Research On Injection Compression Molding Of Microlens Array And Molded Part Birefringence

Polymer microlens array is one of the most important optical elements in micro-optical system for its advantages such as highly compact design and easy assembly. However, microlens array fabricated by traditional injection molding, the most widely used process for polymer microlens array fabrication, is found to have unacceptably large residual stress and birefringence. This paper carries out preliminary research to realized high volume and high quality production of microlens array by introducing injection compression molding to minimize residual stress and birefringence. Mold design and process parameters have been investigated by both simulation and experiments.Residual stress of injection compression molded microlens array was simulated in commercial software package MPI/Injection-compression. The processing state parameters affecting residual stress, i.e. melt pressure, shear rate and volumetric shrinkage were studied in comparative approach. Orthogonal experiment design was used to optimize and investigate the effect of the injection parameters, then the effect of compression parameters, i.e. compression distance, compression speed and compression force were investigated through single factor experiment.The compression and mold design plan of injection compression mold was reviewed and investigated. According to the structure and forming requirement, and taking into consideration of compression method, compression region, cavity layout and gate type, an injection compression mold based on the compression frame concept was fabricated and assembled.Injection compression molding experiment was carried out. The birefringence distribution and value of injection compression molded and injection molded microlens array was compared. It was found that the average birefringence of injection compression molded microlens array is significantly smaller than that by injection molding. Single experiment was carried out to investigate the effect of compression parameters on part birefringence. The injection compression molded microlens array under the best parameters is further reduced than before optimization and that by injection molding.