Abstract
Strontium ferrite/polyamide12 (PA12) magnetic composite fused deposition modeling filaments made using a twin-screw extruder are being studied to be used for Magnetic Field Assisted Additive Manufacturing (MFAAM) of permanent magnets. A previous study shows that the magnetic viscosity (MV) varies with field and field angle. The Mx MV is maximum in the easy plane (1.1%) and decreases with a factor of two along the filament’s macroscopic hard axis [1]. The My MV is nearly zero along the hard axis and in the easy plane but maximum for intermediate angles. Here we study the time dependence as a function of temperature using a MicroSense EZ9 biaxial VSM to determine the melt viscosity.Samples were made by loading ~14 mg of SrO(Fe2O3)6/PA12 (26 or 40 wt%) cylindrical-shaped filament parts in pyrex glass tubes. The filament is compacted with PEEK stoppers on both sides to secure its position and avoid direct exposure to the atmosphere while heating. Our aim is to study the rotation of the magnetic particles in the molten PA12 matrix. We first heat the sample to 200 oC to orient all the particles in the y-direction using a field of 22 kOe. Then the sample temperature is lowered to the target temperature after which the field is lowered to zero. Finally, a field step is applied in the x-direction and Mx and My are monitored as a function of the time. The rotation of the moment vector with time is calculated from the data. Measurements are done for the fields of 500 and 2000 Oe, and at the temperatures of 120, 124, 128, 132, and 136°C.Up to 128°C, no strong time dependence was observed. Between 128 and 132°C, the magnetic particles orientations are no longer fixed in the matrix, but they can rotate and line up along the field direction. The time dependence of the rotation of the magnetic particles in the PA12 matrix is calculated from Mx and My (Fig. 1 and Fig. 2). The time constant decreases with temperature and field. The melt viscosity of the molten PA12 matrix at 136 oC is determined from the data is 1.4E5 Pa.s at 500 Oe field.  
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