Glass manufacturing – Processes – Reshaping or surface deformation of glass preform
Patent
1997-05-28
1999-04-13
Silverman, Stanley S.
Glass manufacturing
Processes
Reshaping or surface deformation of glass preform
65106, 65107, 651822, 65279, 65291, C03B 23035
Patent
active
058939415
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method of bending and tempering of a glass sheet, wherein in the first step the glass sheet to be bent is heated to its tempering temperature in the second step the glass sheet is bent to the bending configuration, and in the third step the glass sheet is tempered. During application of the method, the glass sheet is supported on a substantially annular supporting means. During application of the method, the glass sheet is subjected to a different pressure effect on its upper and lower surfaces.
BACKGROUND ART
In view of the prior art reference is made to publication U.S. Pat. No. 4,437,871 disclosing a bending and tempering method, where in the glass sheet is heated in a roller furnace in which the glass sheet is transported forward on rotating ceramic rollers. When the glass sheet is heated and reaches its softening temperature, its bending downwards between the rollers is prevented by the continuous movement of the glass sheet, wherein the location of supporting points, supporting the glass sheet from below, is continuously changed. In this manner, the glass sheet remains straight.
According to the method, heating of the glass sheet is continued until the glass sheet reaches also the tempering temperature which is approximately 620.degree. C. In the next step of the method, the glass sheet is supported by rollers in the furnace apparatus at a precisely predetermined point, a vacuum mould at said point above the rollers grips the glass sheet and elevates it upwardly apart from the rollers. This step of the method requires very high precision, because the positioning of the glass sheet must be precisely determined both in view of directional displacements and rotation. Positioning errors at this step result in faulty dimensions in the finished bend and tempered glass, particularly at its margins. An excessive position error will naturally damage the whole glass. The glass sheet is elevated by the vacuum mould up from the rollers to such an extent that a bending/tempering frame can be brought under the glass sheet from outside the furnace apparatus. The glass sheet is dropped onto the bending/tempering frame from a certain height, which is usually about 30 mm. After the dropping, the glass sheet is bent by gravity into the mould shape determined by the bending/tempering frame, and it is pulled out from the furnace apparatus. In the next step, the tempering is conducted with air jets. The bending of the glass sheet is relatively quick; with the most common glass thicknesses of 3-4 mm the bending will take about 1 to 3 seconds.
The method known from U.S. Pat. No. 4,437,871 is advantageous in certain respects, particularly in large-scale production for manufacturing simple bent and tempered glass shapes in large series. However, the method usually involves several disadvantages, the most important of which is the requirement that the glass sheet must be precisely positioned at the elevating step, which is difficult in quick serial production and causes losses, as mentioned above. Another disadvantage is the fact that the vacuum mould must be coated with a soft, heat-resisting coating material to prevent damage to the surface of the hot glass sheet. This coating material must be replaced from time to time, which will naturally cause production losses and replacement costs. Furthermore, application of the method to screencoated glass sheets will reduce the replacement time, because the silk screen ink used will stain the coating material, thus increasing the need for a replacement. Another disadvantage is also the fact that the glass sheet must, in practice, be dropped from a height of at least 20-25 mm, which may cause deformation of the glass sheet particularly at its margins.
Further in view of the prior art, a method is known from Finnish Patent No. 86054 where the glass sheet is first bent by gravity to a desired shape, using an edge mould. Thus the temperature of the glass sheet, particularly in its central areas, is clearly below 600.degree. C. After
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Silverman Stanley S.
Vincent Sean
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