Christopher. J. Hansen

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.

Digital light processing (DLP)-based additive manufacturing has emerged as a powerful technique for fabricating structures from filled resin systems, in which the light scattering behavior is critical to the dimensional fidelity of the cured part. Recently created low density filled resins that incorporate hollow microspheres introduce a third optically active phase, producing yet more complex scattering and cure behaviours that existing empirical relationships cannot predict. This study simulates light scattering in these systems via Mie theory and a novel Monte Carlo model, providing insight into the relationship between filler volume fraction and cured dimensions, and proposes an inversion parameter for predicting film dimensions. Cured resin geometry dimensions such as cured depth (C_D) and cured width (C_W) are predicted using the developed model for 10, 30, and 50 vol% hollow glass microsphere filled resin systems. In contrast to standard two-phase models, our three-phase model predicts a positive relationship between cured depths and half-widths and the filler volume fraction, consistent with experimental data. By elucidating the intricacies of light scattering in three-phase systems, this work provides valuable insights for advancing DLP-based additive manufacturing and designing filled resin formulations to achieve the desired cured dimensions.