JPH08217467A - Manufacturing method of cover glass for wristwatch - Google Patents

Abstract

(57) [Summary] [Purpose] Not only does the problem of microscopic microcracks remaining on the glass surface disappear when the glass is hot-pressed, but it is also hot-pressed by a press die with a mirror-finished temperature difference. By being molded, the glass surface is rapidly cooled and subjected to heat strengthening treatment, and the breaking strength of the glass is increased without using the chemical strengthening method by ion exchange. [Constitution] To heat the glass to a working temperature range of 700 to 1000 ° C, press-mold the softened glass by the pressing method, and transfer it to the mirror surface shape of the press die surface to quench the glass while molding. Thus, a compressive stress on the glass surface and a tensile stress inside the glass are obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for increasing the strength of a cover glass for a wristwatch and a strengthening treatment method, and a cover glass for a wristwatch having a small diameter and a thin diameter without using a usual chemical strengthening treatment method by ion exchange. The present invention relates to a glass forming process capable of strengthening glass.

[0002]

2. Description of the Related Art Conventionally, the types of materials for cover glasses for wristwatches are blue plate (soda glass) and white plate (B-27).
0), BK-7 (borosilicate glass), and special glass (high-strength glass) with alumina added, and various glasses suitable for the purpose of use are diverted to the wristwatch cover glass.

Generally, a method of processing a cover glass for a wristwatch is such that one large fixed-size plate glass is cut into a rough size of a size required for a watch cover glass, and a shape is formed by cutting to an outer diameter shape and an outer diameter dimension. After processing, further polishing the glass thickness direction of the upper and lower surfaces to complete the appearance mirror surface,
In order to increase the strength of the glass material, it is necessary to strengthen it in the final process.

This strengthening treatment is carried out in order to make up for the drawback of the glass that the watch cover glass is so thin that the general thickness is in the range of 0.8 to 1.5 mm, so that it easily breaks at the time of impact or collision of a drop. A chemical strengthening method is well known as a strengthening method for increasing the strength of thin glass that is easy to do.

The chemical strengthening treatment is called an ion exchange method, and is generally a molten salt solution of potassium nitrate (KNO ₃ ) (about 400
° C.) was immersed for several hours, Na ⁺ and K ⁺ is replaced in potassium nitrate present in the glass composition, by entering the glass surface with a slightly larger K ⁺ several μm depth of the ionic radius, the glass surface The compressive stress increases, the surface hardness increases, and the breaking strength of the entire glass plate is dramatically increased.

In addition to this, the air-cooling strengthening treatment is generally known as a means for strengthening glass. Wind-cooling tempering is a process of quenching heat treatment of molded glass in a high temperature state with cold air gas.This can be applied to large glass plates such as window glass of buildings and window glass of automobiles, It is impossible to process with the thin and small size of the cover glass,
Under the present circumstances, only the above-mentioned chemical strengthening method is substantially used as the strengthening method for the watch cover glass.

Further, not only the watch cover glass but also the surface of general glass which has been polished on the upper and lower surfaces may have minute microcracks and internal strain on the surface of the glass when the polishing is insufficient. Yes, it is one of the causes of cracking due to reduced strength. The microscopic microcracks remaining on the glass surface can be removed only after annealing by heating the glass itself to near the glass softening point after the forming process and then performing an annealing treatment, but after the annealing treatment, the strength of the glass increases again. Therefore, it was necessary to carry out chemical strengthening treatment.

[0008]

DISCLOSURE OF THE INVENTION The present invention not only eliminates the problem of minute microcracks remaining on the glass surface by hot press molding of glass, but also mirror-finished a press metal sheet with a temperature difference. A method for strengthening glass and a method for manufacturing the same, in which the glass surface is rapidly cooled by a mold to carry out a heat strengthening treatment to increase the breaking strength of the glass without using a chemical strengthening method by ion exchange.

As the glass material, glass that can be generally melt-formed such as white plate, blue plate, and BK-7 is used, and the glass is heated to a working temperature range of 700 to 1000 ° C., which is near the softening point of each glass material, By press-molding the glass with a mold set to a temperature that is -50 to 100 ° C lower than the transition temperature of each glass material, the softened glass will be affected by the cooling temperature difference between the inside of the glass and the glass surface during press molding. ,
Compressive stress is obtained on the glass surface, and the breaking strength of the glass is increased as in the case of the air-cooling tempering.

That is, since the glass surface is in a softened state by being heated to a high temperature immediately before press molding, when it is press molded, it is rapidly cooled while being transferred to the mirror surface shape of the mold. For this reason, compressive stress remains on the surface of the molded glass plate and tensile stress remains inside the glass, so that the breaking strength increases due to the strengthening treatment effect similar to the air-cooling strengthening.

The heating temperature range of the glass is 700 to 1000 ° C., preferably 800 to 900 ° C., the glass is not softened at 700 ° C. or less and cannot be molded, and the glass at 1000 ° C. or higher cannot be completely formed. It will melt. Also, the press die temperature is minus the glass transition temperature of each glass material.
If there is no temperature difference within the temperature setting range of 50 to 100 ° C., the strengthening effect of the present invention will be weakened.

