Stress spots are also called wind spots. Due to the uneven heating and cooling processes in the glass tempering process, different stress distributions are generated on the surface of the glass plate. Under the irradiation of polarized light, optical path difference occurs due to uneven stress distribution, and stress spots appear. This unique light interference phenomenon. Stress spots are also caused by uneven stress. For example, in the heating process, there is a temperature difference between the edge of the hearth and the center, resulting in uneven stress. Stress spots are not yet completely eliminated, but well-designed tempering equipment and scientific processes can greatly reduce the visibility of stress spots.
1, the cause of the formation of stress spots
The phenomenon of stress spots caused by the tempered and semi-tempered glass as anisotropic materials can be understood by the light path diagram: When a polarized light passes through a tempered glass, a stress difference exists in the glass, which causes the surface particles to become dense and the center particles are loose and in various positions. Differently, this beam of light has a larger refraction angle at a relatively light density, and a relatively smaller angle of refraction at the light-repellent area, and is decomposed into two polarized lights with different propagation speeds. When two lights formed at a certain point intersect with light beams formed at another point, there is a phase difference at the intersection of the beams due to the difference in the speed of light propagation. At this point, the two lights will produce an interference phenomenon when the two lights are When the amplitude direction is the same, the light intensity is strengthened, resulting in a bright field of view, ie, a bright spot; when the direction of the light amplitude is opposite, the light intensity is weakened, resulting in a dark field, ie, a dark spot.
2, the impact of heating on the stress spot
The characteristics of the physical tempered glass is that the glass is heated first and then air-cooled to make the glass form internal and external stress difference. At present, the mainstream tempering equipment is batch-type convection ovens (single chamber furnaces, dual chamber furnaces, and bending furnaces) and continuously produced radiant furnaces (continuous furnaces). Convection furnace glass is affected by convective wind pressure, heating wire and roller heat conduction. The ideal state is that the temperature of each point is the same at the end of the heating of the glass, or it is understood that the temperature field uniformity of the heating is Good.
Because of the existence of these three factors, the change of any one of these factors will cause the change of the temperature field and directly affect the heating effect of the glass. If the temperature of the glass is not uniform, it will enter the same air-cooling system. Because the cooling shrinkage is inconsistent, the stress spot is obvious. Therefore, the primary factor for reducing the stress spot is to ensure uniform heating. When the glass is baked, the thermal imaging shows that the glass is unevenly heated, and the uneven stress spots are obvious.
3, how to obtain a uniform temperature field
First of all, we must consider whether the power value of the heating furnace wire is even. The inspection method can be used to judge the intensity of the current through the furnace wire. Furnace aging during use, affecting the power to play, or using different manufacturers of furnace wire will also have a larger power deviation. Take the TAMGALSS convection oven as an example, there is a deviation in the value of the passed current.