Glass manufacturing – Processes – Reshaping or surface deformation of glass preform
Reexamination Certificate
2000-11-27
2001-11-20
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes
Reshaping or surface deformation of glass preform
C065S025100, C065S025400, C065S286000, C065S287000
Reexamination Certificate
active
06318125
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a process for bending glass sheets with a surface-shaping mould, against which the glass sheets are pressed using a pressure drop produced by sucking out the air from the space lying between the moulding face and that face of the glass sheet on the opposite side from the latter, the pressure drop being a maximum at the edge of the glass sheet and decreasing towards the middle of the latter, because of the fact that air is introduced into the region of the moulding face via air-inlet apertures, as well as to a device suitable in particular for implementing such a process.
DISCUSSION OF THE BACKGROUND
Document EP 0 241 355 B1 discloses a relevant device in which a skirt surrounds the outer edge of the shaping mould with a peripheral annular slot. Through this annular slot it is possible to suck out the air lying in the space between the glass sheet and the face of the shaping mould. The resulting difference in pressure with respect to the atmosphere acting on the free face of the glass sheet presses the latter against the moulding face. Next, a pressing ring is applied against the free face of the glass sheet, the said pressing ring giving the edge of the sheet the desired final outline.
Direct contacts between metal moulding faces and surfaces of glass sheets pressed against the latter are prevented, in a known manner, by means of interlayers of heat-resistant fabric (see EP-Al-0,767,146). However, in the presence of high application pressures and/or large air pressure differences, it is not possible to completely avoid visible traces of the fabric in the surface of the glass sheet (which is malleable at the shaping temperature).
In an alternative form of the known device, hot air under pressure is therefore blown in between the moulding face and the surface of the glass sheet via channels uniformly distributed over the face of the shaping mould. Thus an air cushion is supposed to be formed, on which the glass sheet floats. It is also possible, with this mould, to shape glass sheets whose central zone is enamelled, or alternatively provided with screen printing, and/or to obtain particularly good optical quality without traces of fabric.
U.S. Pat. No. 5,669,952 discloses a process and a device for sagging glass sheets particularly deeply, in which an upper shaping mould with an entirely convex surface interacts with a lower shaping ring. The glass sheet, deposited on the ring and already preformed by gravity, is subjected to a cushion of pressurized air via outlet channels in the central region of the moulding face, after having been brought close to the upper shaping mould. This cushion of air is used here as a deep-drawing punch, as it is supposed to deepen the shaping of the glass sheet even further without surface contact with the moulding face. In an alternative embodiment, the upper shaping mould has several compartmentalized chambers. It is thus possible to create various pressure levels over the outline of the moulding face. In another alternative embodiment, exhaust apertures are provided in the peripheral region of the face of the shaping mould. They emerge in free pressure chambers (in communication with the atmosphere) inside the upper shaping mould. In this way, the blown-in air can again escape.
Shaping moulds having several vacuum chambers are also known (U.S. Pat. No. 4,877,437). Generically, these create a vacuum over the entire face of the glass sheet applied against the shaping mould. The pressure may again be lowered locally for particularly short radii of the face of the shaping mould, so as here again to ensure good application of the face of the glass sheet against the moulding face and to exclude any bulging. With these known shaping moulds acting purely under vacuum, it is also not possible to remove the impressions of the fabric when several different pressure levels are provided. Even with a mould which has suction apertures only in the edge region, pressure contact of the glass sheet with the moulding face cannot be avoided in the intermediate region. In fact, the shorter the distance between the glass sheet and the mould, the more rapidly the air still present flows and the greater the pressure difference in the region in question with respect to atmosphere.
Finally, document EP 0,182,638 B1 discloses a plane transport plate for glass sheets, the working face of which includes both suction apertures, for lifting and supporting the weight of the glass sheets by vacuum, and air-inlet apertures for producing an air cushion serving as a spacing element between the glass sheets and the working face. This arrangement, in which the suction and air-inlet apertures are uniformly mixed over the entire face, is also supposed to prevent degradation of the sensitive surfaces of the glass sheets by avoiding any mechanical contact.
SUMMARY OF THE INVENTION
The object of the invention is to further optimize a process for shaping glass sheets, in which a pressure level is produces, which varies from the edge towards the centre, between the moulding face and the glass sheet for the purpose of avoiding any trace of the pressing operation, as well as to provide a device which is suitable in particular for implementing this process.
According to the invention, this objective is achieved by the characteristics of Claim
1
with regard to the process. The characteristics of Claim
6
indicate a corresponding device. The characteristics of the secondary claims respectively associated with the independent claims indicate advantageous variants of the said subject-matter.
Preferably, hot air is delivered, specifically in so far as there prevails, in the region in question, as low a pressure as possible with respect to the pressure level acting beyond the glass sheet—and therefore with respect to atmospheric pressure or to a holding pressure for example. In order to avoid impressions of the fabric, it is absolutely not sufficient to bring the air-inlet chambers, or parts of the moulding face, into communication only with the atmosphere, as is known in another respect from the prior art examined. In order to reduce the vacuum, a certain positive introduction of air at a limited, or in other words adjustable, pressure is necessary. Moreover, contact of the glass sheet heated to the shaping temperature with insufficiently heated atmospheric air could have a negative impact on the result of the shaping because of undesirable cooling.
By limiting the injected pressure between the face of the glass sheet and the moulding face and by sucking out the hot air introduced which is still within that region of the moulding face which is covered by the glass sheet, any bulging is avoided, which cannot be completely excluded in the prior art. Of course, controlling the pressure ratios requires a great deal of care. As a result of the controlled delivery of air provided according to the invention, preferably towards the centre of the glass sheet, the contact forces between the glass and the mould, or the fabric, in the central region are minimized in a reproducible manner.
Particularly preferably, the air introduction zone is located in the central region of the moulding face and of the corresponding glass sheet, and the air introduced is again evacuated around this region. It has proved particularly effective in this regard to subdivide, into three zones, the level of pressure existing here in the space between the moulding face and the glass sheet. The outermost of these zones, which extends along the edge of the shaping mould, is, as previously, subjected to a high vacuum, so as to fasten the glass sheet to the moulding face. The intermediate zone is subjected to a lesser reduced pressure, so as, on the one hand, also to contribute to some extent to the retention of the glass sheet and, on the other hand, and mainly however, to extract again along a short path the air introduced into the inner or central zone.
If the moulding face itself of the shaping mould includes, in addition to the air-inlet apertures, suctio
Dahlhoff Knut
Diederen Werner
Heining Klaus-Peter
Korsten Wilfried
Ollfisch Karl-Josef
Colaianni Michael
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Saint-Gobain Vitrage
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