## 3 Results and Discussion  
### Surfactant Selection  
For an adequate surfactant selection and feedstock formulation, the presence of melamine should be considered. Surfactants in the feedstock are key components to ensure the compatibility between the binder and the powder and minimize segregation in the feedstocks. Furthermore, side reactions with other binder components should be avoided. For this reason, the interactions between the surfactant and the melamine (1:1 ratio) have been firstly analyzed by DSC measurements (TA Instruments, DSC25). Results with stearic acid as the surfactant (named as A), an acid-based surfactant typically used in MIM, are shown in Figure 1a, where the endothermic peak of stearic acid (A) becomes two endothermic peaks in the mixture with melamine (MM), as can be seen in the MM+A vs. A curves, meaning that an acid-base chemical reaction has occurred. However, no significant changes between the surfactant and the mixture peaks are observed if an ester-based surfactant (named as E) is used (Figure 1b: surfactant E vs. mixture MM+E curves). This latter surfactant was preferably used, as it is expected to keep melamine activity in the following processing steps, while surfactant A is detrimental to helping the subsequent melamine-formaldehyde reaction, in addition to losing its powder wettability function. The interaction of melamine with the rest of the binder components (POM and HDPE) has been also studied by DSC in a 1:1 ratio (Figure 1c,d). This will help us to further understand possible changes in the flow behavior of mixtures at the typical processing temperatures during the injection stage (190 \({}^{\circ}\)C), which will be commented on later. The melting enthalpy (Table 1) found in the endothermic peaks of both pure polymers (HDPE and POM) are reduced when they are mixed with the melamine, this effect being more pronounced in the case of HDPE. Although the melting temperature is maintained in both cases, between the pure polymer and the melamine mixtures, this reduction of \(\Delta\)H\({}_{\text{m}}\) is related to lower energy required to produce the melting process in the presence of melamine, which was reported before [13]. The peak narrowing of MM+HDPE and MM+POM compared to the pure polymers also demonstrates this effect (Figure 1c,d). The differences observed between the HDPE and POM mixtures can be encountered in the fact that melamine interactsmore efficiently with POM, presumably by H bonding, through N-H and O functional groups, respectively. It should be noted that melamine presents no peaks during the heating cycle up to 300 \({}^{\circ}\)C, which leads us to conclude that it is acting as an extra solid filler to the polymer, in the same manner as the metallic powder. In the following section, all the components having been selected and thermally characterized, the feedstock's fabrication and its characterization are described. Together with the FA feedstock described in the Materials section employed as the reference feedstock, three other feedbacks were prepared in this study: FE, containing the ester-based surfactant E; FA-MM, containing surfactant A and melamine; and FE-MM, containing surfactant E and melamine (see Table 2). The concentration of FeNi8 powder, POM and HDPE was kept as in FA. In the melamine-added feedstocks (FA-MM and FE-MM), the concentration (wt.%) of melamine was 2.77 times lower than that of POM, according to a study reported elsewhere [10]. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\mathbf{\Delta H_{\text{m}}}\) **(f/g)** & **T\({}_{\text{m}}\) (\({}^{\circ}\)C)** \\ \hline
**HDPE** & 176.6 & 133.8 \\
**MM + HDPE** & 54.6 & 133.1 \\
**POM** & 143.9 & 165.0 \\
**MM + POM** & 116.7 & 166.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Melting enthalpy and melting temperature of the pure polymers in the binder (HDPE and POM) and mixtures with melamine (MM) determined by DSC.