Patent FR-2909372-A1 (FR)
Title: Manufacturing non-planar glass-ceramic product useful in e.g. windows, comprises heating sheet in precursor glass of glass-ceramic in conventional furnace, forming heated sheet, cooling formed sheet, and ceramming formed and cooled sheet
Inventor: FREDHOLM ALLAN
IPC: C03B5/16
Filed: 2006-12-05
Abstract: The method for manufacturing a non-planar glass-ceramic product comprises heating a sheet in a precursor glass of glass-ceramic in a conventional furnace (3) so that the sheet is formable while avoiding its ceramming, forming the heated sheet, cooling the formed sheet (2b), ceramming the formed and cooled sheet, determining a range of working viscosities of a reference sheet in the precursor glass, and determining a range of furnace operating temperatures, delimited by a lower temperature and an upper temperature. The sheet has a thickness of 3-7 mm. The method for manufacturing a non-planar glass-ceramic product comprises heating a sheet in a precursor glass of glass-ceramic in a conventional furnace (3) so that the sheet is formable while avoiding its ceramming, forming the heated sheet, cooling the formed sheet (2b), ceramming the formed and cooled sheet, determining a range of working viscosities of a reference sheet in the precursor glass, and determining a range of furnace operating temperatures, delimited by a lower temperature and an upper temperature. The sheet has a thickness of 3-7 mm. The working viscosities at which the reference sheet is able to deform before the onset of ceramming. The lower temperature allowing the reference sheet to be brought to a deformable state of less than 5 minutes, and the upper temperature corresponding to the maximum temperature of the furnace to which the reference sheet is exposed, for 30-90 seconds, before the onset of ceramming. The determination of the range of working viscosities comprises determining pairs of temperature/viscosity values (10 2>-10 1> 2> Pa.s (10 3>-10 1> 3> Poises)) for the precursor glass, deducing, from the viscosity and temperature values, the coefficients A and B of the equation representing the curve relating viscosity to temperature of Log(Log(mu v))=A-BxLog(T v), where T v is the temperature of the precursor glass in degrees Celsius, mu v is the viscosity in poises and Log is the decimal logarithm or base 10 logarithm, determining the highest temperature of the reference sheet in the precursor glass to which the reference sheet is brought, after it is placed in a furnace for 30-90 s, without showing any onset of ceramming, and the lowest temperature of the reference sheet, and deducing a range of working viscosities in which the reference sheet is able to deform without initiated ceramming, from the viscosity curve and the temperatures. Other precursor glass has a composition close to that of the precursor glass, reasoning by analogy and considering that the range of working viscosity of the reference sheet is the same as that of the sheet in other precursor glass. Heating a conventional furnace containing a mold whose working surface is connected via channels (6a, 6b), through the structure of the mold, to a vacuum-creating device, up to a temperature (TF) lying within the range of operating temperatures, placing a sheet in the precursor glass on the mold, leaving the sheet on the mold in the furnace at the temperature (TF), until it reaches a viscosity lying within the range of working viscosities, applying a vacuum, at the channel of the mold, to the sheet that has reached the viscosity, for enabling the sheet to assume the shape of the mold, and removing the non-planar sheet (2b) obtained from the furnace and leaving it to cool, and proceeding with ceramming the non-planar sheet in the precursor glass, to obtain the desired non-planar product in glass-ceramic. The working surface of the mold has a shape adapted to impart the desired shape for the glass-ceramic to the sheet of precursor glass. The sheet is preheated before its introduction into the conventional furnace. The cooling of the non-planar sheet is controlled. The precursor glass is a precursor glass of a glass-ceramic containing a solid solution of beta-quartz or beta-spodumene as main crystalline phase. The range of working viscosities of the reference sheet is 10 6> .> 3> 0>-10 7> .> 7> 5> Pa.s (10 7> .> 3> 0> and 10 8> .> 7> 5> Poises) and/or the working viscosity of the furnace is 10 5> .> 6>-10 7> .> 4> Pa.s (10 6> .> 6>-10 8> .> 4> Poises). The conventional furnace comprises a lower part (3a) and an upper part (3b) able to be separated from each other in part. The sheet placed on the mold, before application of the vacuum via the channel arranged in the centre of the mold, is stabilized on its periphery by mechanical means and/or by application of a vacuum via channel arranged on the periphery of the mold.