## 3 Results and Discussion
### Feedback's Fabrication and Its Microstructural, Thermal and Rheological Properties
When the metallic powder is involved (Figure 5b), in all cases the feedstocks show a solid-like viscoelastic behavior, with G\({}^{\prime}\) > G\({}^{\prime\prime}\) in the whole frequency range. The intersection between G\({}^{\prime}\) and G\({}^{\prime\prime}\) curves does not occur even at the lowest frequency values, indicating that the powder-binder interactions dominate and the particles orientate in the shear direction, hindering the previously observed melamine-surfactant organization in the case of the binders shown in Figure 5a. Comparing the results according to the surfactant employed, surfactant E lowers the difference between G\({}^{\prime}\) and G\({}^{\prime\prime}\), confirming that the presence of both E and melamine leads to a predominant behavior of lower viscoelasticity compared to A and melamine. This is beneficial for the feedstock rheological properties  
Figure 5: Dynamic storage modulus (G\({}^{\prime}\)) and loss modulus (G\({}^{\prime\prime}\)) for (**a**) the neat binder samples (triangle symbol) and (**b**) the feedstocks (square symbol) obtained in dynamic frequency sweep. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Binder FA/FA** & **Binder FE/FE** & **Binder FA-MM/FA-MM** & **Binder FE-MM/FE-MM** \\ \hline \(\tau_{y}\) [\%] & 39.6/0.01 & 39.6/0.01 & 0.1/0.01 & 0.1/0.02 \\ \hline G\({}^{\prime}_{y}\) [Pa/10\({}^{5}\)Pa] & 87.0/159.6 & 86.2/105.2 & 601.6/42.8 & 527.0/66.4 \\ \hline tan (\(\delta\)) & 11.6/0.2 & 13.3/0.3 & 5.5/0.5 & 5.5/0.4 \\ \hline \end{tabular}
\end{table} 
Table 3: Rheological parameters by amplitude–strain-sweep oscillatory measurements: elastic limit (\(\tau_{y}\)), elastic moduli (G\({}^{\prime}\)_y_) and loss tangent (tan (\(\delta\))) for both the neat binders and the feedstocks (Binder/Feedstock). since the solid particles embedded in the polymer matrix (both the melamine and the metallic powder) are causing a reduced effect in the case of surfactant E.  
Finally, the complex viscosity (\(\eta^{*}\)) calculated at low shear rates by oscillatory rheometry and the apparent viscosity (\(\eta_{\text{app}}\)) calculated at high shear rates by capillary rheometry are displayed together in Figure 6. The viscosity decrease upon shear rate increase corroborates the shear thinning character of this kind of mixture, even if the melamine additive is incorporated, as well as in the E-containing feedstocks. At low shear rates (1 \(\times\) 10\({}^{-7}\)-1 \(\times\) 10\({}^{-1}\) s\({}^{-1}\) in Figure 6), the presence of melamine does not change the viscosity behavior, while the most influencing factor is the use of surfactant A or E, meaning that the powder-binder interactions are dominating. Conversely, it is at high shear rates (1 \(\times\) 10\({}^{-1}\)-1 \(\times\) 10\({}^{4}\) s\({}^{-1}\) in Figure 6) when the presence of melamine is critical. Apparently, melamine favors polymer chain alignment and powder-binder compatibility, improving the flow properties of both feedstocks FA-MM and FE-MM. The values of apparent viscosity achieved at the highest shear rates are in accordance with those recommended for the injection process, below 1000 Pa-s [12], and the curves in both shear rate ranges show good continuity, which is related to the absence of agglomerates in the feedstocks. The viscosity criteria for indirect printing technologies, such as MEX, are under development in the literature, and some differences with MIM might be expected since the shear rates occurring at the printing extrusion are lower (100-500 s\({}^{-1}\)). The present study suggests that this alternative feedstock formulation, FE-MM, shows adequate rheological properties to be used as MIM and MEX raw material. Further investigations regarding the behavior during the green-parts shaping, debinding and sintering are in progress.