Influence of Carbonyl Iron Particle Loading on the Dimensional and Rheological Properties of 4D-Printed TPU-Based Magnetorheological Elastomers

Abstract

The advancement of 4D printing technologies has created new opportunities for fabricating smart materials with tunable properties, including magnetorheological elastomers (MREs). This study investigates the fabrication and characterization of thermoplastic polyurethane (TPU)-based MREs with varying carbonyl iron particle (CIP) loadings (10–50 wt.%) using the fused filament fabrication (FFF) method. MRE filaments were produced through a solvent-assisted mixing and extrusion process, ensuring consistent diameters within 1.75 ± 0.10 mm. Test specimens were printed under fixed processing parameters to assess printability, dimensional fidelity, mechanical hardness, and rheological behaviour. Results showed that all FFF- printed MREs achieved high dimensional accuracy, with deviations below 5.5%, and exhibited a progressive increase in Shore A hardness and off- state storage modulus (G') with higher CIP content. Under an applied magnetic field, all samples displayed the magnetorheological effect, with stiffness enhancement proportional to CIP loading. Comparative analysis with previously reported fused granulate fabrication (FGF)-printed MREs revealed that FFF yields higher baseline stiffness but a comparatively lower MR effect, attributed to anisotropic particle alignment during filament extrusion. These findings highlight the trade-offs between dimensional precision, inherent stiffness, and field responsiveness in additively manufactured MREs, providing valuable insights for their application in adaptive structures, tunable damping systems, and soft robotic actuators.

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