Direct ink write 3D printing of high solids loading bimodal distributions of particles

Abstract

As additive manufacturing (AM) expands as a processing technology for structurally customizable materials, there is increasing interest in printing with high particle contents. For suspensions with particle contents of over 50 vol%, there are significant formulation and processing challenges due to increased interparticle friction and suspension complexity. We focus on suspensions with bimodal particle distributions and two common binder systems, a high molecular weight polymer in a solvent that solidifies via solvent evaporation and a monomer mixture that cures via ultraviolet irradiation. We examine the interplay between formulation and processing and show that the formulation effects are particularly important at optimal printing parameters, but that they are overcome by processing-related defects at sub-optimal parameters. By understanding the processing and formulation effects specific to direct ink write AM of high solids suspensions, new customized inks can be designed for a wide range of applications, including construction, energetics, and ceramics.

Research, standards, and data needs for industrialization of ceramic direct ink writing

“…The viscosity rises rapidly with concentration in this regime (e.g., from ≈58% by volume upwards) – so one should aim for the extrusion paste to coincide with the onset of this increase. At the same time, the concentration ranges quoted above can vary markedly with particle surface state and geometry, and also with particle – solvent interactions and solvent fluid (effects reminiscent of the behavior discussed here and captured by the diagram in Figure 1 have been reported for non‐ceramic systems) 5 . Ideally, on deposition, any drying should move the deposited bead from the pseudo‐plastic into the dilatant state, but not too quickly as it needs to support the next layer while also merging with it.…”

“…Courtesy of J. Cesarano concentration ranges quoted above can vary markedly with particle surface state and geometry, and also with particle -solvent interactions and solvent fluid (effects reminiscent of the behavior discussed here and captured by the diagram in Figure 1 have been reported for non-ceramic systems). 5 Ideally, on deposition, any drying should move the deposited bead from the pseudo-plastic into the dilatant state, but not too quickly as it needs to support the next layer while also merging with it. As solvent is removed and the pseudoplastic-to-dilatant transition occurs, much larger shear stress is required to register a significant shear strain, enabling the DIW print process to proceed without compromising earlier parts of the build due to the weight of new layers deposited on top.…”

Material extrusion additive manufacturing of dense pastes consisting of macroscopic particles

“…For example, bimodal distributions of particles near the ideal packing ratio for the particle sizes have been shown to lead to fluids with lower tan-delta and improved flowability. [ 10 ] And monomodal distributions of small particles as well as elongated particles, such as needles, have been shown to have significantly higher viscosities due to the high surface area for interactions with other particles. [ 28 , 29 ] Thorough characterization of particles, especially their size distributions and shapes, is essential to designing a formulation for DIW, but different particles require different characterization techniques and careful consideration of how to best characterize is required.…”

“…1 ). A major advantage of DIW is the flexibility and customizability of the ink; gels, [ 9 ] pastes, [ 10 , 11 ] liquid crystalline elastomers [ 12 ] , and resins [ 13 ] have all demonstrated success as long as the rheology of the ink can be adapted to flow in the nozzle and maintain shape as a filament. With this wide array of materials that can be printed via DIW, both understanding how the formulation affects printability and developing design rules for the customized inks are particularly important.…”

“…Thus, there is a rich parameter space for designing AM processes for dense pastes, including not just the vol% particles, but also the particle surfaces (roughness [ 17 , 18 ] and chemistry [ 19 ] ), binder choice, [ 3 ] particle shape [ 20 ] , and whether a mixture of particles is employed. [ 10 ] One of the most important parameters, however, is the size of the particles. We can consider them in two regimes: macroscopic particle size (> 1 µm) and colloidal particle size (1 nm to 1 µm).…”