## 2 Materials and Methods
### Filament Materials  
A 5-component polymer-bonded magnetic compound was developed for the fabrication of a filament optimized towards a high-powder loading and mechanical flexibility. The components consisted of a magnetically isotropic NdFeB powder, an isotropic NdFeB powder/Polyamide-12 (PA12) compound used in injection molding, polyoxymethylene (POM) as the main matrix material, a dispersing agent, and Aerosil as the rheology optimizing additive. Table 2 gives details on the materials, manufacturers, and the compound composition. The diameter and shape of the NdFeB powder particles (MQP-S-11-9) were given by the manufacturer as spherical with a diameter of 42 \(\pm\) 13 \(\upmu\)m. The NdFeB/PA12 compound had a specified magnetic powder filling of 60 vol.%. Details on the size and shape of the compounded magnetic particles were not disclosed by the manufacturer. The remanence, intrinsic coercivity, and maximum energy product of the NdFeB powder and NdFeB/PA12 compound are presented in Table 3. Both the NdFeB/PA12 compound and POM were obtained as granules. As both polymers (PA12 and POM) have a high melt flowability with excellent mechanical strength and toughness, they are suited for processing as highly-filled plastics. To further reduce the viscosity of the composite and to ensure a homogeneous particle distribution in the matrix, a high molecular weight-dispersing agent was used. It was further intended to serve as corrosion protection and for mitigating fire risk of the dry NdFeB powder, and was added as 3% of the powder weight. To improve powder flowability and reduce friction between the magnet particles, a small amount of fumed silica was added. Initial experiments revealed that the order of mixing influenced the processability and homogeneity of the compound. The following order proved to be beneficial: First, mixing the NdFeB and fumed silica powders and, separately, the POM granules with the dispersing agent. Then, combining the two mixtures with the NdFeB/PA12 compound granules, for further processing. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Extruder temperature & 220 \({}^{\circ}\)C \\ Layer height & 0.15–0.25 mm \\ Printer speed & 25 mm/s \\ Infill density & 100\% \\ Infill pattern & Rectilinear \\ Bed temperature & 60 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table} 
Table 1: Empirically-found printer parameters for the developed polymer-bonded compound filament (MagFil). \begin{table}
\begin{tabular}{c c c c} \hline
**Ingredient** & **Type** & **Weight Fraction** & **Volume Fraction** \\ \hline NdFeB powder & \multicolumn{2}{c}{MQP-S-11-9 (NdFeFeCoTiZrB),} & \multirow{2}{*}{66.03\%} & \multirow{2}{*}{42.34\%} \\ NdFeB/PA12 compound & \multicolumn{2}{c}{Magnetquench (Singapore)} & \multirow{2}{*}{24.46\%} & \multirow{2}{*}{23.07\%} \\  & BMNFi-60 fH, Bomate (Hori, Switzerland) & & \\ POM & F10-01, Keptital (Wiesbaden, Germany) & & \\ Dispersing agent & Edaplan 93\%, Manzing (Absatt, Germany) & & 1.98\% & 9.35\% \\ Fumed silica & Aerosil, Evonik (Essen, Germany) & approx. 0.20\% & approx. 0.44\% \\ Total magnet powder 1  & - & 88.53\% 2/91.40\% 3  & 56.19\% 2 \\ \hline \end{tabular}
\end{table} 
Table 2: Composition of the developed flexible polymer-bonded compound filament (MagFil). The compounding of the final mixture and filament extrusion was performed on a parallel twin-screw extruder with a nozzle diameter of 2.5 mm (Process 11, ThermoFisher Scientific, Karlsruhe, Germany). The 8 temperature zones (Z) were set with increasing temperatures from 200 \({}^{\circ}\)C at the feeder (Z1) to 220 \({}^{\circ}\)C at the nozzle (Z8) (Z1/Z2: 200 \({}^{\circ}\)C, Z3/Z4: 210 \({}^{\circ}\)C, Z5/Z6: 215 \({}^{\circ}\)C, Z7/Z8: 220 \({}^{\circ}\)C). The extrudate was collected by a conveyer belt and cooled down with an air stream for accelerated solidification. A filament diameter of 1.75 mm was targeted by adjusting the speed of the conveyer belt.