Enhanced machining of micron-scale features in microchip molding masters by CNC milling

An improved approach for the generation of aluminum masters used in the replication of polymer-based microfluidic devices is proposed. It is commonly assumed that conventional computer numerical controlled (CNC)-milling yields insufficient specifications for use in micro-milling. As molding masters suitable for microfluidic device replication require features whose dimensions are on the order of tens to hundreds of microns, the previous limitations of CNC-milling include machining tolerances that exceed 25% of the machined feature and inability to mill intersecting features. Here, we present several novel and advanced machining techniques to overcome existing limitations of a CNC-milling based approach. To achieve microfeatures with tolerances up to a factor of ten better than expected from the CNC-mill used, external calibration and measurement of the milling process was employed. To reduce the unwanted volume occurring from two intersecting features, 102 μm dia. high-density micrograin carbon endmills were used with all milling steps externally calibrated and measured. To generate masters with abutting features of different widths, keyway slots were milled into the master that could accommodate various size molding master keys. In addition to adding flexibility to the downstream embossing step, the use of keys eliminated dead-volume effects. A variety of molding masters have been machined and characterized in this work. Taken together, these approaches illustrate that conventional CNC-milling using equipment commonly found in university machine shops is a viable alternative to other more expensive master generation processes. Molding masters can be machined to yield high aspect ratio microfeatures with a tolerance of ±4 μm, and these masters can then be used in conjunction with casting or hot embossing to produce polymer microchip platforms suitable for analytical use.

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