Printability Metrics in Direct Ink Writing: Critical Review of the Literature and Novel Perspective Based on Dimensional Analysis

Direct Ink Writing (DIW) is a versatile additive manufacturing technique widely used for processing complex inks, particularly particle-loaded formulations. Despite its broad applicability, the field lacks standardized criteria for defining and assessing printability. Existing approaches, typically based on rheological thresholds such as yield stress and elastic modulus, often lead to system-specific, inconsistent results and rely heavily on subjective visual inspection. This work presents a systematic framework for analyzing DIW printability, grounded in the unpacking of the printing process into five subfunctions: extrudability, single filament accuracy, planar accuracy, buildability, and ability to produce suspended structures. Each subfunction is examined in detail through dimensional analysis to identify the parameters responsible for its success. This approach enables a deeper understanding of the interactions between material properties and process variables and highlights how certain parameters can have opposing effects in different subfunctions. To support an objective evaluation, a standardized printability test based on a serpentine pattern is introduced. By quantifying key geometric features (filament width, height, curvature, and deflection) the test enables a reproducible and scalable comparison of inks across a wide range of formulations. This method replaces qualitative assessments with measurable, normalized metrics, providing a robust tool for ink development and printer calibration.