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Thermoplastic bonded magnetic composites combine the cost-effectiveness, low mass density, and manufacturing flexibility of conventional thermoplastics with the unique characteristics of magnetic powders/ fillers to form multifunctional magneto polymeric composites that offer superior properties to conventional materials. At elevated temperatures, the magnetic properties change significantly, and the polymer matrix no longer secures the magnetic particles and can rotate freely with respect to an externally applied magnetic field. This often happens at temperatures significantly below the melting point of the polymer. To extend the thermal window of bonded magnets beyond 175 ◦C (the typical temperature of rotors in motors and generators), poly- mers such as polyetheretherketone (PEEK), polyetherimide (PEI), or other high-temperature thermoplastics have been considered suitable binders for magnetic fillers. Another suggested approach is using a surface treatment to increase the adhesion between the polymer matrix and magnetic particles. In this review, the fabrication pro- cesses to make bonded magnets by injection molding and fused filament fabrication were discussed as well as their thermal, mechanical, and magnetic performance obtained via analytical and materials characterization methods. The magnetic properties of bonded permanent magnets manufactured via different techniques were discussed in terms of the most important single magnetic parameter known as “the maximum energy product- (BH)max, which can serve as a performance index for manufacturing bonded magnets. The energy product normalized on cost or mass density are used to provide insight on the performance of bonded magnets for ap- plications driven by cost or inertia. Finally, applications of high-performance thermoplastic-based magnetic composites that can be viable for stringent engineering devices such as sensors, actuators, motors, and generators were highlighted. 

Magnetic Field Assisted Additive Manufacturing (MFAAM), 3D printing in a magnetic field, has the potential to fabricate high magnetic strength anisotropic bonded magnets. Here, 10, 35, and 54 wt% strontium ferrite bonded magnets using polyamide 12 binder were developed by twin screw compounding process and then printed via MFAAM samples in zero, and in 0.5 Tesla (H parallel to the print direction and print bed). The hysteresis curves were measured using a MicroSense EZ9 Vibrating Sample Magnetometer (VSM) for 3 different mount orientations of the sample on the sample holder to explore the magnetic anisotropy. The samples printed in zero field exhibited a weak anisotropy with an easy axis perpendicular to the print direction. This anisotropy is caused by the effect of shear flow on the orientation of the magnetic platelets in the 3D printer head. For the MFAAM samples, the S values are largest along the print bed normal. This anisotropy is caused by the field. The alignment of the magnetic particles happens when the molten suspension is in the extruder. When the material is printed, it is folded over on the print bed and its easy axis rotates 90° parallel to the print bed normally. Little realignment of the particles happens after it is printed, suggesting a sharp drop in temperature once the composite touches the print bed, indicating that field-induced effects in the nozzle dominate the anisotropy of MFAAM deposited samples. 

The magnetic anisotropy of strontium ferrite (SF)/PA12 filament, a popular hard magnetic ferrimagnetic composites that is used for 3D-printing of permanent magnets, is studied by vibrating sample magnetometry. The studied filaments have a composition of SF/PA-12 thermoplastic composite with a 40% wt.  ratio of SF. SF particles are non-spherical platelets with an average diameter of 1.3 um and a diameter to thickness ratio of 3. Filaments are produced by a twin-screw extruder and have a diameter of 1.5 mm. SEM images show that the SF particles are homogeneously distributed through the filament. VSM measurements on different parts of the filaments show that the outer part of the cylindrical filament has a higher anisotropy, and the core is mostly isotropic. This conclusion is consistent with computational work by others which suggest that particle alignment predominantly takes place near the walls of the extruder die where shear flow is maximum. Additional hysteresis curve measurement of the outer cylindrical part of the filament parallel to the r and ϕ directions indicates that the squareness of the hysteresis curve (S) is larger in the r-direction. This indicates that the outer surface of the filament has a strong easy axis in the r-direction. We conclude that the SF platelets line up parallel to the walls of the extrusion die.