Due to the tolerance of drastic temperature changes, annealing basic rational glass is in the forming process. The inner and outer layers produce a temperature gradient, and due to the differences in the shape, thickness, and degree of cooling of the finished product, irregular thermal stresses are generated in the product. Such thermal stress can reduce the mechanical strength and thermal stability of the product, and also affect the optical uniformity of the glass. If the stress exceeds the ultimate strength of the finished product, it will break on its own. Therefore, the existence of uneven thermal stress in glass products is a serious mistake. Annealing is a heat treatment process that can eliminate or reduce the thermal stress in the glass to an allowable value as much as possible. Except for glass fibers and thin-walled small hollow products, indeed all glass products must be annealed. Thermal stress in glass products can be divided into temporary stress and permanent stress according to its characteristics.
Due to its poor thermal conductivity, ① temporary stress. When glass is heated or cooled below the strain point temperature. Each part will form a temperature gradient, which will generate inevitable thermal stress. This kind of thermal stress, with the existence of the temperature difference, the greater the temperature difference, and the greater the stress temporarily, and disappear with the disappearance of the temperature difference. This thermal stress is called temporary stress.
But before the temperature is out of adjustment, what should be paid attention to, of course, the stress can be eliminated by itself. When the stress value exceeds the ultimate strength of the glass, the glass will rupture itself abnormally, so the heating or cooling speed of the glass in the brittle temperature range should not be too fast.
Thermal stress caused by temperature difference, ②Permanent stress. When the glass starts to cool from above the strain point temperature. After the glass is cooled to room temperature and the temperature of the inner and outer layers is out of adjustment, it cannot be completely dissipated. There is a certain stress in the glass imitating as before. This stress is called hydrostatic stress. The magnitude of the permanent stress depends on the cooling rate of the finished product when the temperature is above the strain point, the viscosity of the glass, the thermal shortening factor and the thickness of the product and other glass machinery.
is the annealing of the glass by reheating the glass product with permanent stress to a temperature at which the internal particles of the glass can move. The displacement of the holding mass points to disperse the stress (called stress relaxation) to eliminate or weaken the permanent stress. The rate of stress relaxation depends on the temperature of the glass. The higher the temperature, the faster the relaxation rate. Therefore, a suitable annealing temperature range is the key to the excellent annealing quality of the glass. In the actual production process, it is impossible to completely eliminate the permanent stress. Through annealing, the residual stress is wished to increase or homogenize to the minimum limit, so as to enhance the mechanical properties of the glass. Strength and thermal stability of the characteristics of thick glass annealing The production of thick glass mainly includes the reverse method and the baffle method. Here, it is important to negotiate the annealing of the baffle method for the production of thick glass. The main principle of thick glass produced by the baffle method is to “thaw” the baffle area of the molten glass. The important features of annealing are as follows, 1. The thick glass produced by this method has cold edges and thin glass edges, so the edge compressive stress is very large. 2. The thicker the glass, the longer the heat stays in the glass. 3. In order to meet the cutting request, the tempering stress must be reduced. 4. It is necessary to prevent the edges from cooling too fast in the post-annealing zone, which will cause higher temporary tensile stress at the edges, which will lead to longitudinal crack loss.
The advantages and disadvantages of annealing for thick glass at the edge of the control are closely related to the thickness curve of the glass. Annealing improves the thickness curve. Extraordinary is the thickness of the edge of the glass plate. The individual requires that the thickness curve is to ensure that the thickness at the edge of 25 mm is about 1 mm thinner than the even thickness. In the past production, I often failed to do this. The first thing is that the edges are too thin to reduce the load to reduce the tempering stress. The increase in thickness of the thick glass production leads to a delay in the connection of heat in the glass body. If it is produced under high load Thick glass will increase the tempering stress in the glass body, which will bring difficulties to subsequent cutting and customer re-cutting. It will also cause troubles in annealing, increase bursts, and reduce yield. Therefore, the load must be reduced to meet the appropriate tempering stress. Increase the gas spray gun in the A zone and increase the chilled water after the washing machine. Zone A and Zone B advance the temperature of the edge as far as possible, because the gap spray gun has been added at the entrance of the annealing furnace, so it is a good choice to add a pair of gas spray guns in the zone A to advance the tensile stress of the glass edge. In the same way, adding chilled water after the washing machine can also increase the weight and tensile stress of the edge, and achieve the purpose of improving the slitting.
The longer the heat stays in the glass, the longer the temperature in zone A should be lowered. As mentioned above, the thicker the glass. Corresponding to its annealing time is also longer. Because the annealing length of the annealing furnace is fluid, it is possible to delay the annealing zone by properly lowering the temperature of the A zone, and finally achieve the purpose of reducing the tempering stress of the glass. The RET zone adds a side gas spray gun. In the production of thick glass, the largest loss of longitudinal cracks in the glass, if the longitudinal crack occurs, it will take as little as half an hour, and if the longitudinal crack occurs, it may or may take a few hours. Therefore, I have to take special precautions against similar situations that occur in the production of thick glass. Maybe take measures to improve the thickness curve to control the edge thickness; reduce the load; increase the gas spray gun in the A zone and increase the chilled water after the washing machine; properly reduce the temperature in the A zone; increase the edge gas spray gun in the RET zone to achieve the best yield and The best cutting quality.