(234a) Printability Criterion of Highly Filled Pastes for Direct-Ink Writing Based on Small-Amplitude Oscillatory Shear (SAOS) Experiments and Packing Fraction

Direct-ink write is an additive manufacturing technique that enables the creation of reproducible and complex hardware by depositing a viscous, shear-thinning liquid onto a substrate in a custom-pattern via extrusion through a syringe. The rheology of these inks is tailored through the addition of various filler materials. To successfully print highly-filled inks, we need to understand the effect of filler morphology, size, loading, and packing fraction on the ink rheology and corresponding printability. More importantly, characterization methods that accurately capture the ink’s rheological properties that correlate to printability is imperative. We present results which demonstrate why traditional characterization of these systems through steady-shear tests are difficult and often unreliable. We then offer a more reliable methodology to determine printability. Various filler particles and volume loadings of particles were dispersed in Polydimethylsiloxane (PDMS, Sylgard® 182) and extruded through a syringe to determine the printability of each material separated into three categories: Thin, Printable, and Thick. We then correlate measurements made through small-amplitude oscillatory shear to the printability of each finished product and show that some of the common metrics (linear regime storage modulus and complex viscosity) as well as often ignored metrics (axial force) can distinguish between materials which are printable and materials which are too thick to be extruded. We calculate the rate of change of the strain amplitude and axial force in time, to see how fast materials are changing behavior, which we have paired with images which show that the axial force drops due to material leaving the gap between plates. Finally, we used the linear regime loss modulus, which is associated with energy dissipation, to fit the Krieger-Dougherty to determine the maximum packing fraction of each particle. In doing so we have determined a printability criterion based on packing fraction normalized by the maximum packing fraction to separate materials which are printable from those that are too thick or too thin.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.