A rheology roadmap for evaluating the printability of material extrusion inks

Thanks to its ability to process highly viscous inks with a range of rheological properties, Chemical Reaction Bonding Material Extrusion (MEX-CRB) Additive Manufacturing (AM), commonly referred to as Direct Ink Write (DIW), has been used to process a wide variety of materials. However, there does not yet exist a methodology to guide the creation and screening of novel inks for this process. Leveraging a series of rheology experiments and rheology-to-printability relationships, a “Rheology Roadmap” is presented that guides material designers through systematic evaluation of an ink’s compatibility for MEX, independent of the ink’s specific attributes. In this work, printability is defined as an ink's ability to be processed via DIW, and is assessed by using measured rheology data to predict its behavior across the three critical sub-functions of the MEX process: extrusion, extrudate solidification, and layer support. The Roadmap synthesizes the broad range of reported rheology analyses in literature into a systematic decision framework that presents a repeatable order for conducting the rheology experiments and evaluating the associated criteria for relating each experiment’s results to printability. Within this framework, an ink is either deemed printable following each rheology experiment, and the ink is further characterized in the next experiment, or the experiment reveals that the ink is not printable and the evaluation process is stopped. The Roadmap can characterize a range of inks, independent of their specific attributes, including yield-stress, time dependent solidification, and combined solidification inks for a range of MEX process embodiments. Quantifiable criteria based on rheological properties is provided to classify ink type. To demonstrate the practical application of the Roadmap, a silicone ink with both yield-stress and reactive curing behavior, and urethane acrylate inks with photocuring behavior, are screened for processing via MEX and then printed using a dual-component extruder. By following the Rheology Roadmap, users with minimal expertise in rheology, MEX, and/or materials formulation can assess new inks for printability in a standardized manner, thus assisting in the creation of the next generation of MEX materials.