CONCLUSION
The motivation of this dissertation research was to investigate and develop a methodology on precision polymer optics fabrication by injection molding that can be used for high volume and low cost lens manufacturing. Injection molding polymer optical components have long been used for its high volume, low cost and lightweight capability over traditional glass optics. Injection molding is an inherent freeform process thus complex geometry (including aspherical and freeform design) may be readily manufactured. However, the process has not been readily accepted in precision optical fabrication industry because several difficult issues related to the injection molded optics have hindered the implementation of injection molding process in high precision applications. These issues include geometry deviation and inhomogeneous index distribution due to thermal shrinkage; birefringence incurred during the molding process also limited the adoption of polymer optics in certain polarization sensitive optical systems; thermal instability of molded polymer lenses can also render the optics less effective in application where temperature changes become large and frequent (such as
research involved in polymer injection molding was focused on the determination of process parameters in order to optimize part quality but did not address the issues concerning mold compensation for high precision polymer lenses. Also the optical effects from process conditions of lens injection molding such as index distribution, residual stress/birefringence and optical scattering were not studied systematically. Furthermore, with the high precision requirement of the optical system, freeform optics including microlens array and diffractive optics can provide a practical solution for some design and manufacturing problems. The success of the process relies on the fabrication of the mold inserts and measurement technology. Fewer articles discussed the advanced mold fabrication and measurement issues. It is necessary for current researchers to make efforts to improve the injection molding process on precision optical component production.
This dissertation research involved fundamental and systematic study of precision polymer optics fabrication by injection molding. The study included both experimental approach and numerical modeling in order to identify the proper polymer lens manufacturing processes. The scope of this research includes investigation in optical design, mold and lens fabrication, as well as optical metrology issues related to polymer lens manufacturing to obtain precision macro and micro polymer freeform optics with accurate geometry and proper optical performance by state-of-the-art mold fabrication and molding technology.
In Chapter 3, with the aid of DOE and DEA methods, the critical process parameters including packing pressure, mold temperature and melt temperature were narrowed down for other process and performance studies and the optimal condition was found for compensation study both by the plano lens molding experiment and measurement results. The mold compensation methodology was developed based on advanced freeform measurement and data analysis technology and STS freeform mold insert fabrication.
In Chapter 4, the effects of the process parameters to optical performance such as birefringence, index distribution and surface scattering were carefully studied by theoretical and empirical analysis. Lower packing pressure, higher mold temperature and melt temperature were better setup for lens molding with lower birefringence. Lower packing pressure and higher mold temperature were proven to be better for lens molding with smaller index deviation. Higher packing pressure, lower mold temperature and higher melt temperature were better for lens molding with lower optical scattering. Due to the complexity of the injection molding process, single process condition cannot fulfill all the requirements for lens quality requirements so process parameters need to be selected as a compromise for desired specification.
In Chapter 5, macro Alvarez lens and micro Alvarez lens array were fabricated. The mold inserts were successfully machined using slow tool servo and broaching process. The injection molded Alvarez lens can fulfill the requirement of vision test and
molded micro Alvarez lens array and design was around 0.2 μm with the P-V value of the design at 10 μm. The average error of the adjustable focal length was only 5.2%.
In Chapter 6, diffractive lenses and Fresnel lenses were fabricated. The fabrication of the multilevel DOEs with STS has proven that STS can provide an easy and quick solution without expensive and time-consuming mask making and lithography in cleanroom. The measurement results of the Fresnel lens showed that more accurate lens profile can be obtained under higher mold temperature, higher packing pressure and higher melt temperature. The mold temperature is also critical to the lens geometry accuracy. The same conclusion was drawn from optical performance simulation.
In addition, simulation using Moldflow was implemented to verify the experiment results, for example, plano lens warpage and birefringence. The tendency of the simulation results was similar as the experiment results. However, accurate predictions can not be easily obtained using commercial software in all cases.
This dissertation research was an attempt to create a methodology for injection molding process for high precision polymer lens manufacturing. Experimental study and process modeling were conducted to develop a fundamental understanding of the process. The feasibility of lens compensation using freeform mold were fully tested. Other functional freeform optical elements were fabricated and numerical simulation was utilized to predict the optical performance of the molded elements. The contributions of
• Performed experiments (both axisymmetric lenses and freeform lenses) and evaluated surface geometry and optical performance to investigate the feasibility of using injection molding process to manufacture high precision polymer lenses.
• Explored the effects of process variables and material property for specific objective function (surface shape deviation, birefringence, optical retardation, optical scattering) for lens performance optimization.
• Utilized current measurement methods and developed freeform data analysis method for real surface shape, part thickness and optical performance measurement.
• Designed and fabricated multiple freeform mold inserts and obtain functional injection molded freeform optics including compensated lens, Alvarez lens, micro Alvarez lens arrays and diffractive lenses.