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As a result of the undergo drastic temperature changes, annealing fundamentals of the glass in the forming process. Causes the inner and outer layer to produce the temperature gradient, and because of the finished product shape, the thickness, by the cooling degree and so on glass machinery divergence, causes the product to produce the irregular thermal stress. This thermal stress can reduce the mechanical strength and thermal stability of the product, but also affect the optical uniformity of the glass, if the stress exceeds the ultimate strength of the finished product, it will rupture itself. Therefore, the uneven thermal stress in glass products is a serious error. Annealing is a heat treatment process that allows thermal stress in the glass to be removed or reduced to allowable values as much as possible. In addition to glass fiber and thin-walled small hollow products, it is true that all glass products are necessary to perform annealing.
The thermal stress in glass products is divided into two kinds, which are stress and permanent stress, according to their existing characteristics. Because of its poor thermal conductivity, ① temporary stress. The glass is heated or cooled below the strain point temperature. Each part will form a temperature gradient, resulting in the inevitable thermal stress. This kind of thermal stress, with the existence of temperature difference, the larger the temperature difference, the higher the stress, and disappear with the temperature difference.
This thermal stress is called a stress. But before the temperature is maladjusted, the stress of course should be stressed may be eliminated by itself.
When the stress value exceeds the ultimate strength of the glass, the glass anomaly will rupture itself, so the glass in the brittle temperature range of heating or cooling speed should not be too fast. Thermal stress generated by temperature difference, ② permanent stress. When the glass starts to cool above the strain point temperature. Glass cooling to room temperature, internal and external temperature imbalance, and can not be completely dissipated, in the glass in imitation of the remaining a certain amount of stress, this stress is called water long stress.
The permanent stress depends on the glass machinery such as the cooling rate, the viscosity of the glass, the heat shortening coefficient and the thickness of the product when the finished product is above the strain point temperature. It is to reheat glass products with permanent stress to the temperature at which the particles in the glass can move, and the annealing of the glass. The displacement of the control particle causes the stress to disperse (called stress relaxation) to eliminate or weaken the permanent stress.
The stress relaxation speed depends on the glass temperature, the higher the temperature, the faster the relaxation speed. Is that, in a suitable annealing temperature range, the glass gets a critical actual production process for the quality of the fine-mining annealing, the total elimination of permanent stress is impossible by annealing wishes for residual stresses to be added or uniformly to the minimum constraints, In order to enhance the mechanical strength of glass and thermal stability of the thick glass annealing characteristics of the production of thick glass is important for the reverse method and baffle method, here is important to negotiate the baffle method for the production of thick glass annealing. The main principle of the thick glass produced by baffle method is to "thaw" the baffle area of the glass liquid. The important characteristics of the annealing are as follows:
1.This method produces thick glass because of the cold side, the edge of the glass thickness is thin, so the side of the compressive stress is very large.
2. The thicker the glass, the longer the heat is connected to the glass.
3. In order to meet the cutting request, the tempering stress must be reduced.
4. In order to prevent the Benzi from cooling too fast in the back fire zone, the edge of the side produces a higher tensile stress, resulting in longitudinal crack loss.