Rheological modeling of frontal-polymerization-based direct ink writing of thermoset polymers

Ensuring a high degree of dimensional accuracy of the printed part is critical in the development of additive manufacturing techniques. The advent of computational tools to simulate additive manufacturing has provided a robust way to predict part quality during the entire process. In this paper, a multiphysics model is introduced for the novel additive manufacturing technique of frontal polymerization-based direct ink writing, which enables rapid printing of complex free-form thermoset polymeric structures with excellent strength and stiffness. In direct ink writing, the fluid should possess suitable yield stress behavior to be successfully extruded and cured, and swelling of the fluid upon extrusion impacts the dimension of the printed filament. This model describes the thermo-chemo-rheological behavior of the printing gel during extrusion and polymerization, based on experimental measurements of the nonlinear viscoelastic and cure kinetics behavior. This model is applied to the FP-based printing process by considering an Eulerian domain that follows the motion of the printer head. Simulation results are presented for the vertical printing of both cylindrical filaments and annular tubes. The effect of process parameters on the dimensional accuracy of the printed part and the position of the polymerization front downstream from the printer head are investigated under steady and unsteady print speeds. Measurements from printing experiments validate the accuracy of the model.