Voxel-Scale Conversion Mapping Informs Intrinsic Resolution in Stereolithographic Additive Manufacturing.

In high-resolution stereolithography, printed parts deviate significantly from the projected photomask because of the reaction complexity on the voxel scale. To better understand the reaction process, we have developed a technique to measure local photopolymerization rates using a nanocylinder-tipped atomic force microscope cantilever. The drag force experienced during cantilever oscillations can be correlated to the viscosity and extent of reaction. Fluid dynamics simulations show micrometer-scale measurement localization, and the resonance-based measurement allows submillisecond temporal resolution. In a thiol-ene resin exposed to patterned light, oligomer diffusion length scales are significant compared to the size of printed structures. Consequently, part resolution is dictated by the competition between polymerization and diffusion. Because of the radical polymerization mechanism, increasing the light intensity at a constant dose decreases the local conversion, even though the diffusion length decreases. In the case of printed test structures with features matching the measured diffusion lengths, increased light intensity causes increased geometric aberration from the projected mask. Overall, the results indicate a need for enhanced control over polymerization and diffusion to obtain dimensionally accurate and mechanically homogeneous parts.