The goal is to eliminate permanent stress and structural imbalances in glass products. Glass is a poor thermal conductor. After the product is molded, the surface layer and the inner layer produce a temperature difference during the cooling process.
When the surface layer is solidified and the inner layer is viscous, the temperature difference exists and the stress relaxation does not exist. This temperature difference is when the surface layer is cooled to room temperature. The inner layer continues to cool down and shrinks, and the surface layer hinders the generation of tensile stress. At the same time, the surface layer generates compressive stress and is permanently present.
The magnitude and distribution of the permanent stress caused by the thermal process at each part of the product will not be uniform. Therefore, the strength of the glass product will be affected, and even the stress concentration will cause it to rupture. Annealing can eliminate the harmful tension inside the glass and prevent new stress from being generated. During annealing, the glassware is heated or maintained at the annealing temperature after thermoforming, so that the original stress is relaxed and eliminated, and then slowly cooled to below the strain temperature.
After the glass completely enters the rigid body state, the temperature difference between the inner and outer layers only generates temporary stress. . Due to certain properties and functions of the glass (such as optical glass, thermometer glass) in the transition temperature range, the large viscosity makes the structure particles move slowly, causing its performance to reach the equilibrium state with the temperature, using slow balance There is a change in performance, so it must be kept at the annealing temperature for a sufficient period of time.