Further, by transferring the mirror surface shape of the press die, the occurrence of fine scratch microcracks due to the processing of the glass surface also becomes a factor to increase the strength. The mold material used is preferably one having excellent heat resistance, welding resistance and releasability, and one having a low coefficient of thermal expansion is preferable, and a cemented carbide, a ceramic, a sintered body of a metal oxide or the like is suitable. Further, these may be coated with a platinum alloy or the like.

[0013]

[Embodiment 1] An embodiment of the present invention will be described with reference to FIG.
First, white plate (B270) glass 1 is 2.5 mm thick and has an outer diameter of 50 mm.
After cutting into pieces and preheating, the pieces are set on the stainless steel frame 2 of the press molding machine and heated with the burner 8 to a predetermined temperature 83
Heated to 0 ° C. At the same time, an electric heater 7 is provided with a press die 3 in which a platinum alloy is coated on a cemented carbide material (WC) whose glass contact surface portion is mirror-finished in a shape as shown in FIG.
While heating at, the temperature is set to 520 ° C., which is lower than the transition point temperature of the white plate which is the material of the glass 1 to be pressed.

After both the temperatures have been set, pressure is applied to the softened glass 1 in the stainless steel frame 2 of the press molding machine from the direction of the arrow by the press mold 3, and the mirror surface portion of the press mold 3 as shown in FIG. Press molding glass 1 while cooling with
Box-shaped glass 4 was obtained. In this step, the press pressure was 15 to 20 kg / cm ² , and the press molding treatment time was about 8 seconds.

At this time, due to the temperature difference between the set temperature of the press die 3 and the glass temperature heated to the glass softening temperature, the glass surface is rapidly cooled at the contact surface of the press die 3 (39
(° C / sec), while the inside of the glass is kept warm without being affected by the mold temperature from the surface contact portion, so the glass is cooled from the contact surface.

Press molding was repeated in the same manner, and the outer diameter of the molded box-shaped glass 4 was ground to a shape (φ23 mm) as shown in FIG. 3 to prepare about 320 sheets of glass 4.

Using the glass 4 as a strength test sample, a breaking strength test was carried out using a load tester. As comparative samples, those subjected to normal mechanical polishing were chemically strengthened and those not reinforced were subjected to a load fracture test, and the measurement results are summarized in Table 1. Figure 4 shows the load test method
Was carried out by pressing the center portion of the glass 4 with the load measuring terminal 5 shown in Fig. 3 and the maximum load value at the time when the glass 4 was broken was recorded.
A total of 300 lots of 300 lots were actually measured with 100 lots of each glass 4 as one lot, and an average value for each lot was calculated.
(In Table 1, the glass of the present invention is A, the chemically strengthened glass is B, and the unstrengthened glass is C.)

[0018]

[Table 1]

[0019]

[Embodiment 2] Similarly, press forming was performed on glass plates having a glass plate thickness of t = 1.0 mm and t = 0.8 mm, and both had an outer diameter φ.
A 23 mm strength test sample glass 4 was prepared and subjected to a breaking strength test, and the results are summarized in Tables 2 and 3, respectively.

[0020]

[Table 2]

[0021]

[Table 3]

From the results of Table 1, Table 2 and Table 3, the chemical strengthening B
Although it has a slight difference compared to the unstrengthened C, it shows a breaking strength value that is clearly different from that of the unreinforced C, which is a strength that is sufficiently satisfied within the standard range as a watch cover glass, and is actually mounted in a watch case. Therefore, there were no problems in tests such as waterproof pressure application and drop impact.

[0023]

According to the method of manufacturing a cover glass for a wristwatch of the present invention in which glass is hot press-molded, a glass in a softened state is hot-molded by a press die having a mirror-finished surface, so that a minute glass on the glass surface can be formed. There is no problem of microcrack remaining, and at the same time, it can be strengthened by quenching heat treatment.
The breaking strength of glass could be increased without using the conventional chemical strengthening method by ion exchange.

[Brief description of drawings]

FIG. 1 is a schematic view of a press molding machine for manufacturing a watch cover glass of the present invention.

FIG. 2 is a diagram showing a press-molding process of the watch cover glass of the present invention.

FIG. 3 is a view showing a glass shape after press molding and after shaping.

FIG. 4 is a schematic view showing a load test method for glass.

[Explanation of symbols]

 1 Glass 2 Stainless Steel Frame 3 Press Mold 4 Glass 5 Load Measuring Terminal 6 Thermocouple 7 Electric Heater 8 Burner

Claims (3)

[Claims] 1. A glass is heated to a working temperature range of 700 to 1000 ° C., the softened glass is pressure-molded by a press method, and the glass is rapidly cooled while being transferred and molded in the mirror surface shape of the press die surface. By doing so, a compressive stress on the glass surface and a tensile stress inside the glass are obtained. 2. The wristwatch according to claim 1, wherein the temperature of the pressing press die from above and below or from the left and right during press molding is maintained at -50 to 100 ° C. below the transition point of the glass material. Of manufacturing cover glass for automobile. 3. A die material used for press molding is a cemented carbide having a low coefficient of thermal expansion, which is excellent in heat resistance, welding resistance, and releasability.
The method for manufacturing a cover glass for a wristwatch according to claim 1, which is made of ceramics or a metal oxide sintered body.