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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. 

To better understand Magnetic Field Assisted Additive Manufacturing (MFAAM) the effect of a magnetic field on the orientation and distribution of magnetic particles in a molten magnetic composite was studied. Vibrating Sample Magnetometer (VSM) measurements were made on Sr-ferrite/PA12 fused deposition modeling filaments of different packing fraction (5 and 40 wt. %). The rotation of the sample’s magnetic moment upon application of a field perpendicular to the easy axis was monitored with a biaxial VSM above the PA12’s softening temperature. The observed magnetic moment transients depend on the temperature, the applied alignment field, the packing fraction, and the initial field-anneal procedure. Longer field-anneals result in larger time constants and seem to induce a hurdle that prevents complete alignment at low temperatures and/or for small fields. Results indicate the molten composite is a non-Newtonian fluid that can support a yielding stress. Scanning Electron microscopy (SEM) images taken on field-annealed samples at 230 °C show strong chaining with little PA-12 left between individual Sr-ferrite particles suggesting that direct particle to particle interaction is the reason for the observed non-zero yielding stress. The melt viscosity of the composite increases with the number of thermal cycles above the melting temperature (T m ). Room temperature (RT) torque magnetometry measurements show that magnetic anisotropy depends on the field annealing process through induced shape anisotropy contributions originating from magnetic particle agglomerates.