Increasing Printable Solid Loading in Digital Light Processing Using a Bimodal Particle Size Distribution

Digital light processing (DLP) is rapidly growing in popularity as an additive manufacturing method for the fabrication of composite structures, and is an effective way to prepare high-resolution filled parts, such as ceramic green parts or composite magnets. Yet, higher solid loadings of resins and the resulting dramatic increases in viscosity limit DLP printing for applications that depend upon maximization of filler content. In this work, we investigate the capacity of a bimodal particle size distribution to enable the printing of a photosensitive resin containing up to 70 vol% of fillers. Formulations with unimodal and bimodal solid loadings ranging from 50 to 72 vol% are prepared and their viscoelastic properties measured through rotational rheometry. The zero-shear viscosity of these formulations is fit with a Krieger-Dougherty model, and the dense random close packing fraction of particles determined from the fit is found to be 76.3 vol%. Parts with fine positive and negative features are printed to evaluate the resolution achievable with highly filled photosensitive resins. The part printed with a 70 vol% formulation shows negative features as small as 860 μm, and positive feature as small as 380 μm. Finally, parts with complex patterns are prepared with previously tested formulations, and thermogravimetric analysis results show that the filler content within the parts remains relatively constant over a 3-h print, with a decrease of 2 vol% of solids for the final printed layers.