Modeling Reaction and Transport Effects in Stereolithographic 3D Printing

Abstract

Continuous stereolithography has recently emerged as a leading technology in additive manufacturing (3D printing). Though several methods for continuous printing have been reported, they all share the benefit of reducing forces on the growing part and eliminating adhesion to the resin bath due to the introduction of the dead zone, a region where polymerization does not occur. The recently developed dual-wavelength approach, in which photoinitiation and photoinhibition of polymerization are controlled via different wavelengths of light, has achieved unprecedented vertical print speeds via expansion of the dead zone. We address several limitations in dual-wavelength continuous printing (and some within continuous stereolithography more broadly) via theoretical and computational modeling and the use of spatially varying exposure patterns. First, we address the problem of cure-through, undesired curing along the axis of exposure, which is more significant in continuous stereolithography than in traditional layer-by-layer stereolithography. Recognizing that the use of highly absorbing resins to improve layer resolution inherently limits achievable print speeds, we developed a method to improve part fidelity in low- to moderate-absorbance resins through modification of the images projected during printing. We derive a mathematical model to describe dose accumulation during continuous printing, describe the resulting grayscale-based correction method, and experimentally verify correction performance. Using optimized parameters with a high absorbance height resin (2000 um), feature height errors are reduced by over 85% in a test model while maintaining a high print speed (750 mm/h). Recognizing the limitations of this model, we developed a kinetics-based curing model for dual-wavelength photoinitiation/photoinhibition under variable intensities. The model is verified via experimental characterization of two custom resins using cured height and dead zone height experiments. For the two custom resins characterized, the model achieves R2 values of 0.985 and 0.958 for fitting uninhibited cure height data and values of 0.902 and 0.980 for fitting photoinhibited dead zone height data. The model is also applicable to resins in standard layer-by-layer stereolithography, and for commercial resin cure height data, our model performs similarly to the standard Jacobs model, with all R2 values above 0.98. Finally, we introduce the complexities of resin flow during continuous printing. The kinetic curing model is used in a computational fluid dynamics model to analyze dead zone uniformity, which we find is greatly affected by exposure intensity ratio, while print speed and part radius have minor effects. We find that relatively small variations in the intensity ratio (25%) can have large effects, going from good printing conditions to print failure (curing to the window) or to significant nonuniformity (maximum dead zone height over three times the minimum). We optimize exposure conditions to maximize dead zone uniformity, finding that the ability to pattern light sources is critical in generating uniform dead zones: for a 10 mm radius cylinder, over 90% of the dead zone is near the optimized value when using patterned intensity functions, compared with only 18% when using constant intensity values. In printing experiments, we find that an optimized intensity function can, without modification, successfully produce difficult-to-print parts. Taken as a whole, the work advances our understanding of the dual-wavelength approach in continuous stereolithography, improves printing performance, and motivates future research into the wide range of physical phenomena affecting the system.