BACKGROUND The present invention relates to an improved apparatus for tempering glass sheets and a method of tempering utilizing that apparatus.

Many methods of treating glass to cause it to break in small, harmless pieces, rather than large, elongated shards which can cause serious injury, have been practiced. The goal of early tempering methods to treat relatively small sheets of glass was to uniformly distribute the airflow over the entirety of the surface of the glass sheet. As single glass sheets used, for example, in vehicles became larger, and the analysis of stresses in glass became more sophisticated, methods of differentially treating areas of a glass sheet were devised. One particularly intractable problem has been the elimination of inadequately tempered areas, relatively near the center of large sheets of glass such as automotive backlights.

Thus, those skilled in the art of glass tempering have continued to search for a way to improve tempering, on a consistent basis, during time- critical automotive glass manufacturing operations. As noted, glass tempering or heat treatment is the subject of many patents, for example:

U.S. Patent No. 4,314,836 describes a construction and arrangement of a tempering medium supply system that is said to provide relatively wide escape paths in the direction of the glass sheet thickness for blasts of tempering medium applied toward the central portion of the glass sheet compared to the escape paths provided in said direction for blasts of tempering medium applied to the portion of the glass sheet beyond the central portion. The resulting tempered glass sheets utilizing the process of the present invention are said to produce fragments meeting code requirements in the central portion as well as the portion beyond the central portion when subjected to destructive testing.

U.S. Patent No. 4,323,385 describes a process for uniformily tempering the entirety of a glass sheet utilizing plenum chambers having aperture walls facing the opposite major surfaces of the glass sheet, wherein the apertures are constructed and arranged to be smaller and more closely spaced in the portion of the plenum that faces the central portion of the glass sheet during tempering than the apertures provided in the wall outside the central portion. Nozzles extending from said apertures are constructed and arranged so that nozzles extending from the relatively small diameter, more closely spaced apertures of the central portion of the wall are longer and narrower and have their ends closer to the major surface of the glass sheet than the relatively widely spaced nozzles of relatively larger cross-section that extend from the remaining apertures in the aperture wall.

U.S. Patent No. 4,508,783 describes a method of differentiated rate of hardening of a glass sheet by blowing a cooling gas in the direction of the sheet having two zones, from nozzles. According to the invention, a simultaneous solidification of the glass sheet is said to be produced at both zones of the sheet in spite of the differentiated treatments applied to establish a difference of hardening between the different zones of the sheet.

U.S. Patent No. 4,773,926 describes a tempering process for glass sheets of ± 2mm in thickness in which process, a plurality of nozzles from which a gas flow emanates, are located so that their free ends are directed toward the glass sheet in a position spaced slightly apart from the glass surface. Each nozzle is pierced by a narrow, calibrated orifice and the feed pressure of each nozzle is maintained for purposes of blowing the gas, generally air, at least at sonic speed toward the surface of the sheet. The configuration of the nozzles are said to ensure that the speed of the blowing gas is, at least, at sonic speed at the surface of the glass sheet, thus optimizing coating power. One measure to increase the evacuation of the blown air is said to be accomplished by the use of nozzles having longer tubular bodies extending toward the free end. The nozzles may be longer in relation to the width of the tempering station. Additionally, or alternatively, the flow of blown air may be increased in volume per unit surface area.

U.S. Patent No. 6,412,390 describes a glass quenching apparatus including a first nozzle group for jetting blower air onto opposite surfaces of the glass sheet, the first nozzle group being stationary nozzles, and a second nozzle group being movable nozzles for jetting compressor air onto at least one of the opposite surfaces of the glass sheet, which nozzles are capable of moving parallel to a plane of the glass sheet. The movable nozzle group can be located, it is said, at an optimum position with respect to, for example, a hard-to-cool portion of a glass sheet.

SUMMARY OF THE INVENTION

The present invention relates to a glass tempering apparatus comprised of at least one and preferably of two glass tempering modules, at least one such glass tempering module being capable of selectively delivering specified volumes of a tempering medium at increased velocity to designated areas of a moving glass sheet to create desired stress differentials in such designated areas, by the selective arrangement and utilization of quench nozzles at different distances from the surface of the glass sheet to be tempered. In particular, nozzles arranged in rows parallel to the direction of travel of the glass sheet, spanning a specified distance on either side of the centerline of the glass tempering apparatus, certain selected nozzles being at a reduced distance from the surface of the glass sheet to be tempered delivering tempering medium at an increased velocity, have been found to substantially reduce the incidence of inadequate tempering of, in particular, large sheets of glass. '

A method of improved tempering utilizing the apparatus of the present invention is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a plan view of a glass tempering line in accordance with the invention.

Fig. 2 is a plan view of a representative glass sheet showing areas prone to insufficient tempering. Fig. 3 is a perspective view of a conventional blasthead assembly.

Fig. 4 is a perspective view of a blasthead assembly with variable length nozzles in accordance with the invention. Fig. 5 is a cross-sectional view of first and second complementary tempering assemblies in accordance with the invention.

