There is a growing tendency to require glass used for large projects be HST tested. Regulations become more stringent and investors better aware of potential consequences caused by spontaneous fractures of already installed glazing systems. Fractured glass panes must be promptly replaced, a very costly procedure often calling for a lifting crane or other specialized equipment.
A broken pane may leave facilities with restricted access wide open. Fracture of a tempered façade pane occurring at a high elevation may cause injuries when glass shards, despite their small size and blunt edges, hit people who happen to be close to the building, and that may mean legal problems for the building manager.
HST (Heat Soak Testt) is also known as homogenization treatment, commonly known as “detonation”. Heat Soak Test 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 a heating medium, which is called “Heat Soak Test” abroad, or HST for short, which is literally translated as Heat Soak Test.
Difficulty in heat soaking. In principle, Heat Soak Test 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 vary, 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 the temperature during operation of the hot dip furnace is so important, and three years ago, the temperature difference in most domestic hot dip furnaces even reached 60°C during hot dip and heat preservation, and it was 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.
The new standard will be more effective. 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-dipping furnace are constantly being improved, and the heating uniformity has also been significantly improved, which can basically meet the requirements of the hot-dipping 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 in the 120,000-square-meter super-large project of Guangzhou New Airport.
Although the Heat Soak Test 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.
