Polishing Pmma And Other Optical Polymers With Magnetorheological Finishing

POLISHING PMMA AND OTHER OPTICAL POLYMERS WITH MAGNETORHEOLOGICAL FINISHING Polishing PMMA and Other Optical Polymers with Magnetorheological Finishing Introduction Polymer optics are typically manufactured by injection molding (thermoplastics, high volume, economical), compression molding (thermosets, higher precision, and larger sizes), or diamond turning. Once formed by these techniques, polymer components are used “as manufactured,” usually without further cold working to improve surface finish or figure. This is because optical polymers are soft and possess high linear expansion coefficients and poor thermal conductivities.1 Attempts to improve surface finish and figure using conventional grinding and polishing processes usually result in scratching, the embedding of abrasive particles, the formation of “orange peel,”2 and degradation to surface figure. There are circumstances when it would be desirable to perform a classical polishing operation on a polymer surface. In some instances it is desirable to drive rms surface roughness values below 2 to 4 nm in preparation for deposition of a coating. In other applications, reduced surface form errors are required. It is advantageous to eliminate the unwanted flare from diamondturning marks on polymer optics in order to test prototype imaging system designs. Magnetorheological finishing (MRF) is a new polishing process that was invented and developed by an international group of collaborators at the Center for Optics Manufacturing (COM) in the mid-1990s3 and commercialized by QED Technologies, Inc. in 1997.4 MRF is based on a magnetorheological (MR) fluid consisting of nonmagnetic polishing abrasives (typically CeO2 or nanodiamonds5) and magnetic carbonyl iron (CI) particles in water or other carriers. With the appropriate MR fluid, MRF has successfully polished a variety of materials to subnanometer rms surface-roughness levels with peak-to-valley (p–v) form accuracies to better than 20 nm. Polished materials include optical glasses (fused silica, BK7, SF6, LaK9), hard crystals and polycrystalline glass-ceramics (silicon,6 sapphire,7 Zerodur, and Nd: YLF8), soft UV and IR materials (CaF2,9 AMTIR-1,10 polycrystalline ZnS,10 and soft phosphate laser glass), and soft, water-soluble potassium dihydrogen phosphate (KDP) frequency-conversion crystals.11 LLE Review, Volume 96 A normal force of the order of 0.01 N between the abrasive particle and the part is the key to removal in most classical polishing processes. In MRF, however, there is almost no normal load.12 Figure 96.38 shows a schematic of a part being placed into a ribbon