Fig. 6 is a graph/chart of quench air delivered to glass across the width of the blasthead assembly in accordance with a preferred embodiment of the invention.

Fig. 7 is a graph/chart comparing the occurrence of splines in a glass sheet utilizing the tempering technology of the present invention, to a conventional tempering apparatus.

Fig. 8 is a graph/chart comparing the minimum glass particle count which occurs on selective breakage testing of a glass sheet utilizing the tempering technology of the present invention, to a conventional tempering apparatus.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a glass tempering apparatus 10 and to a method of tempering glass sheets utilizing such apparatus. More specifically, the invention relates to an apparatus 10 for, and method of selectively delivering desired volumes of tempering medium at increased velocity to one or more areas of at least one major surface of a glass sheet 12. The glass sheet 12 is, for example, adapted for use as a vehicle window. In particular, the apparatus 10 of the present invention allows for significant improvement in tempering of large glass sheets 10, such as vehicle backlights, by directing specified volumes of tempering medium, at increased velocity, toward specified areas of the glass sheet 10 where glass quality testing has shown that tempering may have been insufficient. To remedy this situation, applicants utilized the well known principle that tempering is creating stresses in the amorphous glass structure, and that areas where compressive stresses predominate make the glass stronger than in areas where tensile stresses predominate. While not wishing to be bound by any theory, applicants believe that the apparatus of the present invention reduces the undesirable occurrence of splines by as much as 90%, due to the creation of a number of areas of increased stress differentials. More particularly, the inventors have found that use of quench nozzles selectively closer to the surface of the glass sheet to be tempered create stress differentials of 1OmPa or greater, which has been found to be beneficial in reducing spline length in selected areas.

As shown in Fig. 2, in large glass sheets 12, the area found to be most prone to insufficient tempering resulting in deviations from acceptable tempering break patterns, i.e., break patterns of elongated glass shards known as "splines", rather than small, rounded particles, occurs a relatively consistent distance (distance "A") transversely from, and on either side of, the centerline of the glass tempering apparatus. The width of the area in which the splines typically occur tends also to be relatively consistent, and is sometimes referred to herein as distance "B". The distance from the outer edge of distance "B" to the outermost edge of the glass sheet, transversely from its centerline, is sometimes referred to herein as distance "C". Tempering apparatus such as the present invention are sometimes referred to as "blastheads", "quench modules" or "quench boxes." It is an advantage of the present invention that the tempering apparatus

10 can be used as a component of a typical glass tempering line 14, shown schematically in Fig. 1. A conventional blasthead assembly is shown in Fig. 3. As can be seen, the quench nozzles are configured to follow the contour of the shaped glass sheet. As such, the distal ends of the quench nozzles remain substantially a uniform distance from the surface of the shaped glass sheet. A tempering apparatus 10 in accordance with the present invention is shown in Fig. 4. As will be explained in greater detail, it is an objective of the present invention to position the distal ends of selected quench nozzles closer to the glass surface, in order to increase the velocity of the tempering medium from those selected nozzles, which contacts the surface of the shaped glass sheet. Still with reference to Fig. 4, and in the direction denoted therein as the direction of glass travel, there is illustrated a plurality of nozzles from which tempering medium, preferably air, is emitted and directed toward a major surface of the glass sheet. In the direction of travel of the glass sheet 12, in accordance with the invention, the glass sheet 12 first encounters a first zone having a first plurality of nozzles 16, preferably arranged in staggered rows, sometimes known as a "domino five" pattern, although other nozzle patterns are within the scope of the invention. The moving glass sheet 12 then encounters a second zone having a second plurality of nozzles 18 arranged in parallel rows, sometimes known as a "striper" or a modified striper. As can be seen, the length of the nozzles varies in an area on either side transversely of the centerline of the apparatus. Distances "A", "B" and "C" as designated on the glass sheet 12 of Fig. 2 are superimposed on the corresponding area of the tempering apparatus 10, according to the embodiment of the present invention illustrated in Fig. 4. Finally, in its route of travel past the tempering apparatus, the glass sheet 12 encounters a third zone having a third plurality of nozzles 20 in staggered rows, such as the domino five, or modified domino five pattern.

The length of the nozzles in the first, second and third pluralities of nozzles 16, 18, 20 are predetermined to substantially conform to the shape of the glass sheet 12 to be tempered.

Still referring to Fig. 4, in the modified striper portion 18 of the tempering apparatus 10, and in a direction transverse to the direction of glass travel, the modified striper, according to the present invention, can be described as parallel rows of nozzles from which cooling air typically at a temperature of from 50 ⁰ F to 15O ⁰ F is emitted toward the glass sheet 10.

As shown, for example, in Fig. 5, the distance of the end of the nozzles from the surface of the glass sheet in the parallel rows transverse distance "A" on either side of the centerline of the glass tempering apparatus can be designated as x, andjs preferably on the order of 80-90mm. These nozzles are typically about 6-12mm, preferably about 6-9mm, in diameter.

