GB2232978A

GB2232978A - Method of tempering glass sheet by quenching

Info

Publication number: GB2232978A
Application number: GB9007046A
Authority: GB
Inventors: Shin-Ichi Aratani
Original assignee: Central Glass Co Ltd
Current assignee: Central Glass Co Ltd
Priority date: 1989-04-05
Filing date: 1990-03-29
Publication date: 1991-01-02
Anticipated expiration: 2010-03-29
Also published as: JPH0623068B2; FR2645528A1; FR2645528B1; GB9007046D0; DE4010718A1; JPH02267131A; GB2232978B

Abstract

In quenching a glass sheet heated to a temperature above the strain point by blowing jets of air onto the opposite surfaces of the glass sheet from two sets of nozzles respectively protruding from opposed air chambers, the quenching is made in a two-stage process by first blowing cool air which is producing a shock wave onto the glass surfaces for a relatively short period of time such that the heat transfer coefficient on each surface becomes greater than 300 and not greater than 1000 kcal/m<2> &cirf& h &cirf& DEG C and successively blowing cool air onto the glass surfaces for a relatively long period of time such that the heat transfer coefficient on each surface becomes 100-300 kcal/m<2> &cirf& h &cirf& DEG C. By this method even glass sheets thinner than 3.5 mm can be tempered so as to meet the regulations on tempered glass sheets for use in automobile windows.

Description

22 s:j-. -:q.7 '-ES METHOD OF TEMPERING GLASS SHEET BY QUENCHING This

invention relates to a method of tempering a glass sheet by heating the glass sheet to a temperature above.the strain point and quenching the heated glass sheet with blasts of cool air, the method being particularly suitable for use in tempering a relatively thin glass sheet, viz. glass sheet about 1.5-3..5 mm in thickness, for use as an automobile window glass.

As the recent automobiles are designed to reduce the gross weight there is a growing demand for tempering of relatively thin glass sheets for use in automobile side and rear windows. To ensure an adequate margin of safety to the drivers and passengers in case of breakage of automobile window glasses there are official regulations which specify the manner of fragmentation of tempered glass sheets. Typical regulations require that the number of glass fragments contained in any 5 cm x 5 cm square traced on the glass sheet (except in a circular area with a radius of 7.5 cm around the point of impact and marginal areas 3 cm in breadth) should fall in the range from 60 to 400, that the fragments should not be larger than 3 cm 2 in surface area and that the fragments should not include elongated particles longer than 75 mm (such elongated particles are called fisplines").

However, it is not easy to temper glass sheets It k thinner than about 3.5 mm by conventional air quenching methods so as to fully meet the aforementioned regulations. In general, the quenching is for producing a center-to-surface gradient of temperature through the thickness of the glass sheet to result in that permanent compressive stresses are produced in the surface layers of the glass sheet with compensating tensile stresses in the center of the glass thickness. In the case of the thin glass sheet it is difficult to create and maintain a suitable gradient of temperature in the glass sheet during the quenching process.

For the tempering of relatively thin glass sheets by air quenching there are some proposals with a view to enhancing the cooling efficiency. For example, USP 4,578,102 proposes directing jets of a mixture of air and atomized water onto the heated glass surfaces by means of Laval nozzles. Air is supplied to the Laval nozzles at such a pressure that the jet velocity at the exit of each nozzle becomes at least sonic, while water is introduced from a radial direction into the constricted throat section of each nozzle. The mixture of air and atomized water has a higher specific heat than air, and it is intended to rapidly extract heat from the glass sheet surfaces by using two-phase jets high in both velocity and specific heat. However, from a practical point of view the use of water besides air offers complicacy, and very high precision of equipment is required for complete atomization of water and thorough mixing of atomized water with air during the passage of the two fluids from the nozzle throat to the nozzle exit. Besides, the relative pressure of air supplied to the nozzles must be at least 0.91 bar (about 0.93 kg/cm 2) in order to make the jet velocity sonic at the nozzle exit, and there is a possibility that droplets of water hit the heated glass sheet to cause breakage of the glass sheet.

JP-A 60-145921 relates to quenching of a heated glass sheet with air jets and proposes to determine the air pressure and the nozzle configuration such that the maximum drop of the cooling air pressure takes place at the exit of each nozzle and such that the air jet volocity at the nozzle exit becomes sonic. The pressure of air supplied to the nozzles is at least 0.9 bar (about 0.92 kg/cm 2) by gauge pressure. A disadvantage of this method is that fluctuations of the air supply pressure in the quenching equipment are liable to be transmitted to the glass sheet surfaces so that the glass sheet under quenching is liable to be distorted in the case of a relatively thin glass sheet. Besides, in this method it will be necessary to give very careful consideration to the arrangement of the quenching nozzles.

