Evaluating the viscoelastic properties of glass above transition temperature for numerical modeling of lens molding process

Glass molding process has emerged as a promising way to produce complex optical elements with high precision. Glass material shows explicit viscoelasticity at molding temperature, therefore studying the viscoelastic properties of glass at elevated temperature is important for the molding process. In this paper, Young's modulus and viscosity of glass were tested by compressing cylindrical glass gobs above the transition temperature, and obtained by curve fitting using the Burgers model of viscoelastic deformation. Based on the viscoelastic parameters obtained from experiments, a numerical model was developed to simulate the glass molding process with the help of a commercial finite element method software package. The simulation results provide an easy way to analyze and understand the molding process in detail, such as temperature distribution, stress, strain and strain rate, which are difficult or impossible to measure in experiments. A good agreement between the calculated pressing loads and the experimental results verified the validity of the numerical model, which can be applied to predict the pressing load during complex-shape glass lens molding process.