A dual-cure approach for the ultraviolet-assisted material extrusion of highly loaded opaque suspensions

Abstract

Ultraviolet-Assisted Material Extrusion (UV-MEX) is an additive manufacturing (AM) technology in which a viscous ink, is selectively extruded and solidified via in-situ UV irradiation (photocuring) in a layer-by-layer fashion to create three-dimensional parts. UV-MEX is an attractive AM technology due to its ability to (i) extrude highly viscous inks that are not processable via other photo-enabled AM technologies, (ii) leverage the broad photopolymer material library and chemistries established for other photo-enabled AM technologies and (iii) process a wide range of loaded inks to enable fabrication of metal, ceramic, polymer, bio-based, and multi-material parts. UV-MEX is well positioned to process highly loaded and highly viscous photocurable inks, a class of materials that cannot be processed by other AM modalities due to their process-induced viscosity constraints. However, opaque particles in the highly loaded systems may absorb and scatter UV irradiation, which will hinder final part properties due to limited cure depth and degree of conversion of the photopolymer matrix. To enable the robust UV-MEX printing of highly loaded inks containing opaque fillers, this work introduces a framework for formulating and characterizing resins that are both photo (UV irradiation) and thermally (heat) curable. This dual-cure strategy is enabled by the facile addition of free radical photo- and thermal initiators into acrylate-based resins. The resin’s responsiveness to UV irradiation enables rapid solidification and stiffening of the resin to reduce the spreading and slumping of the extruded bead. However, due to the absorption of the UV irradiation by the opaque fillers, only a thin cured skin (20-100um) is formed on the surface of the extruded bead. Thus, the secondary thermal cure is used to fully cure the ink and develop the final properties of the printed part. Heating during the printing process is shown to provide in-situ thermal curing of the deposited ink, which further strengthens the deposited material and enables the printing of tall structures. In this work, the dual-cure ink formulation strategy is assessed for its ability to enable the printing of dimensionally stable and high-aspect ratio parts from inks highly loaded with opaque solids. A methodology for characterizing the curing of the highly loaded inks using rheological and differential scanning calorimetry experiments is presented, along with a framework for assessing their printing performance in UV-MEX. The dual-cure ink concept and screening methodology is demonstrated on a dual-cure urethane acrylate ink loaded with 60 vol% aluminum particles.