Transparent thermoplastics: Replication of diffractive optical elements using micro-injection molding

Small plastic components with sub-micron and micron gratings for diffractive optics were prepared by micro-injection molding. The aim of the work was to improve the filling of binary diffractive gratings with high aspect ratio by varying the molding parameters. Tests were made under conventional processing conditions with four transparent thermoplastics: polycarbonate (PC), cyclo-olefin polymer (COP), styrene–acrylonitrile copolymer (SAN), and hexafluoropropylene–tetrafluoroethylene–ethylene terpolymer (HFP–TFE–Et). Melt and mold temperatures were kept as recommended by the manufacturer. Other molding parameters (injection speed, shot size, vacuum, holding pressure, and injection plunger diameter) were varied, and their effect on the profile of the gratings was measured by atomic force microscopy. The filling of the gratings (500 nm and 1000 nm) was clearly affected by injection speed, shot size, and injection piston diameter, but the most significant factor was the type of material. Replication fidelity was highest with PC and lowest with SAN.