GB 2,185,476 relates to quenching of a heated glass sheet and proposes to produce a shock wave in the A X air chamber in each of two opposed blast heads from which nozzles protrude by forcing compressed air to rapidly expand in each air chamber such that the pressure (gauge pressure) of air rapidly drops from a predetermined first pressure ranging from 2 to 8 kg/cm 2 to a predetermined second pressure ranging from 0.05 to 2 0.5 kg/cm By virtue of the propagation of the shock wave through the air chamber and the nozzles, the air jets have high kinetic energy at the moment of impinge- ment on the glass surfaces and hence are high in the initial cooling effect. By this method even glass sheets thinner than 3 mm can be tempered so as to meet the regulations on tempered glass sheets for use in automobile windows. However, this method requires an air quenching apparatus of relatively large capacity. JP-A 64-3029 relates to tempering of a glass sheet by using the method of GB 2,185,476 and proposes to first quench a central region of the heated glass sheet and then gradually directing the quenching air jets toward the edges of the glass sheet. This proposal is for relatively mildly tempering glass sheets about 3-5 mm in thickness and is not suitable for sufficiently tempering glass sheets for use in automobile windows.

It is an object of the present invention to provide an improved method of tempering a glass sheet, which may be thinner than about 3.5 mm and may be for use as an automobile window glass, by quenching with air jets spouted from simple nozzles.

The present invention provides a method of tempering a glass sheet, the method having the steps of heating the glass sheet to a temperature above the strain point of the glass and quenching the heated glass sheet by blowing jets of cool air onto the opposite surfaces of the glass sheet from two sets of nozzles prodtruding from oppositely disposed two air chambers, respectively, and the method being characterized in that the quenching is made in a two-stage process by first blowing cool air which is producing a shock wave onto the two opposite surfaces of the heated glass sheet for a first period of time such that the heat transfer coefficient on each of the glass surfaces becomes greater than 300 kcal/m 2. h.0 C and not greater than 1000 kcal/m 2. hoo C and successively blowing cool air onto the two surfaces of the glass sheet for a second period of time which is longer than the first period time such that the heat transfer coefficient on each of the glass,surfaces falls in the range from 100 to 300 kcallm 2. h. 0 C.

In the two-stage quenching process according to the invention the initial stage is using the quenching method disclosed in GB 2,185,476 only for a short period of time with limitation of the heat transfer coefficient on each surface of the glass sheet within a specified range. At the initial stage of the quenching cool air producing a shock wave is spouted from the nozzles protruding from the respective air chambers so that the air jets impinge on the glass sheet surfaces with high kinetic energy. Therefore, a heat transfer suppressive laminar film that exists on each surface of the heated glass surface is immediately broken up or greatly diminished in thickness, and heat is rapidly and efficiently dissipated or extracted from the glass surfaces. By virtue of the enhanced cooling effect at the initial stage of quenching a center-tosurface.gradient of temperature is surely produced in the glass sheet. After that it is unnecessary to use air jets having high kinetic energy, and it is no longer necessary to produce a shock wave. It suffices to maintain a thicknesswise gradient of temperature in the glass sheet and suppress relieving of the compres.sive stresses created in the surface layers of the glass sheet until completion of the quenching. Therefore, at the later stage of the quenching the glass surfaces is blown with relatively weak jets of air.

Since the initial stage of the two-stage quenching process is relatively short in duration, the air chambers and the related components for producing a shock wave do not need to be made very large in capacity.

k Glass sheets of various thicknesses can be well tempered by the method according to the invention. Even relatively thin glass sheets ranging from about 3.5 mm to about 1.5 mm, in thickness can efficiently be tempered by this method so as to meet the current regulations on tempered glass sheets for use as automobile side or rear window glasses. Furthermore, this tempering method is applicable to the manufacture of tempered glass sheets for various uses such as railroad vehicle window glasses, building window panesy furniture glasses and electronic device substrates. Both flat glass sheets and bent or curved glass sheets' can be tempered by this method.

When the method according to the invention is used distortion or deformation of the quenched glass sheet is rarely, and the probability of breakage of the quenched glass sheet greatly reduces, because the quenching can be accomplished with little swaying of the glass sheet under quenching. This is particularly valuable in the case of tempering a thin glass sheet since, in general, liability of glass sheet to deformation or distortion augments approximately inversely proportional to the square of thickness.

In tempering a glass sheet by a method according to the invention the first step is uniformly heating the glass sheet to a temperature above the strain point of the glass and below the softening temperature, e.g. to 600-700 0 C. This is similar to the heating in the conventional quench tempering methods.

