Study Of Residual Stresses In Compression Molding Of Precision Glass Lenses

As an alternative method for fabricating glass lenses, compression molding is a very attractive process. Compared to the conventional glass fabrication techniques, compression molding is an environmentally conscience process since it is an near net-shape process, high productively and low cost, suitable for mass production of glass lenses in industry.Compression molding is a hot forming method in which a glass blank was pressed by molds to create the lens shape. Residual stresses were frozen in the glass lens due to the nonuniform of temperature and complicated process when the glass lens went through its glass transition region during the cooling process. The existence of residual stresses in glass lenses lead to birefringence and refractive index variation which affect the optical performance of the glass lens.In this research, both experimental and numerical simulation methods were employed to study the residual stresses inside compression molded glass lenses. And annealing experiments of molded aspherical glass lenses were investigated to reduce the residual stresses and refractive index variation in the glass lenses.Residual stresses inside a cylindrical glass lens and compression molded aspherical glass lenses were measured in this research. The residual stresses inside the glass lenses were measured by a circular polariscope. With six-step left right phase shifting technique, isoclinic angle and optical retardation of the glass lens under test can be measured. Taking advantage of axisymmetrical properties of the residual stresses and weak birefrignece of the glass lens, the residual stresses can be reconstructed using the relations between stresses and parameters of photoelasticity, equilibrium equation and generalized sum rule.Residual stresses inside the glass lenses were also studied by numerical simulation. The simulation results demonstrated a reasonable agreement with experiment results. It shows that numerical simulation can be used to predict the residual stresses of compression molded glass lenses.The influences of heat capacity and the coefficient of thermal expansion of the glass on residual stresses in numerical simulation were studied. The influences of cooling rates were also studied by numerical simulation. Based on the results, the cooling rate before break temperature of BK7Glass plays an important role in the cooling process. As the cooling rate decreased, the residual stresses became smaller but the surface deviation became bigger.In order to reduce the residual stresses inside the compression molded aspherical glass lenses, annealing experiments were conducted. The influence of annealing on the aspherical glass lenses was evaluated by the change of residual stresses, refractive index vaiation and aspherical surface curve deviation of the glass lens pre and post annealing experiement. The residual stresses were measured by the circular polariscope. The refractive index variation was measured by an optical setup based on Mach-Zehnder interferometer. And the aspherical surface curve deviation was measured by a profilometry. The results showed that the residual stresses can be reduced when the annealing temperature is lower than the glass’s strain point, however the refractive index experienced a minor change. If the annealing temperature is near the glass’s annealing point, both residual stresses and refractive index variation of the glass lens can be reduced. While the aspherical surface curve deviation has a minor change after annealing experiments.Based on the experiments and simulations shown in this research, a new compression molding process can be proposed. Once the glass lens was fabricated by compression molding, an annealing process could be used to reduce the residual stresses and the refractive index variation in the glass lens.