Thermal annealing system and process design to improve quality of large size glasses

Large size optical glass has attracted much attention due to its applications in large optical devices. Fine annealing is widely employed to improve mechanical and optical quality of glass products. During fine annealing, the temperature distribution in glasses is crucial to the final quality of optical glasses as a result of its influence on thermal stress and optical homogeneity. To ensure its high performance, the temperature distribution in glasses should be homogeneous and symmetric, and the maximum temperature difference should be small. In this study, two approaches are proposed to improve the temperature uniformity in glasses during fine annealing. Firstly, the glass blocks are packaged with heat exchange blocks on the top and bottom surfaces, and an insulation layer on the side. Simulation results show that the homogeneity and symmetry of temperature distribution in glasses can be improved. Temperature difference in the horizontal direction can be further reduced in the case using 10 mm copper heat exchange block together with 50 mm insulation layer. Secondly, a muffle apparatus is utilized. The symmetry of temperature distribution in glasses can be improved. In conclusions, the first approach can be used to achieve more symmetric and homogeneous temperature distribution, while the second approach can be used for changing the symmetry of temperature distribution in glasses. Combining two approaches can be used to design thermal system and industrial fine annealing process of optical glasses upon which the overall stress level can be reduced and the symmetry of the residual stress can be improved.