Rheology-Guided Material Extrusion of Highly Filled Sm₂Fe₁₇N₃ and SrFe₁₂O₁₉ Magnetic Filaments Using a...

Highly filled magnetic filaments require binder systems that balance powder loading, melt flow, filament strength, and strand stability during material extrusion (MEX). Here, a polyolefin-based binder composed of a thermoplastic elastomer and grafted polyolefin (TPE + gPO) was used to prepare Sm₂Fe₁₇N₃- and SrFe₁₂O₁₉-based magnetic filaments. The feedstocks were compounded, extruded into 2.85 mm filaments using a single-screw extrusion line with automatic spooling, and printed by MEX into ring-shaped specimens. Thermogravimetric analysis confirmed high powder contents, with residual masses of approximately 89 wt.% for Sm₂Fe₁₇N₃ and 85 wt.% for SrFe₁₂O₁₉. The filaments showed good dimensional control, with a diameter of 2.85 ± 0.07 mm and ovality ≤ 0.017 mm. Small-amplitude oscillatory rheology at 200-240 °C showed pronounced shear-thinning behavior and an elastic-dominant response (G' > G″) over 0.01-100 Hz, supporting flow through the nozzle and shape retention after deposition. Mechanical testing confirmed sufficient handling performance, with a maximum flexural stress of 36.1 ± 0.1 MPa for SrFe₁₂O₁₉ and a flexural modulus up to 5.5 ± 0.2 GPa for Sm₂Fe₁₇N₃. The results demonstrate that the TPE + gPO binder provides a suitable processing window for highly filled magnetic filaments for MEX-printed bonded magnets.