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After the glass tube is formed, the temperature difference between the surface layer and the inner layer occurs in the process of cooling, so as to eliminate the permanent stress and structural imbalance in the glass tube. Glass is a bad heat conductor. When the surface layer is solidified and the inner layer is viscous, the temperature difference exists and the stress relaxation does not exist. When the surface layer is cooled to room temperature, the inner layer continues to cool and contract, and the surface layer obstructs the formation of tensile stress, while the surface layer produces compressive stress. Annealing can eliminate harmful tension and prevent new stress in glass tube. During annealing, the glass tube is heated or maintained to annealing temperature after hot forming to relax and eliminate the original stress, and then slowly cooled to below the strain temperature. After the glass tube completely enters the rigid body state, the temperature difference between the inner and outer layers only produces temporary stress. In the range of transition temperature of some performance and function glasses, the huge viscosity makes the structure particles move slowly, so that its performance can not reach the balance state consistent with the temperature, and the performance will change due to the slow balance in use. Therefore, it is necessary to maintain sufficient time at the annealing temperature.

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