The purpose of glass annealing is to reduce and prevent excessive residual internal stress and optical inhomogeneity in glass products, and to stabilize the structure inside the glass.
The annealing of glass can be divided into two main processes: one is the weakening or disappearance of the internal stress in the glass, and the other is to prevent the re-generation of the internal stress. The weakening and elimination of internal stress in glass is based on relaxation theory. The so-called internal stress relaxation refers to the process of material dissipating internal stress under the action of molecular thermal motion. The relaxation rate of internal stress is largely determined by glass. the temperature at which it is located.
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1. Asymmetric cooling of the upper and lower surfaces of the glass ribbon
1.1 Asymmetric cooling of glass sheets in the annealing zone
The glass plate is in the annealing area. If the cooling strength of the upper and lower surfaces is different, when the glass is cooled to room temperature equilibrium, the stress distribution will be asymmetric, and the compressive stress will shift to the side that cools faster, and the cooling is fast (cooling). If the original glass sheet is large enough, the unbalanced stress distribution in the glass will inevitably cause deformation - bending or warping. If the lower surface of the glass plate cools faster than the upper surface, the side with the larger compressive stress is on the lower surface, and the plate bends upward; otherwise, it bends downward. If the generated stress exceeds the allowable value, bursting will occur.
1.2 Asymmetric cooling of glass below the annealing zone temperature
When the temperature of the glass is below the annealing zone temperature, the cooling rate of the upper and lower surfaces is inconsistent, and temporary stress will be generated.
2. The lateral temperature of the glass ribbon is not uniform
2.1 Inhomogeneous distribution of temperature laterally symmetrical
The temperature distribution is symmetrical about the centerline, but the edges are cooler than the center or the edges are hotter than the center. There is a temperature difference in the width direction of the glass ribbon, which also generates thermal stress.
When the edge of the glass ribbon is cooler than the middle, if it is in the annealing temperature region, the edge (cool) is subject to compressive stress, and the middle (hot) is subject to tensile stress; if the glass is below the annealing temperature, the edge (cool) is subject to tensile stress , the middle (thermal) compressive stress.
When the edge of the glass plate is hotter than the middle, the stress in the direction of the plate surface is opposite to the above.
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The stress generated by the above conditions can be measured with a stress meter. A qualitative test can usually be done with fingers, that is, at a considerable distance from the position of the cutting machine, gently lift the edge with two or three fingers. If the edge is wavy and easily lifted, the edge is under compressive stress and the middle is under tensile stress; if the edge is tight and not easily lifted, the edge is under tensile stress and the middle under compressive stress.
It is well known that the compressive strength of glass is about ten times greater than the tensile strength, so the glass plate usually first cracks on the surface of the plate under tensile stress, and then the crack may continue to develop into the compressive stress area. Therefore, the temporary stress is the most destructive to the glass. As long as the tensile stress generated is equal to or slightly greater than the tensile strength of the glass, the glass will also rupture by itself without any external force.
2.2 The uneven distribution of temperature in the lateral direction of the glass plate
The lateral temperature distribution of the glass plate is asymmetric, that is, the cooling speed of the two sides of the glass plate surface is not the same, which causes the temperature on one side to be high and the temperature on the other hand to be low, and the stress distribution on the surface of the glass plate is also asymmetric. If it is in the annealing temperature region, the cold side will also become longer under compressive stress, and the hot side will become shorter under tensile stress, and the glass ribbon will shift to the hot side; if the glass is under the annealing temperature, the cool side will be under tensile stress, The hot edge is under compressive stress, and bursting occurs when the temporary stress exceeds the allowable value.
At this time, the theoretical stress value of the glass plate surface cannot be measured, because part of the stress is eliminated due to deformation.
When the glass ribbon leaves the annealing area, it can be cooled at a relatively fast rate, because the glass will no longer generate water stress. However, it must be pointed out that if the cooling rate is too fast or the lateral temperature difference is too large, excessive temporary stress or unbalanced stress distribution will result in the glass breaking in the annealing kiln, which often occurs in actual production.
2.3 Conclusion
In summary, within the annealing temperature range, the lateral temperature of the glass ribbon is not uniform. When the glass is cooled to room temperature, permanent stress will be generated. Some are under pressure. In the annealing temperature range and below, the hotter part of the plate is in a state of compression, and the cooler part is in a state of tension. From the point of view of the cracking of the glass, the glass ribbon is below the annealing area, and both sides are cold. Too fast is more disadvantageous than cooling down the middle, so it must be prevented. This situation is often caused by the leakage of cold air into the shaft ends of the rollers. Therefore, the shaft seal is crucial.
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