## 3 Results and Discussion  
### Material Characterization  
The extruded filament had sufficient flexibility to be spooled to a conventional filament spool with a diameter of 10 cm and could be processed on a Bowden principle FDM printer, in which filaments undergo significant bending. This is an improvement compared to published polyamide-based compositions (PA11 [3, 4], PA12 [5]) that yield brittle filaments processable only with large bending radii. Initial experiments with only POM as the matrix phase yielded bendable filaments that were very soft and ground up in the extruder of the printer. The addition of PA12 in the form of a NdFeB/PA12 compound increased the resistibility in the extruder of the printer while simultaneously maintaining sufficient bendability. A diameter within the range of 1.65 and 1.85 mm complying with the required tolerance could be achieved (Figure 2a). SEM imaging confirmed the expected spherical morphology and close packing of the NdFeB particles (Figure 2b). The average particle size was 48 \(\pm\) 15 \(\upmu\)m. The residual mass of 91.40 wt.% recorded by TGA analysis was comparable to the amount of filler used in the composite (88.53 wt.%, Table 2), which indicates that compounding was able to provide good dispersion of the filler within the matrix. The comparison between the measured filament density of 4.27 g/cm\({}^{3}\) with the theoretical value of 4.78 g/cm\({}^{3}\) indicates a certain amount of porosity (approx. Pfilament = 10.6 vol.%). Printing of precise objects with smooth surfaces was achieved. The minimum feature size determined by the extrusion width of a single printed line was approximately 500 \(\upmu\)m. The surface of the printed cylinders was smooth, with an average roughness of R\({}_{\text{a}}\) = 23.7 \(\upmu\)m (SD = 35%) on vertical surfaces and R\({}_{\text{a}}\) = 12.4 \(\upmu\)m (SD = 17%) on the top surfaces. This surface roughness is well within the range of reported values for standard plastic FDM parts [17, 18, 19, 20]. The cylinder size and shape were very good, in accordance with the corresponding 3D model (on average 3.5% and 2% larger in diameter and height, respectively). From visual comparison with images of various printed magnets published by [PERSON] et al., it is apparent that the achieved surface quality and geometric precision is equal or better to them [3, 4]. The density of printed samples 3.67 g/cm\({}^{3}\) (SD = 7%) indicated a further increase of porosity during printing by approximately 14% to about Pcylinder = 24%. This decrease in density due to the printing process was also reported by [PERSON] et al. (15% [5], 22% [3, 4]). The remanence of the printed cylinders was B\({}_{\text{r, cylinder}}\) = 353 mT (SD = 6%) which is slightly higher than the published values for printable NdFeB compound filaments (314 mT [5] (given), 301 mT [4]  
Figure 2: (a) Polymer-bonded magnetic filament flexible enough to be spooled on a standard filament spool with a diameter of 10 cm; (b) SEM micrograph of the extruded polymer-bonded magnetic compound, NdFeB particles inside the POM/PA12 matrix. (estimated from given compound remanence and density of printed parts)). A theoretical value B\({}_{\text{r, th, dense}}\) = 466 mT can be calculated under the assumption of a dense composite and a linear relation of the volumetric powder filling with the remanence (analog to [PERSON] et al. [5]). Therefore, the measured remanence of the printed cylinders B\({}_{\text{r, cylinder}}\) exhibits the same deviation of approximately 24% from the theoretical calculation as the porosity P\({}_{\text{cylinder}}\). Hence, it is likely that the decrease in remanence is mainly caused by porosity, and other factors, like oxidation, are insignificant. Figure 3 shows the achieved remanences in the context of conventionally manufactured and 3D printed magnets, as well as theoretical values for the MagFil material based on the volumetric powder filling.