(603g) Cure-through Correction in Continuous Stereolithographic 3D Printing

Recently, continuous stereolithographic 3D printing (SL) technologies have been developed which significantly increase print speeds. By introducing a dead zone adjacent to the window where polymerization does not occur, continuous SL eliminates several time-consuming steps present in traditional SL and enables speeds up to 2000 mm∙h^-1. Two methods have been reported to generate the dead zone: (i) oxygen inhibition, and (ii) dual-wavelength exposure with complementary photoinitiation and photoinhibition.

In continuous SL, the print speed must be limited to ensure high fidelity of the printed part. Accuracy along the vertical axis is affected by the penetration depth of light into the resin. When light penetrates too deep into the resin bath, unwanted curing (known as cure-through, overcure, the backside effect, or print-through error) can occur, particularly on the backside of exterior surfaces and into internal voids. Cure-through also contributes to dose heterogeneity, introducing internal stresses which can deform the part and further reduce fidelity. Cure-through has typically been mitigated by adjusting the resin formulation to control penetration depth; however, highly absorbing resins require either higher-intensity light sources or, more commonly, slower print speeds.

Alternatively, high fidelity can be achieved without reduced print speeds by modifying slices to account for expected cure-through. This approach requires both a curing model and a slice correction algorithm. While slice correction has been developed for both external features and internal voids in non-continuous SL, existing models for continuous printing are not tailored to this purpose.

We have developed a dose-based curing model and a slice correction algorithm to precisely tune exposure levels and minimize cure-through artifacts in continuous SL. Our approach differs from previous slice-modification methods in several ways. First, we are considering a continuous system, where cure-through is a more significant and complex problem. Second, we are developing corrections using greyscale, while previous approaches were restricted to white and black. Finally, our correction uses an exact mathematical solution, while previous approaches have used heuristics and optimization.

The mathematical approach will be presented along with theoretical and corresponding experimental results from a dual-wavelength continuous SL printer. Our model allows prediction of the printed part resulting from a given set of slices and analysis of the contributions of each slice to the accumulated optical dose at each point. We have also experimentally investigated the effects of dose heterogeneity to determine optimal dose profiles. The method is demonstrated to yield marked improvement in print fidelity while maintaining high print speeds.