Evaluation of the viscoelastic behaviour and glass/mould interface friction coefficient in the wafer based precision glass moulding

Abstract

Recent improvements in the manufacturing process of camera lenses have introduced the use of a new technology involving wafer based precision glass moulding. The utilization of this technology has some important advantages such as cost reduction, supply chain simplification and higher image quality. However, the required accuracy for the final size and shape of the moulded lenses as well as the complexity of this technology call for a high level of process understanding and numerical simulation is a very important part of achieving this goal. The viscoelastic parameters of the optical glass as well as the glass/mould interface friction coefficient play a key role in deformation behaviour and stress distribution of the moulded glass lens. Therefore, a proper evaluation of these parameters is the first important step in numerical modelling of the precision glass moulding process.The current paper deals with characterization of the interfacial glass/mould friction coefficient and viscoelastic behaviour of the L-BAL42 glass material above the glass transition temperature. Several glass rings are pressed at three different temperatures to various thicknesses and the experimental force, displacements, internal diameter and thickness of the rings are measured during the tests. Viscoelastic and structural relaxation behaviour of the glass are implemented into the ABAQUS FEM software through a FORTRAN material subroutine (UMAT) and the FE model is validated with a sandwich seal test. Then, by FE simulation of the ring compression test and comparison of the experimental creep with the simulated one in an iterative procedure, viscoelastic parameters of the glass material are characterized. Finally, interfacial glass/mould friction coefficients at different temperatures are determined through FEM based friction curves combined with experimental data points. The obtained viscoelastic parameters and interfacial friction coefficients can later be employed for prediction of the final shape/size as well as the stress distribution in the glass wafer during a real wafer based precision glass moulding process.