Notably, however, in the area designated by transverse distance "B", the distance of the end of the second plurality of nozzles 18 from the glass can be expressed as y, where y is less than x. Preferably, the nozzle distance from the glass surface is on the order of 60-70 mm. The diameter of the nozzles is typically about 6-12mm, preferably about 6-9mm. Thus, the velocity of tempering medium that can be delivered to a selected area of the surface of the glass sheet is substantially increased, preferably to on the order of 15% or more over the velocity delivered by conventional striper modules. A graphical representation of the variation in the velocity of tempering medium delivered in the modified striper module is shown in Fig. 6. The temperature of the tempering medium generally is from 50 ⁰ F to 150 ⁰ F.

Additionally, the heat transfer, away from the glass surface in the transverse distance "B" where the nozzles are y distance from the glass surface, is on the order of 10% or more greater than in areas where the nozzle distance from the glass surface is x.

In the area designated as transverse distance "C" the distance of the end of the nozzles from the substrate surface is x, i.e. on the order of 80- 90mm. These nozzles are typically about 6-12mm, preferably 6-9mm, in diameter. In accordance with the invention, in a direction transverse to either side of the centerline of the glass tempering assembly, transverse distances A, B, and C will be dependent on the size and geometry of the glass sheet 12 to be tempered.

The theory of the present invention may be better understood by reference to the following equations:

(1 ) V _f = V ₀ K ^ D _τ where

V _f = final quench air velocity at the substrate surface V ₀ = individual nozzle quench air exit velocity K = velocity constant for jet flow

D ₀ = individual nozzle diameter D _t = distance from end of individual nozzle to substrate surface.

(2) h ≤ (V) ^{0 8} where h = heat transfer coefficient

V = V _f from equation (1 )

EXAMPLES

The benefits of the present invention can be seen by reference to the data presented in Tables 1-4. Conditions for the Comparative Examples and Examples according to the invention were as follows: Line speed = 240 in/min.

Quench module pressure was between 54" and 47" of water column. Glass heating furnace temperature was 652°C-657°C. Mold closed time = approx. 1 sec. Distance between tips of upper and lower quench modules = 89mm

Blasthead Air Temperature = 104 ⁰ F, 110 ⁰ F Glass thickness ranged from 3.44 - 3.47mm

One way of determining the effectiveness of a glass tempering process is by comparison with certain regulatory standards for minimum particle count and maximum spline length when the tempered glass is fractured. For purposes of this application, tempering is deemed to be satisfactory if minimum particle count is greater than 40 particles per 50mm ² , while no spline is more than 75mm in length. By spline is meant a generally elongate glass shard which remains essentially intact after the glass is fractured.

TABLE 1

Table 1 shows results of fracturing five glass samples tempered utilizing a set of conventional quench modules, that is upper and lower quench modules, having alternating areas of nozzles in domino five and striper patterns, the nozzles conforming to the shape of the glass being tempered. No selectively extended length nozzles were present in either the upper or lower quench nozzles.

As can be seen, utilizing the conventional technology, while minimum particle counts easily exceed the minimum number of 40/50mm ² , spline length is, regularly, greater than 75mm, indicating tempering is, generally, insufficient.

TABLE 2

Quench Modules - Extended Nozzles in Upper and Lower Modules without Differential Heating

Table 3

Quench Modules - Extended Nozzles in Upper and Lower Modules without Differential Heating

Tables 2 and 3 show results of fracturing of a number of glass samples tempered utilizing a set of quench modules having extended length nozzles in selected areas of both upper and lower quench nozzles. Additional heating (differential heating) of the area of the glass substrate affected by the extended quench nozzles was tested in, Examples 6-11. No such differential heating was utilized for Examples 12-21. Differential heating in the areas affected by the extended nozzles appears to somewhat increase the minimum particle counts observed, but did not significantly affect spline length. Minimum particle counts are well above the minimum number of 40/50mm ² . Likewise, maximum spline length for Examples 6-21 is well below 75mm. TABLE 4

Quench Modules - Extended Nozzles in Upper Module

Table 4 shows results of fracturing of a number of glass samples tempered utilizing a set of quench modules having extended nozzles in selected areas of the upper quench module only, according to the invention. Minimum particle counts and maximum spline lengths are as good as, or better than, those where extended nozzles were utilized in both upper and lower quench modules as in Examples 6-21. Thus, it has, surprisingly, been found that extended nozzles in both upper and lower quench nozzles are not necessary to achieve good tempering in areas of large glass sheets where obtaining such results has been difficult with conventional quench modules.

In a preferred configuration, a second tempering apparatus 22, having all nozzles approximately the same distance from the surface of the substrate and a shape substantially conforming to the shape of the glass sheet to be tempered, and complementary to the first glass tempering apparatus is spaced opposite and apart from the first tempering apparatus 10 a distance sufficient for the glass sheet 12 to be tempered to pass therebetween, the second glass tempering apparatus 22 being capable of directing desired volumes of tempering medium at a predetermined velocity toward the second surface of the glass sheet.

Those skilled in the art will appreciate that changes and modifications to the invention are possible in light of the preceding description. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.