JPS6238289B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for strengthening plate glass, and in particular to a method for strengthening plate glass in which the generation of sharp edges (elongated fragments) is extremely small when glass plates are crushed. When tempered glass is shattered, it appears to be broken into fine pieces almost uniformly, but in some cases long thin pieces called sharp edges may be generated. As automobiles become lighter, tempered glass is becoming thinner, and as tempered glass becomes thinner, sharp edges tend to occur more easily, and countermeasures against sharp edges are becoming more important. To briefly describe the fracture phenomenon of tempered glass, the fragment density (number of fractures per 50 mm square) Fd is a function of Fd = f (σ ₀ , σ), where the stress value σ of the tempered glass and the stress value σ ₀ at the point of fracture start As shown in Figure 1, the fracture of tempered glass is expressed in the form of a crack 2 that advances radially from the fracture starting point 1, and after a certain distance, it reaches a branching point 3.
It forms and branches, and then this phenomenon is repeated and it is crushed into small pieces. Furthermore, sharp edges tend to occur more often when the strengthened glass is fractured at the center than at the edges. Conventionally, in the production of curved tempered glass, a sheet glass is bent and then strengthened using a press die that is equipped with a wave-shaped heating element to prevent cooling due to contact with the press surface of the sheet glass ( Japanese Patent Application Laid-open No. 51-97553), and a method in which sheet glass is bent and then strengthened using a press die in which a plurality of heating elements are arranged in parallel on the surface of the press die (Japanese Patent Laid-Open No. 55-95629) are known. ing. However, although it is possible to increase the degree of reinforcement of sheet glass and reduce the occurrence of sharp edges by providing a heating element in the press mold as in the above-mentioned known example, this is still not sufficient. The present invention solves the conventional drawbacks, namely, that sharp edges are more likely to occur when the tempered glass is broken from the center, and that the sharp edges can be effectively reduced by particularly heating the center of the glass sheet. The present invention was developed by paying attention to the fact that this can be done. That is, the present invention provides a method for strengthening plate glass in which a plate glass heated above the strain point is pressed against a pair of press dies, and then a cooling medium is sprayed to strengthen the plate glass. The gist of this invention is a method for strengthening a plate glass, which is characterized by heating the plate glass by providing heating elements concentrically toward the plate glass. Here, the plate glass heated above the strain point is, for example, one heated to a temperature of 600 to 700°C. A pair of press dies may include a convex press die and a concave press die for forming curved glass, or a press die with a flat press surface or frame for strengthening flat sheet glass. It's okay. Air or water mist is used as the cooling medium to strengthen the sheet glass using a normal cooling method. A concentric heating element facing outward from the center is, for example, a heat-resistant substrate with a plurality of heating elements concentrically facing outward, and the temperature is increased toward the center. is preferable, and it is preferable that the heating element closer to the center generates a larger amount of heat, or that the density of the arrangement of the heating elements is higher closer to the center. The individual heating elements may also be circular, oval, spiral or polygonal. In this way, by providing the heating elements concentrically outward from the center of the press mold, a highly strengthened part is formed concentrically on the sheet glass surface corresponding to the heating element, and the tempered sheet glass is heated further away from the center. When crushed, the cracks generated radially are dispersed by the above-mentioned high-strength portions, and the occurrence of sharp edges is suppressed. In addition, when sheet glass is tempered, the central region of the sheet glass is usually less likely to be strengthened compared to the outside, and by arranging the heating element as described above, it is easy to adjust the temperature of the central region, and the central region and the surrounding region can be easily adjusted. The degree of reinforcement can be made uniform, and since the peripheral area of the glass plate is not heated more than necessary, the glass plate is not stretched at the hanging part. Next, the method for strengthening plate glass of the present invention will be explained based on the drawings. FIGS. 1, 2, and 3 show one embodiment of the present invention. The plate glass 4 is suspended by a hanging tool 5 and heated in a heating furnace to a temperature higher than the strain point, for example, 600 to 700°C, and then transferred to a press device 6 for bending. The press device 6 consists of a convex press mold 7 and a concave press mold 8, and the press molds 7 and 8 are arranged by cylinders 9 and 10 so that the plate glass 4 can be pressed against or separated from both. The convex press mold 7 is a plate glass 4
The contact surface is formed in a convex shape, and a glass cloth or the like is adhered to the surface. Further, the concave press die 8 has a press frame 11 on the outer periphery so as to correspond to the convex press die 7, and inside the press frame 11,
A plurality of heating elements 1 made of, for example, nichrome wire are arranged concentrically outward from the center on a heat-resistant substrate 12.
3 are provided at intervals of, for example, 30 to 70 mm, and each heating element 13 is electrically connected via a transformer 14. It is preferable that the heating element 13 generates more heat toward the center. After bending, the plate glass 4 is strengthened using a commonly used cooling device using air or the like. Note that the heating elements 13 may be arranged on the heat-resistant substrate 12 so as to be denser toward the center as shown in FIG. 5, or may be arranged in a spiral shape outward from the center as shown in FIG. . Example 1 A plate glass of 1200 x 700 mm and 3.0 mm thick heated to a temperature of 670 to 680°C was prepared as shown in Figures 2, 3 and 4.
After bending using the apparatus shown in the figure, air cooling strengthening treatment was performed at an air pressure of 2500 mmAq and an air flow rate of 560 Nm ³ /min. In addition, for comparison of the present invention, tests were conducted using a press mold in which a plurality of heating elements were provided in parallel (Comparative Examples 1 and 2) and one without a heating element (Comparative Example 3). Table 1 shows the results. The degree of reinforcement of plate glass is expressed by the number of fractures when the plate glass is broken at the center and corners, and the number of sharp edges is determined by the length of the broken pieces.
The length should be 60 mm or more, and the length to width ratio should be 4 or more.

[Table] According to the present invention, the number of sharp edges can be extremely reduced by providing heating elements concentrically outward from the center of the press die.
In addition, it is easy to adjust the temperature at the center of the glass plate.
This method has the remarkable effect of producing tempered glass with a uniform degree of reinforcement throughout the glass plate.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the breaking phenomenon of tempered glass. FIG. 2 is a schematic side view of a press device used in carrying out the present invention. FIG. 3 is a schematic front view of the concave press die shown in FIG. 2. FIG. 4 is a schematic diagram showing the wiring of the heating element of FIG. 3. 5 and 6 are schematic front views of other embodiments showing the arrangement of heating elements of the present invention. 4...Plate glass, 6 ...Press device, 7...Convex press type, 8...Concave press type, 12...Heat-resistant substrate, 13...Heating element.

Claims (1)

[Claims] 1. In a method for strengthening plate glass in which plate glass heated above the strain point is pressed against a pair of press dies and then strengthened by spraying a cooling medium, at least one of the pair of press dies is formed with concentric circles extending outward from the center. 1. A method for strengthening plate glass, which comprises heating the plate glass by providing a heating element in the shape of the plate.