Press-bending method and apparatus for bending glass sheets

Abstract: The invention relates to a press-bending method and apparatus for bending glass sheets. The press bending is effected between an upper curved mould surface (12) and a lower ring mould (13). Air is blown through pores included in an upper mould (2) into a space (3) between mould surface (12) and a glass sheet (11), air being vacuumed from the edges of said space for developing in said interspace (3) a negative pressure (P₂) capable of bearing a glass sheet (1) in the immediate proximity of mould surface (12), yet without contacting said mould surface (12).

Description: The present invention relates to a press-bending method for bending glass sheets. The invention relates also to a press-bending apparatus for bending glass sheets. The Applicant's earlier Patent publications US-5,066,320 and US-5,079,931 disclose a perforated mould provided with blow and suction channels for holding and, if necessary, also for supporting a glass sheet in the immediate proximity of an upper mould surface. Such a double-channeled perforated mould has been found highly functional and capable of bending also complicated bending shapes. However, this prior known mould technique involves certain defects and problems. First of all, the double-channeled mould is quite inconvenient to manufacture and, thus, in view of costs, one and the same mould should be capable of producing long series. Another drawback is the major pressure fluctuation caused by adjacent blow and suction perforations over a short distance. The size and relative disposition of the perforations as well as the distance of a glass sheet from the mould surface are difficult to adjust in a manner that local pressure peaks would not cause deterioration of the optics. An object of the invention is to provide a press-bending method and apparatus, which has been improved so as to overcome the above problems. The object in respect to the method is achieved by a method according to claim 1 or claim 2. The object in respect to the apparatus is achieved by an apparatus according to claim 4. One exemplary embodiment of the invention will now be described in more detail with reference made to the accompanying drawings, in which fig. 1 shows an apparatus of the invention in vertical section, and fig. 2 shows an enlarged detail in the same apparatus. The apparatus includes a mould 2, located above a glass sheet 1, having a curved mould surface 12 which matches a desired bending shape for glass sheet 1. Mould 2 is made of a porous material such that air can be blown through mould 2 into an interspace 3 between surface 12 and glass sheet 1. The edges of mould 2 are surrounded by a ring-like wall 4, defining an suction port 5 between the wall 4 and the edge of mould 2. The wall 4 is sealed against a glass sheet edge 14 and/or against a zone adjacent to edge 14. The narrow port 5 opens into a larger channel 6 which is connected with a suction channel 7 to the suction port of a fan 8. A channel between the pressure port of fan 8 and a chamber 10 is provided with an air deflector 9, deflecting some the air coming from fan 9 to the environment. Just some of the circulation air finds its way into chamber 10. The chamber 10 is defined by walls 11 and on one side by mould 2. Since some of the circulation flow is deflected by means of damper 9 to the environment, the circulation flow has a static pressure P1, P2, P3 lower than ambient pressure P0 at any point of the circulation flow, wherein pressure P1 prevails in the chamber 10 and is confined to the back side of the mould, pressure P2 prevails in the interspace between the mould surface 12 and the glass 1 and pressure P3 prevails in the suction channel 7 between the suction port 5 and the fan. In circulation flow, said mould 2 produces a certain pressure loss P 1 - P 2 . When pressure difference P 1 - P 2 is selected to be equal to or more than the weight of glass 1, said mould 2 carries the weight of glass 1 even though the entire surface area of mould 2 is solely covered by the blow, while the suction occurs primarily at the edges of mould 2. Naturally, the mould surface can be provided with grooves joining said suction port 5 and the central area of mould 2 can also be provided with a larger individual suction port if this is allowed by the glass shape (in other words, glass 1 has no tendency of pressing against mould surface 12 at this suction port). Ambient pressure P o can be substantially equal to normal atmospheric pressure. The invention can be applied particularly in cases where a glass sheet 1 is supported on a ring mould 13 at least at the initial stage of bending. When using a method and mould apparatus of the invention, a glass sheet 1 can be heated throughout to a sufficiently high temperature for bending or tempering. In this case, the negative pressure prevailing in interspace 3 prevents excessive sagging of the central area of glass sheet 1. If, on the other hand, as mould surface 12 approaches glass sheet 1, the gap therebetween diminishes beyond a certain critical point, the pressure between surface 12 and glass 1 rises within said area preventing the glass from coming into contact with mould surface 12. The position of deflector 9 can be used for regulating a pressure difference P o - P 2 for obtaining a desired bearing effect on glass sheet 1. This pressure difference can be readily brought to a sufficient value for having a glass sheet 1 entirely, without a lower support, carried by mould 2. However, a lower support is required at the initial stage of bending for bringing a glass sheet 1 close to mould surface 12 and for sealing said edges 14 against wall 4. The complete tightness between edges 14 and wall 4 is not required since, despite a certain runoff, a sufficient negative pressure can be produced in interspace 3 relative to ambient pressure P o . An essential feature in the invention is that glass 1 is carried partially or entirely by an upper mould surface 12 such that only blowing occurs therethrough. The manufacture of such a mould is very simple since there is no need for parallel double-channeling. Most preferably, the mould can be made of a porous material which allows the flow-through of air without separate perforation. Thus, no local pressure peaks are produced in interspace 3 over a short distance, either, even when glass 1 is approaching surface 12. Thus, the only pressure variations in interspace 3 are caused by the variations of a gap between surface 12 and glass 1. If surface 12 is provided e.g. with perforations having a diameter within the range of 1 - 2 mm, glass 1 shall not be subjected to pressure peaks if the gap between surface 12 and glass 1 slightly exceeds the diameter of the blow perforations. Thus, a pressure peak shall disperse equalize on the glass surface but, if said gap decreases dramatically, the effect of pressure peaks will also increase at the perforations. Thus, this can be completely avoided by using a porous mould structure, wherein the perforations in surface 12 are very small. A porous mould can be manufactured e.g. such that, during the manufacturing operation, the interior of a mould material (e.g. ceramic mass) is furnished with minute pellets or grains which disappear upon heating (melt and/or gasify). The degree of porosity can be selected by selecting the size and/or amount of pellets or grains. This is an easy way of creating a mould having its degree of porosity vary as desired within various sections of the mould. Thus, it is possible to blow more air through a certain section than through some other section. The perviousness of a mould can also be affected by the mould thickness, which can be made varying as desired within various sections of the mould. An entire mould assembly of the invention is placed in a hot furnace and temperature of the air to be circulated through the mould corresponds substantially to the bending temperature of glass 1. It is often necessary to apply heat in a locally varying manner to various sections of the glass. For this local heating the interior of mould 2 made of a ceramic material can be provided with heating resistances 15, e.g. in the form of a meshed resistance mat. It is possible to employ one or a plurality of individual resistance mat sections 15. Alternatively, the resistance mat 15 can be extended over the entire surface area of mould 2 and the subdomains of the resistance mesh mat can be individually switched on and off. In practice, it is generally sufficient to have a possibility of further heating at one or more predetermined spots where a complicated glass bending shape requires further local heating.

