The need for ultra-precision optical components with intricate geometric profiles has grown rapidly in the last few decades. Applications of precision optical lenses range from consumer electronic products to optical sensing instruments, microscopy, astronomy, etc. Typically, polymer-based lenses have been used and have dominated the industry so far, but due to the advantages of using glass components, the demand for ultra-precision glass aspherical components has been steadily raising. In fact, it is estimated that the demand for aspherical glass lenses will grow at a rate of 6.5% in the next five years. However, the conventional manufacturing processes when used for producing aspherical glass components become time-consuming and expensive.
Precision glass molding (PGM) technology provides an alternative manufacturing technique to fabricate aspherical glass lenses and irregular optical products. It has the advantages of high forming accuracy, short manufacturing cycles, low cost, and high volume production compared to the traditional manufacturing process. However, the process has a few drawbacks such as lens profile deviations, stress birefringence, etc. Typically, the mold surfaces are machined to be exact negatives of the required lens profile, assuming the lens would take the shape of the molds. But in reality, the complex mechanical behavior of the glass and its high-temperature dependence affects the final lens profile at room temperature. In addition to geometric deviations, the rapid temperature changes, often as much as several hundreds of degrees, in a short time affect the performance of the molded lens. These drawbacks need to be addressed before the glass molding process can be used as a viable option for mass-producing optical components.
As such, in this dissertation, a coupled thermo-mechanical finite element model is established to simulate the precision glass molding process on two different glass types, D-ZK3 (CDGM) and P-SK57 (Schott). The glass is modeled as a thermo-viscoelastic material by defining the stress and structural relaxation parameters. A new testing technique based on the cylinder compression test is developed in this study to extract the viscoelastic parameters at different temperatures. The obtained material parameters when used in the numerical simulations showed a good agreement with the experimental data throughout the testing temperature range. Further, the viscosity of the glass (a highly sought-after property of glass in precision molding) is obtained as a by-product of the proposed material calibration test. Finally, the structural relaxation parameters are obtained from the impulse excitation test based on ASTM standard E1876. All the experiments required for fully calibrating the viscoelastic response of the glass are performed on a precision glass molding machine, Moore Nanotech GPM170 machine. The obtained material parameters are used in the finite element model to predict the lens deviations and the stresses in the molded lens. A mold compensation technique is used to correct the mold profiles for any deviations. The lens molded using the corrected molds is shown to fall within the designer's specifications.
The process parameters used during the molding process play a vital role in determining the profile accuracy and the optical quality of the molded lens. Hence, it is important to determine an optimal parameter set before applying any mold compensation techniques. But due to the obscure and complex nature of the process, determining the parameter sets empirically is a tedious process. As such, in this study, the developed numerical model is used to individually analyze the different process steps and the corresponding process parameters on the profile deviations and the residual stresses in the molded lens. The results obtained in this study can be used as a reference to fast-track the manufacturing process.