mould in the furnace. Figure 1 shows a hinged edge mould on which glass sheet l is installed. Holders 4 of the mould edges are by means of joints 5 hinged to be turning in a way known as such. Due to weights 6 the glass sheet rests at first on several points till the glass sheet bending and proper design of the weights 6 finally turn holders 4 up to their overrunning stops, whereat the edges of the glass sheet middle portion sink onto the mould holders 8. A box according to the invention is placed in the mould so that it is under the glass sheet about in the glass sheet middle portion. Figure 2 is a diagonal representation of the mould in a 1- chamber furnace, which is a typical bending furnace of large windshields. The glass has got bent to its final shape according to the edge mould. In both glass sheet edges there is a small bending radius due to which heat must have been directed at the most to these glass edge portions, so that bending has been possible by the force of gravity. The glass sheet gets heated excessively also in its middle portion which is still without support. By means of a cooled box 3 as per this invention excessive bending of the glass sheet from its middle portion has been prevented in keeping the glass temperature lower. Then the glass sheet stiffness in its middle portion has been retained and there has been no excessive sagging. Cooling compressed air has been conveyed into box 3 when the temperature of glass middle portion has exceeded 550°C. For conveyance of compressed air there are connections 7 in the box. The compressed air is steered off the box along a tube to the outside of the furnace in order to avoid disturbing flows in the furnace. The box causes a cooling effect and heat radiation that is shifted over from the glass to the box and, to a low degree, as convection between these two. The distance of box 3 can be adjusted with respect to the glass sheet, whereat a certain adjustment of capacity is achieved. It is also possible to adjust the compressed air quantity or the temperature and achieve different cooling effects on the glass. If necessary, with the shape and size of the box different cooling effects are produced, likewise in using several boxes disposed in required spots, if necessary. Instead of a 1-chamber furnace a box as per the invention can be used in a series furnace. It can be installed firmly into the bending section, whereat the mould and glass sheet get along a track to its upper side or, transferably, into the bending section, whereat it is moved, when needed, to the glass lower side by a mechanism. A box as per the invention can also be fixed to an edge mould moving along the track in a series furnace, where in the bending section compressed air is connected to it. It can also be in the mould chassis and from there be adjusted into different levels. Instead of the box a plate can be used with a heat conductive coil underneath, through which the compressed air passes.

Claims: PATENT CLAIMS 1. A method to control glass sheet bending in a glass sheet bending furnace, where the glass sheet (1) is supported, for instance, with an edge mould (4,8) and the bending takes place mainly due to the force of gravity, characterized in that the distribution of glass (1) temperature during bending is influenced by blasting air into a substantially flat small-mass box (3) placed in the glass bending direction either on the upper or lower side of the glass sheet in the furnace, in order to produce a cooling effect on the glass sheet (l) and through convection at least at the said box. 2. A method according to patent claim 1 characterized in that air is conveyed to box (3) when the glass temperature is over 450°C. 3. A method according to patent claim 1 and 2 characterized in that the box (3) size, shape or quantity is chosen according to the glass type. 4. A method according to any of the above patent claims 1-3 characterized in that in the bending furnace box (3) is placed on the opposite side of glass sheet (1) with respect to the heating elements. 5. A method according to any of the above patent claims 1-4 characterized in that in a 1-chamber furnace box (3) is installed in connection with the edge mould (4,8), for instance) . 6. A method according to any of the above patent claims 1-3 characterized in that in a multi-section series furnace box (3) is installed into the bending section, either as a fixed equipment or as an equipment moving along a track with the mould. 7. An apparatus to control glass sheet bending in a glass sheet bending furnace, where the glass sheet (1) is supported, for instance, with an edge mould (4,8) and the bending takes place mainly due to the force of gravity, characterized in that the apparatus includes one or several substantially flat small-mass boxes (3) placed in the glass sheet (1) bending direction either on the upper or lower side of the glass sheet and, connected to the box, a compressed-air piping in order to convey air in to and out of the box during glass bending. 8. An apparatus according to patent claim 7 characterized in that the box is flat in the glass sheet (1) direction and built and designed to be firmly fixed to mould (4,8). 9. An apparatus according to patent claim 7 characterized in that in the glass sheet bending furnace box ( 3) is placed in the bending section and takes its position or is moved along a track to the connection of the mould in said section.

Inventors: VAEHAE-ANTILA KARI, VAEHAE-ANTILA, KARI
Applicants: GLASSROBOTS OY, VAEHAE ANTILA KARI, VAEHAE-ANTILA, KARI
Application year: 1997
Main classification: C03B23
IPC codes: C03B23