Viscoelastic characterization and extrusion performance of a novel ink for metal direct ink writing

Purpose This study aims to improve the extrudability and stability of polyvinyl alcohol-polyethylene glycol (PVA-PEG)-based water-soluble binders by modifying their composition with carboxymethyl cellulose (CMC). The primary objective is to determine the optimal CMC concentration that enhances critical solid loading capacity, suppresses binder segregation and phase separation and promotes stable and consistent extrusion. Furthermore, this work seeks to establish quantitative relationships between the rheological properties and extrudability of inks to be used in Direct Ink Writing. Design/methodology/approach A comprehensive analysis of the physical, rheological and mechanical behavior of CMC-modified binders was conducted. Rheological characterization involved the assessment of zero-shear viscosity, extrusion viscosity, yield stress, storage modulus, loss modulus and phase angle. An innovative extrusion testing setup was developed to simulate Solvent Cast Direct Ink Writing (SC-DIW) process conditions, enabling real-time detection of flow instabilities such as clogging and phase separation. The optimum solid loading range was determined based on extrusion force profiles. In addition, three-point bending tests were performed on green parts to evaluate mechanical strength and validate interlayer cohesion after extrusion. Findings The results show a CMC concentration of 1.5 Wt.% improves the stability of the PVA-PEG binder, preventing phase decomposition and separation and ensuring stable flow. The critical powder loading ratio for the binder with 1.5 Wt.% CMC was determined to be within the range of 85–87.5 wt. Moreover, an optimal balance of extrudability and post-extrusion green part strength can be achieved using a binder containing 1.5 Wt.% CMC. In addition, a successful extrusion process can be achieved using CMC-modified binders when G′, τy, µex and µ0 are lower than 3 × 105 Pa, 820 Pa, 100 Pa.s and 50,000 Pa.s., with α values ranging from 0.5 to 0.6. Originality/value This research introduces a novel strategy for stabilizing PVA-PEG-based binders by integrating CMC to suppress phase decomposition and separation and improve extrudability in SC-DIW processes. This study provides, for the first time, a predictive framework linking rheological thresholds to extrusion performance through a custom-designed extrusion simulation test. Findings of this study are expected to significantly advance the design of high-solid-loading inks for extrusion-based metal additive manufacturing.