Rheology and printability: A survey of critical relationships for direct ink write materials design

Direct Ink Write (DIW) is a material extrusion Additive Manufacturing (AM) technique that selectively extrudes material in a layer-by-layer fashion to form 3-dimensional parts. DIW can process a breadth of materials due to the inherent flexibility of its feedstock and the simplicity of its process mechanics (e.g., pressure-induced dispensing through a nozzle, no heat transfer, etc.). As a result, the process has attracted great interest from researchers aiming to develop novel materials for AM. The breadth of materials being introduced for DIW speaks to a need for guidelines for materials design and for a set of analytical characterization techniques that can be used to understand how inks will behave in the DIW process. The DIW process can be separated into three distinct sub-functions: extrusion, solidification, and layer support. Rheological experiments are the primary method used to characterize inks under conditions that mimic the three sub-functions of DIW and predict their printability, e.g. whether they will be successful in DIW printing. A broad range of rheological experiments has been used by researchers to characterize inks and judge their suitability for DIW and this review categorically compiles the predominant techniques. Then, a range of rheology-printability relationships including those derived from observed experimental trends and rigorous analytical models are discussed along with the usefulness and limitations of these relationships. Additional application specific concepts such as bridging and printability maps are further addressed in the context of rheological requirements. Finally, generalizable rheological guidelines of successful DIW inks are provided. This review collectively evaluates rheology-printability relationships in a generalized manner, independent of specific material chemistry or DIW process design. Significant insight into a comprehensive holistic approach for DIW ink development is provided. Lastly, the outlook of this research area is discussed to identify critical gaps in the fundamental understanding of both ink and process design, in the context of a rheological viewpoint.