Casein-based ink for enabling sustainable 3D printed materials

Despite increasing use of waste-derived polymers in extrusion-based 3D printing (3DP), formulation-process relationships governing 3DP fidelity for food waste protein-based inks remain poorly understood. Addressing this gap, we aimed to study the development of 3DP ink using sodium caseinate recovered from expired pasteurized milk for sustainable packaging applications. A formulation-driven approach was adopted by varying sodium caseinate (11–13%), hydroxypropyl methylcellulose (10–12%) and glycerol (15–20%). The inks were described for stability, steady-shear flow at 50 °C, syringe extrudability, mechanical properties at 50 and 25 °C, dimensional accuracy of printed constructs, and deposition cooling dynamics. All formulations exhibited shear-thinning behavior, with concentration-dependent differences in viscosity and shear stress. 3DP fidelity was highest for formulations containing 12% hydroxypropyl methylcellulose, 13% sodium caseinate, and 15% glycerol, which produced constructs with dimensional deviation below 10%. In contrast, formulations with 20% glycerol and 10% hydroxypropyl methylcellulose content showed dimensional deviation exceeding 60%. The extrudability results showed that formulations exhibiting a stable force plateau enabled continuous material deposition, whereas formulations characterized by pronounced force peaks showed uneven extrusion and poor printing accuracy. Mechanical properties at printing temperature (50 °C) indicated that inks exhibiting intermediate hardness (153–224 g) retained their shape after deposition. Infrared thermography revealed deposited filaments exhibiting initial temperatures above 42 °C and cooling toward 30 °C within a layer. In contrast, delayed extrusion was characterized by lower initial layer temperatures, coinciding with reduced interlayer adhesion. This study provides a systematic demonstration of caseins from expired milk as ink for sustainable 3D printed materials.