Frozen-in Birefringence and Optical Performance of Injection/Compression Molded Optical Products: Quantitative Approach

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With the development of information technology and nanotechnology, the manufacturing via molding of precision plastic optical parts such as light guide plates, lenses, disk substrates and other optical components are continuously gaining more importance. The birefringence of molded plastic optical parts introducing optical anisotropy is one of the major problems, lowering their optical quality. Until now, trial-and-error procedures have been required to meet the stringent specifications of the optical parts, because of the lack of science-based technology suitable for the calculation of optical performance of moldings as affected by inherent optical characteristics of polymers and processing conditions used in the manufacturing process. The presently available fundamental tools to evaluate optical performance characteristics of products, such as brightness and aberration, are inadequate and based on geometric ray tracing with the assumption that the media is isotropic. Due to the neglect of anisotropy, inevitable discrepancy appears between the theoretical and experimental evaluation of optical parts. The present proposal attempts to develop novel scientific approaches for predicting optical performance of molded plastic parts with anisotropy by carrying out targeted theoretical and experimental research. Theoretical studies include numerical simulation of frozen-in birefringence and subsequent calculation of the principal refractive index tensor and the directions of the principal axes of the molded parts as affected by processing parameters and measurable inherent optical properties of polymers. This will be done by considering nonisothermal polymer melt flow with solidification using a nonlinear viscoelastic model combined with the stress-optical rule, and a linear viscoelastic solid model, combined with a photoviscoelastic model (optical memory), and free volume relaxation theory. Then a ray-tracing algorithm through anisotropic media will be developed to predict the brightness distribution on light guide plate surfaces and the aberration of lenses and suitable for utilization in practical application for optical products. Extensive molding and characterization experiments will be conducted on optical materials and products to validate the proposed theoretical approaches. The proposed research will have a significant impact on the development of technology for manufacturing of optical products in various industrial areas and will help to develop a methodology to increase the optical quality of molded plastic components. Theoretical modeling and computer simulations of molding processes will aid to optimize the mold design parameters and processing conditions eventually eliminating a trial-and-error procedure in the manufacturing.