The apparatus for-quenching includes a pair of blastheads disposed oppositely such that the heated glass sheet is positioned between the two blastheads. Each blasthead defines therein an air chamber, and a number of nozzles protrude toward the glass sheet from the faceplate of each blasthead. To produce a shock wave in the air chamber in each blasthead the air chamber is connected to a compressor, and usually an air tank is connectqd to both the compressor and the air chamber. The communication of the air chamber with the compressor and the air tank is controlled by a suitable valve means. For the second stage of the two-stage quenching process the air chamber is connectable to a blower by a selector valve.

In preparation for the quenching operation the compressor is operated while the communication of the air chamber with the compressor and the air tank is blocked. The pressure in the air tank is controlled to a predetermined pressure which is usually in the range from 2 to 8 kg/cm 2 by gauge pressure. At the start of the quenching operation the valve means is opened to allow the pressurized air to rush into and expand in the air chamber. Naturally the pressurized air undergoes a considerable reduction in pressure, while the -11 k f -g- atmospheric air in the air chamber is rapidly compressed. Consequently a shock wave is produced at a section close to the entrance to the air chamber, and the shock wave propagates through the air chamber and the nozzles. It is suitable that the expansion of the pressurized air in the air chamber results in a rapid drop in pressure to a predetermined pressure in the range from 0.05 to 0.5 kg/cm 2, and preferably in the range from 0.1 to 0.4 kg/cm 2 9 by gauge pressure. Soon air producing a shock wave jets out from the nozzles of each blasthead to collide against the heated glass sheet. Usually it suffices to continue the initial stage of quenching for 1 to 3 sec.

At this stage of the quenching operation the delivery of cool air onto the glass sheet is controlled such that the heat transfer coefficient on each major surface of the glass sheet becomes greater than 300 2 0 2 0 kcal/m h C and not greater than 1000 kcal/m. h. C. if the heat transfer coefficient is smaller than 300 kcal/m 2. he 0 C it is difficult to achieve sufficient tempering by reason of insufficiency of the initial cooling power of air. If it is intended to render the 2 0 heat transfer coefficient greater than 1000 kcal/m. h. C there arise problems about the quenching apparatus, and such an increase in the heat transfer coefficient offers difficulty in stably performing the quenching as anindustrial operation and is liable to cause breaking of the glass sheet under quenching.

If desired, only a selected region of the heated glass sheet way be blown with cool air, or a plurality of regions of the glass sheet may be blown with cool air in turn or for different peiods of time.

At the later stage of the two-stage quenching process the delivery of cool air onto the glass sheet is reduced such that the heat transfer coefficient on each major surface of the glass sheet falls in the range from 100 to 300 kcal/m 2. h. 0 C. At this stage it is not necessary to use compressed air, so that the air chamber in each blasthead can be connected to a blower. If the heat transfer coefficient is smaller than 100 kcal/m 2. h 0 C it is difficult to maintain a desired gradient of temperature in the glass sheet and suppress relieving of the compressive stresses created in the surface layers of the glass sheet. If the heat transfer coefficient is made greater than 300 kcal/m 2. hao C at - this stage the glass sheet may crack or undergo distortion or degradation of optical characteristics, and the maintenance of such a large heat transfer coefficient for a long period of time is unfavorable for the economy of both the equipment and the operation.

Usually the later stage of the two-stage quenching process is performed for 5-30 sec. In this two-stage quenching process it is suitable that the ratio of the duration of the initial stage to the duration of the later stage is not more than 1/3.

EXAMPLES 1-5

In every example a glass sheet 500 mm x 300 mm in widths was tempered by a method according to the invention. The thickness of the glass sheet was 1.6 mm, 2.3 mm or 2.9 mm as shown in Table 1. In every example the glass sheet was uniformly heated to 670-700 0 C, and the heated glass sheet was held vertically between a pair of blastheads of the above described type.

At the first stage of the two-stage quenching process the primary air pressure in the air tanks was controlled to 2 kg/cm 2, 7 kg.Icm 2 or 8 kg/cm 2 (gauge pressure), and a shock wave was produced in each air chamber by a rapid drop of the primary air pressure to 2 2 2 0.05 kg/cm, 0.3 kg/cm or 0.5 kg/cm The resultant jets of air impinged on the opposite surfaces of the glass sheet for 1 to 3 sec. The blowing of air was controlled such that the heat transfer coefficient on each surface of the glass sheet reached a predetermined level ranging from 350 to 600 kcal/cm 2. h 0 C, as shown in Table 1..

Then the air chamber in each blast head was connected to a blower to further blow the both surfaces of the glass sheet with air for 10-20 sec while control ling the delivery of air so as to keep the heat transfer coefficient on each surface of the glass sheet sheet at a level ranging from 100 to 250 kcal/m 2. h. 0 C, as shown in Table 1. In Example 3 a compressor was used together with the blower to slightly raise the air pressure at the nozzle exit. In Table 1, the character "Blt in parenthesis indicates the use of the blower and the character "C" the use of the compressor.

the glass sheets tempered in Examples 1-5 were subjected to a fragmentation test as described hereinafter.