Claims: A press bending method for bending glass sheets (1), wherein the bending of the glass sheet is effected by using on the one hand an upper, curved porous mould surface (12) on the other hand a lower support in the form of a ring mould (13), a gas bed or alike, comprising the following steps: bringing the glass sheet (1) close to the mould surface (12) of the upper mould (2) by the lower support; blowing air through the porous material of said upper mould into the interspace (3) between the mould surface (12) and the glass sheet (1) substantially over the entire surface area of the mould surface (12); removing the air from the interspace (3) between the mould surface and the glass sheet by applying suction to the edges of said interspace (3); adjusting the relative ratio between said suction on the one hand and the blow occurring through the porous material (2) on the other hand so as to develop in said interspace (3) a glass supporting pressure (P2) lower than the ambient pressure, wherein the pressure between the mould surface (12) and the glass sheet (1) rises in those areas where the distance therebetween decreases to below a certain critical value thus preventing the glass from contacting the mould surface (12). A press bending method for bending glass sheets (1), wherein the bending of a glass sheet is effected by using on the one hand an upper, curved porous mould surface (12) and on the other hand a lower support in the form of a ring mould (13), a gas bed or alike, comprising the following steps: bringing the glass sheet close to the upper curved mould surface by the lower support; supporting the glass sheet (1) by the porous mould (2) at a small distance below the mould surface (12) of the upper mould by blowing air through the pores of the mould surface (12) into the interspace (3) between the mould surface (12) and the glass sheet (1), and by producing; a support-enhancing pressure difference (PO-P2) between the top and bottom surface of the glass sheet by means of suction applied to the edges of the interspace (3) between the glass sheet and the mould surface and wherein the pressure between the mould surface (12) and the glass sheet (1) rises in those areas where the distance therebetween decreases to below a certain critical value thus preventing the glass from contacting the mould surface (12). A method as set forth in claim 1 or 2, characterized in that some of the suction air is recirculated through the mould (2) and some of it is deflected out of the circulation flow such that said circulation flow has a pressure (P1, P2, P3) which is lower than the ambient pressure (PO) at any point of the circulation flow. A press bending apparatus for bending glass sheet (1), the apparatus comprising: an upper, curved-surface mould (2), including a curved mould surface (12) made of porous material, and a lower support element (13) for bearing a glass sheet (1) and for bringing the glass sheet close to the upper curved surface mould (2), mean for blowing air through the pores of the upper curved surface mould (2) into the interspace (3) between the mould surface (12) and the glass sheet (1), means for applying suction to a suction channel (6) wherein the suction channel (6) opens into a suction port (5), said curved mould surface (12) being at least partially surrounded by the suction port (5), for removing air from between the porous mould surface and the glass sheet to be bent when the apparatus is in use via the suction port (5) and the suction channel (6), means for adjusting the relative ratio between the suction and the blowing occuring through the porous material to develop a glass supporting pressure lower than the ambient pressure in the interspace (3). An apparatus as set forth in claim 4, characterized in that said mould (2) or at least a layer thereof adjoining the curved mould surface (12) is made of a porous material, which allows the flow-through of air without separate perforations and that the edges of the curved mould surface (12) of the mould (2) are surrounded by a wall (4) defining a suction port (5) and a suction channel (6) which is sealable against the edge (14) of a glass sheet (1) and/or against an area adjacent to this edge (14) and that said suction port (5) and said suction channel (6) are associated with the intake side of a fan (8) while the pressure side of the fan (8) is provided with an air deflector (9) for deflecting the air coming from the fan (8) partially to the environment and partially into a chamber (10) which is defined on one side by the mould (2). An apparatus as set forth in claim 4 or 5, characterized in that the suction channel (6) is connected via a suction channel (7) to the inlet of the fan (8). An apparatus as set forth in any of claims 4 to 6, characterized in that the interior of the mould (2) is provided with one or a plurality of heating resistance (15) for the local heating of air to be blown through the mould (2). An apparatus as set forth in claim 5, characterized in that the degree of porosity of the mould (2), i.e., the size and/or the density of the porous varies in various sections of the mould and the main diameter of the pores in the mould surface (12) is substantially less than 1mm. An apparatus as set forth in claim 4 or 5, characterized in that the thickness of the mould (2) varies in various sections of the mould.

Inventors: MONTONEN JORI, MONTONEN, JORI
Applicants: TAMGLASS ENG OY, TAMGLASS ENGINEERING OY
Application year: 1993
Main classification: C03B23
IPC codes: C03B23