## 3 Results and Discussion
### Rheological Properties
Although the rheological behavior of feedstock A makes it a proper feedstock formulation, feedstock B was considered the best formulation for further investigation due to its lower viscosity since it has a higher content of PW and SA than feedstock A. This low viscosity makes feedstock B better suited for Nd-Fe-B feedstock because it would allow lower processing temperatures and further increase the powder content. Figure 4: The representative curve of apparent viscosity versus the apparent shear rate for feedstocks (A, B, C, and D refer to the formulations shown in Table 1). Since the formulation of feedstock B was determined to be the most promising for this powder, the powder loading adjustment was performed by loading 55, 57, 58, 59, and 60 vol.% of Nd-Fe-B powder in the selected binder system B (B55, B57, B58, B59, and B60). Figure 6 depicts the representative values of apparent viscosity at different apparent shear rates at 160 \({}^{\circ}\)C. As expected, the increase in powder content increases the apparent viscosity of the feedstocks, especially at lower shear rates. Moreover, the apparent viscosity increase with powder content is not linear and is more pronounced at higher contents. As can be seen, the feedstocks with 59 and 60 vol.% had higher viscosities than 1000 Pa\(\cdot\)s at lower shear rates (below 250 and 750 s\({}^{-1}\), respectively), which are improper formulations from a rheological viewpoint and therefore unsuitable for the MIM process. Both feedstocks with 60 and 59 vol.% solid loadings showed an evident pressure instability (see Appendix A Figure A1). Such pressure instabilities could result from agglomeration at these powder contents and other issues, such as powder-binder segregation at higher shear rates [70]. It must be mentioned that with a powder loading of 65 vol.%, the measured pressure within the capillary rheometer was extremely high, and the viscosity measurement was impossible. Therefore, a higher loading than 60 vol.% was excluded from further investigation. As mentioned, higher values of the n index are preferable for the MIM process. The n values of feedstocks with 55, 57, 58, 59, and 60 vol.% powder loading can be seen in the insert of Figure 6. The feedstock with 58 vol.% powder content had the highest n value (n = 0.67), indicating the lowest viscosity sensitivity to shear rate. The powder loading higher than 59 vol.% caused a considerable decline in the n value (n = 0.59), indicating that a powder loading around 60 vol.% (n = 0.47) may be assumed as critical solid loading [71]. Therefore, the viscosities of the feedstocks with 57 and 58 vol.% powder content are assumed to be the most appropriate for MIM. Higher powder loading in MIM feedstocks results in easier densification, less distortion during sintering [71], and less contamination due to a lower amount of the binder system; therefore, higher magnetic properties are expected. Considering the mentioned phenomena and the rheological properties measured, the feedstock with 58 vol.% and the binder system consisting of 45 vol.% LLDPE, 47.5 vol.% PW, and 7.5 vol.% SA could be considered as the optimal feedstock formulation for the selected Nd-Fe-B powder at the processing temperature of 160 \({}^{\circ}\)C.  
Figure 5: The representative pressure curve of feedstocks.