Finite element modeling of structural relaxation during annealing of a precision-molded glass lens

The precision lens molding process is numerically modeled by incorporating the characteristic structural relaxation phenomenon of glass during the annealing stage. A review of the structural relaxation theory as explained by the Narayanaswamy model (Narayanaswamy, 1971, J. Am. Ceram. Soc., 54(10), pp. 491-498) is provided and is then implemented into the simulation model. The commercial finite element method (FEM) program MSC MARC is utilized for the analysis. Glass material is modeled as a linear Newtonian during the molding stage and as a viscoelastic material exhibiting stress and struc-fluid tural relaxation during the annealing stage. A comparison between the final lens shape and volume results obtained by implementing structural relaxation and thermal expansion coefficient is shown. The results demonstrate the need for including structural relaxation in the simulation model for reliable predictions. The results also show that the FEM can be used to predict the process, improve performance, and also provide a deeper process understanding.