Rheology-Guided Material Extrusion of Highly Filled SmFeN and SrFe₁₂O₁₉ Magnetic Filaments Using a Polyolefin-Based Binder

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 SmFeN- 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 SmFeN 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 maximum flexural stress of 36.1 ± 0.1 MPa for SrFe12O19 and flexural modulus up to 5.5 ± 0.2 GPa for SmFeN. The results demonstrate that the TPE + gPO binder provides a suitable processing window for highly filled magnetic filaments for MEX-printed bonded magnets.