Glass manufacturing – Processes – Glass preform treating
Reexamination Certificate
2000-08-22
2002-04-16
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
Glass preform treating
C065S104000, C065S194000, C065S348000
Reexamination Certificate
active
06370917
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a tempered glass and a method and an apparatus for quenching a glass sheet to obtain the tempered glass sheet and more particularly, the present invention relates to a thin tempered glass having a large surface area and a complicated curved surface, such as a back light glass for automobiles, and a quenching method and a quenching apparatus for tempering the glass sheet of reduced thickness.
DISCUSSION OF BACKGROUND
A tempered glass is used for a window glass for automobiles except for a front windshield glass. There are official regulations on fragmentation of tempered glass from the standpoint of safety so that the driver or a passenger is prevented from injuring. A window glass for automobiles is not permissible to use unless the window glass satisfies requirements described in the official regulations.
For example, in one of the regulations on the tempered glass for automobile windows, there is a regulation concerning a state of fragmentation of glass produced when a localized impact is given to the tempered glass. Specifically, an area in which a number of the fragments of a glass sheet broken by an impact is minimum and an area in which a number of the fragments is maximum are selected, and the minimum and maximum numbers of the fragments in these areas have to fall in permissible ranges. The maximum size of glass particles produced from a fractured glass sheet is determined from a minimum permissible number of the fragments. When the maximum size is small, a danger of suffering injury from larger fragments is reduced. Further, the minimum size of the fragments produced by the fracture of the glass sheet is determined by a maximum permissible number of the fragments. When the minimum size is large, a danger of entering of glass particles into a human body is reduced. ECE standards or JIS standards rule the magnitude and so on of fragmentation of glass sheet when fractured. In ECE standards (E6), for example, it is required that a number of fragments in any 5 cm×5 cm square should be 40 at the minimum and 400 at the maximum (except for a belt-like region of 20 mm from the edge of the glass sheet and a circular region of 75 mm radius having the center which is the point of initiating breakage). In the following description, the maximum value of a number of fragments of glass is referred to as the maximum number and the minimum value is referred to as the minimum number. Further, there are requirements that when a glass sheet is broken, edges of fragments should not be sharp and elongated fragments having a length of 75 mm or more should not be produced. Further, there is a requirement that the surface area of a fragment should not exceed 3 cm
2
.
A tempered glass can be formed by heating a glass sheet to a temperature near the softening point of the glass (usually about 600-700° C.) and quenching it by supplying cooling air. The cooling air is blown to the glass sheet through a plurality of cooling nozzles disposed near both surfaces of the glass sheet. Thus, a temperature difference is given to the glass sheet between a surface portion and the inner portion of the glass sheet at the time of quenching so as to from a compressive stress layer in the glass surface finally solidified, whereby the glass sheet is tempered.
Recently, weight reduction is required for automobiles to reduce fuel cost and so on. With this, there is an increased demand of reducing the weight by reducing the thickness of glass sheets. Using a glass sheet of about 4-6 mm thick, a tempered glass satisfying the above-mentioned requirements can easily be obtained by the above-mentioned glass tempering method (quenching method). However, when a thin glass sheet is to be formed to meet the requirement of weight reduction, it was difficult to obtain a tempered glass satisfying the regulations by the above-mentioned tempering method because a sufficient temperature difference could not be form between the surface and the inner portion of the glass sheet due to the glass sheet being thin.
In concepts, there are considered various measures to increase a pressure of cooling air; to bring the nozzles closer to the glass sheet; to reduce the distance (pitch) between nozzles and so on in order to provide a sufficient temperature difference between the surface and the inner portion of the glass sheet. An attempt of increasing a pressure of cooling air is not realistic because there is a limit in terms of mechanism in a blowing device or a compressor.
It is necessary that the cooling air is supplied to the glass sheet to assure a way of escape of the cooling air after it impinges on the glass sheet. If the cooling air, after impingement, stays there, the cooling air prohibits successively supplied cooling air from impinging on the glass sheet whereby it is difficult to obtain uniform blowing of cooling air to the glass sheet. When the nozzle pitch is reduced or the nozzles are brought closer to the glass sheet, the way of escape of cooling air, after the impingement on the glass sheet, can not be assured.
Further, there has been proposed a method of oscillating the glass sheet at the time of blowing cooling air for tempering the glass sheet, whereby the glass surface is uniformly quenched. In this method, when the nozzles are brought closer to the glass sheet, the oscillated glass sheet may interfere with the nozzles. In particular, when the glass sheet is shaped to have a complicated curved surface, there is a large possibility of interfering of the nozzles with the glass sheet.
There has been proposed to conduct a tempering treatment with a special arrangement of nozzles so that a tempered glass of thin thickness can be obtained. The proposal is to control the propagation of fracture of glass by forming areas of different principal stress in the glass sheet.
Here, description is made as to a direction of the principal stress and a principal stress difference in a glass sheet. First, a plane which is perpendicular to the glass sheet surface (a cross-sectional plane of the glass plate) is selected from the glass sheet and then, a point is selected from the selected plane. Various angles with a line in parallel to the glass surface are selectable from the selected plane. Stresses in a direction perpendicular to the selected plane acting on this point are unequal depending on angles of the selected plane. So, there is one selected plane which has the largest stress and the smallest stress, which are perpendicular to each other, when a certain angle is selected from among the various angles. The principal stress direction is defined as the direction of the largest stress and the smallest stress. Hereafter, the direction of the largest stress is referred to as the principal stress direction, as representative. Further, the largest stress and the smallest stress (i.e., the stress in the direction perpendicular to the direction which indicates the largest stress) is a principal stress difference. In a tempered glass, the principal stress is estimated from the principal stress difference which is obtained with a photoelasticity method. The principal stress difference of the tempered glass corresponds to a value obtained by dividing the sum of values of the difference between the largest stress value and the smallest stress value at points aligned in the glass sheet thickness direction by the thickness of the glass sheet (an average value obtained by dividing an integrated value of the difference between the largest stress value and the smallest stress value by the thickness). Namely, when a certain point is selected in a surface of the glass sheet, an averaged integrated value of the difference between the largest stress value and the smallest stress value at points aligned in the direction of the thickness from the selected point, is referred to as the principal stress difference at the selected point (the principal stress direction in this case is referred to as the principal stress direction at this point).
For the tempered glass in which there
Kato Yasumasa
Nagata Jun
Seto Shigeyuki
Yoshida Satoshi
Asahi Glass Company Ltd.
Vincent Sean
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