COMPARATIVE EXAMPLES 1-4 Glass sheets 500 mm x 300 mm in widths and 2.5 mm or 2.9 mm in thickness were tempered by a conventional quenching method. In every case the glass sheet was heated to 670-700 0 C. In Comparative Examples 1, 2 and 3 air was supplied to the blastheads from a blower, and air was continuously blown onto the opposite surfaces of the glass sheet for 10-20 sec while controlling the delivery of air so as to keep the heat transfer coefficient on each surface of the glass.sheet 2 0 at a level ranging from 150 to 180 kcal/m. h C, as shown in Table 1. In Comparative Example 4 a compressor was used instead of the blower, but the feed of pressurized air into the air chamber in each blasthead was made so as not to produce shock wave. The tempered glass sheets were subjected to the fragmentation test.

FRAGMENTATION TEST The test procedure was generally in accordance with EEC standard ECE R43. The point of impact on each sample of the tempered glass sheets was at the approximate center of the rectangular glass sheet in Table 2) or at a distance of 30 mm from the middle point of a longer side of the glass sheet toward the s center CW' in Table 2). The fragmentation was checked by counting the number of glass particles included in each of many arbitrarily traced 50 mm x 50 mm square areas of the tested glass sheet and the total number of elongated particles (splines) which were longer than 75 mm and in which the length-to-width ratio was greater than 4. However, fragmentation was not checked in a strip 20 mm wide all round the edge of the glass sheet and within a radius of 75 mm around the point of impact. The test results were as shown in Table 2.

1( TABLE 1 Heat Transfer Coefficient (kcallm 2 h. o C) Glass Sheet Thickness (mm) 1st stage 2nd stage quenching quenching Example 1 600 250 (B) 1.5 Example 2 450 150 (B) 2.3 Example 3 350 100 (B & C) 2.9 Example 4 600 250 (B) 1.5 Example 5 450 150 (B) 2.3 Comp. Ex. 1 150 (B) 2.5 Comp. Ex. 2 180 (B) 2.9 Comp. Ex. 3 170 (B).2.5 Comp. Ex. 4 150 (C) 2.9 TABLE 2 Fragmentation Test Results Point of Particle Count Number of Impact Elongated (in 50-mm square) Particles Max. Min.

Example 1 A 205 62 0 Example 2 A 247 78 0 Example 3 A 199 63 0 Example 4 B 179 87 0 Example 5 B 190 92 0 Comp. Ex. 1 A 30 2 1 Comp. Ex. 2 A 240 53 5 Comp. Ex. 3 B 3 1 0 Comp. Ex. 4 B 18 1 0 -is-

Claims

1. A method of tempering a glass sheet, the method having the steps of heating the glass sheet to a temperature above the strain point of the glass and quenching the heated glass sheet by blowing jets of cool air onto the opposite surfaces of the heated glass sheet from two sets of nozzles protruding from oppositely disposed two air chambers, respectively, characterized in that the quenching is made in a two-stage process by first blowing cool air which is producing a shock wave onto the two opposite surfaces of the heated glass sheet for a first period of time such that the heat transfer coefficient on each of said surfaces of the glass sheet becomes greater than 300 kcal/m 2. h. 0 C and not greater than 1000 kcal/m2. h. 0 C and successively blowing cool air onto said surfaces of the glass sheet for a second period of time which is longer than said first period of time such that the heat transfer coefficient on each of said surfaces falls in the range from 100 to 300 kcal/m 2, h. 0 C.

2. A method according to Claim 1, wherein said shock wave is produced by forcing compressed air maintained at a predetermined first pressure in the range from 2 to 8 kg/cm 2 by gauge pressure to rapidly expand in each of the air chambers such that a rapid drop from said first pressure to a predetermined second pressure in the range from 0.05 to 0.5 kg/cm 2 by gauge pressure takes place in 4 each air chamber and such that substantially the whole length of each air chamber and each of the nozzles protruding therefrom serves as a shock wave tube.

3. A method according to Claim 2, wherein said second pressure is in the range from 0.1 to 0.4 kg/cm 2 by gauge pressure

4. A-method according to any of the preceding claims, wherein the heat transfer coefficient at the first stage of the quenching is in the range from 400 to 660 2 0 kcal/m. h. C.

5. A method according to any of the preceding claims, wherein-the ratio of said first period of time to said second period of time is not greater than 1/3.

6. A method according to any of the preceding claims, wherein the thickness of the glass sheet is not more than 3.5 mm.

7. A method of tempering a glass sheet, substantially as hereinbefore described in any of Examples 1 to 5.

Published 1990 a, The Patent Office. StateHouse,66 711-lighHolborr,. London WC1R 4TP. Purther copies maybe obtainedfrom The Patent OfficeSales Branch. St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1'87 1