Three-dimensional grayscale for improving surface quality in projection microstereolithography

A rough surface on mechanical parts results in increased friction and possible breakdown. In particular, surface roughness of micro-scale parts can affect function due to friction. Hence, a material with proper mechanical properties must be determined according to the machining method used. Layered manufacturing technologies have been used to make complete three-dimensional (3-D) microstructures. In projection microstereolithography (PμSL), a 3-D microstructure is made by stacking layers that are polymerized using a patterned beam. Due to the machining characteristics of layered manufacturing, the completed microstructure shows a different surface profile, called a stair-stepping effect, compared to the original CAD model. Generally, stair-stepping effect lowers the surface quality. We propose a 3-D grayscale for improving the surface quality of microstructures in a PμSL base on a digital micro-mirror device (DMD). In this method, dithering is applied to every monochrome cross-sectional image to give a grayscale effect, and additional cross-sectional images between layers are temporarily used to express a more accurate surface profile. Curing experiments using the 3-D grayscale method are performed, and stair-stepping of a layered microstructure using conventional cross-sectional images and 3-D grayscale are compared. Stair-stepping resulted in improvement without additional fabrication time.