Reduce stress value
The distribution of stress in tempered glass is that the two surfaces of the tempered glass are under compressive stress, the core layer of the plate is under tensile stress, and the stress distribution in the glass thickness is similar to a parabola. The center of the glass thickness is the apex of the parabola, that is, where the tensile stress is the largest; the two sides near the two surfaces of the glass are compressive stress; the zero stress surface is located at about 1/3 of the thickness. By analyzing the physical process of tempering and quenching, it can be known that there is a rough proportional relationship between the surface tension of tempered glass and the maximum internal tensile stress, that is, the tensile stress is 1/2 to 1/3 of the compressive stress. Domestic manufacturers generally set the surface tension of tempered glass at about 100MPa, but the actual situation may be higher. The tensile stress of tempered glass itself is about 32MPa to 46MPa, and the tensile strength of glass is 59MPa to 62MPa. As long as the tension generated by the expansion of nickel sulfide is 30MPa, it is enough to cause self-explosion. If the surface stress is reduced, the tensile stress of the tempered glass itself will be correspondingly reduced, thereby helping to reduce the occurrence of self-explosion.
The American standard ASTMC1048 stipulates that the surface stress range of tempered glass is greater than 69MPa; semi-tempered (heat-enhanced) glass is 24MPa to 52MPa. The curtain wall glass standard BG17841 stipulates that the semi-tempered stress range is 24<; δ≤69MPa. The new national standard GB15763 implemented in China. 2-2005 "Safety Glass for Construction Part 2: Tempered Glass" requires that its surface stress should not be less than 90MPa. This is 5MPa lower than the 95MPa specified in the old standard, which is conducive to reducing self-explosion.
The uneven stress of tempered glass will significantly increase the self-explosion rate, which has reached a level that cannot be ignored. The self-explosions caused by uneven stress are sometimes very concentrated, especially the self-explosion rate of a specific batch of bent tempered glass can reach an alarming severity, and self-explosions may occur continuously. The main reasons are the uneven local stress and the deviation of the tension layer in the thickness direction, and the quality of the original glass sheet also has a certain influence. The uneven stress will greatly reduce the strength of the glass, which is equivalent to increasing the internal tensile stress to a certain extent, thereby increasing the self-explosion rate. If the stress of the tempered glass can be evenly distributed, the self-explosion rate can be effectively reduced.
hot dip treatment
Hot dip treatment is also known as homogenization treatment, commonly known as "detonation". The hot dip treatment is to heat the tempered glass to 290℃±10℃, and keep it for a certain period of time, so that the nickel sulfide can quickly complete the crystal phase transformation in the tempered glass, so that the tempered glass that may explode after being used is artificially broken in advance in the factory. Hot dip furnace, thereby reducing the self-explosion of tempered glass in use after installation. This method generally uses hot air as the heating medium, which is called "Heat Soak Test" abroad, or HST for short, which is literally translated as hot dip treatment.
Difficulty in heat soaking. In principle, hot dip treatment is neither complicated nor difficult. But in fact, it is very difficult to achieve this process index. Studies have shown that there are many specific chemical structural formulas of nickel sulfide in glass, such as Ni7S6, NiS, NiS1.01, etc., not only the proportions of various components are different, but also other elements may be doped. The speed of its phase transition is highly dependent on the temperature. Studies have shown that the phase transition rate at 280°C is 100 times higher than at 250°C, so it must be ensured that each glass in the furnace experiences the same temperature regime. Otherwise, on the one hand, the glass with low temperature cannot be completely phase-transformed due to insufficient heat preservation time, which weakens the effect of hot dip. On the other hand, when the glass temperature is too high, it will even cause the reverse phase transition of nickel sulfide, causing even greater hidden dangers. In both cases, the heat-dipping process can be ineffective or even counterproductive. The uniformity of temperature during operation of the hot dip furnace is so important, and the temperature difference in most domestic hot dip furnaces even reaches 60°C during hot dip and heat preservation, and it is not uncommon for imported furnaces to have a temperature difference of about 30°C. Therefore, although some tempered glass has been heat-dipped, the self-explosion rate is still high.
In fact, the hot dip process and equipment have also been continuously improved. The German standard DIN18516 stipulated the holding time for 8 hours in the 90th edition, while the prEN14179-1:2001 (E) standard reduced the holding time to 2 hours. The effect of the hot soaking process under the new standard is very significant, and there are clear statistical technical indicators: after hot soaking, it can be reduced to one case of self-explosion per 400 tons of glass. On the other hand, the design and structure of the hot dip furnace are constantly being improved, and the heating uniformity has also been significantly improved, which can basically meet the requirements of the hot dip process. For example, the self-explosion rate of the hot-dip-treated glass of CSG Group has reached the technical index of the new European standard, and it is extremely satisfied with the 120,000-square-meter Guangzhou New Airport super project.
Although the hot dip treatment cannot guarantee absolutely no self-explosion, it does reduce the occurrence of self-explosion, and truly solves the problem of self-explosion that plagues all parties in the project. Therefore, hot soaking is the most effective method to completely solve the problem of self-explosion that is unanimously recognized in the world.
Products are packed in containers or wooden cases. Each piece of glass should be packed in plastic bags or paper, and the space between the glass and the packaging box should be filled with light and soft materials that are not easy to cause appearance defects such as glass scratches. Specific requirements should comply with relevant national standards.
The packaging marks should comply with the relevant national standards, and each packaging box should be marked with words such as "upward, lightly moved, carefully broken, glass thickness, grade, factory name or trademark".
The various types of transport vehicles and handling rules used in the product shall comply with relevant national regulations.
During transportation, the wooden box should not be placed flat or inclined, the length direction should be the same as the moving direction of the conveying vehicle, and measures such as rain protection should be provided.
The product should be stored vertically in a dry room.
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