Open PhD position: Modeling of residual stresses and deformations of ophthalmic lenses made of thermosetting thermally polymerized materials

The global objective of the current work is to numerically investigate the forming of ophthalmic lenses by the thermal polymerization and identify the occurrences of residual stresses.
ESSILOR company is specialized in the design, manufacture and marketing of corrective lenses. As part of their manufacturing process, the company wishes to better understand the deformations that may appear on its semi-finished glasses after surfacing. Such deformations are linked to the residual stresses generated during the polymerization process related, among other things, to the shrinkage of the thermosetting polymer. After polymerization, the glasses are annealed in order to release the residual internal tensions.

The Center for Material Forming (CEMEF) has strong skills in experimental characterization and in modeling of the fluid and solid behaviors of polymer materials which allow to contribute to the proposed subject.

In this context, CEMEF and ESSILOR are carrying out a joint project on the topic of modeling residual stresses and deformations of ophthalmic lenses made from thermosetting thermally polymerized materials.

The objectives of the study will be:
1) To characterize the parameters necessary for understanding the development of stresses during the polymerization cycle and their changes during the standard annealing cycle,
2) To model the evolution of the material during its polymerization cycle and during its standard annealing cycle,
3) To find the ways of controlling the process and the annealing cycle likely to reduce, ideally eliminate, the residual stresses.
These activities will be based on the characterization methodologies developed at CEMEF concerning the polymerization phase and the characterization of matter in the solid state. They will also involve expertise in the modeling of multiphysics couplings in the laboratory.

TherCast software marketed by TRANSVALOR will be used for this study. The latter makes it possible to simulate solidification processes coupled with mechanics.

The thesis work will aim:
• To propose test protocols for the characterization of polymerization kinetics but also of the thermal and mechanical properties of the various materials in the study;
• To model the polymerization phase of the polymer in the mold and in the annealing cycle phase in order to highlight the levels of residual stresses involved;
• To conduct a reflection in collaboration with ESSILOR on methods for characterizing internal stresses in the material;
• To analyze the capacity of the model to reproduce the different steps of the process, analysis associated with a study of the sensitivity of the model parameters with the aim of proposing parameters of annealing cycles likely to reduce, ideally to eliminate, the residual stresses.