Polishing PMMA and other optical polymers with magnetorheological finishing

Abstract

Magnetorheological finishing (MRF) was used to polish as-molded or diamond turned surfaces of several optical polymers. Materials included polymethylmethacrylate (PMMA), cyclic olefin polymer (COP), polycarbonate (PC), and polystyrene (PS). Parts were nominally plano circular discs of various diameters (~40 mm to 75 mm) and thicknesses (2.5 mm to 25 mm). Polishing trials were conducted with standard CeO₂-based and nanodiamond-based MR fluids, or with MR fluids containing SnO₂, ZrO₂, Al₂O₃, TiO₂, or SiO₂. Excellent results were obtained for PMMA using a ZrO₂-based MR fluid. The diamond turned plano surface of a 38-mm diameter by 8-mm thick puck was improved from an initial p-v wave front error of 4.5 µm to 0.35 µm with two figure correction runs. The average rms surface roughness was reduced from 3.8 nm to 0.47 nm, and the diamond turning marks were eliminated. Mounting and thermalization of polymer parts for in-process and final metrology was found to be a challenge.