## 3 Results and Discussion
### Feedback's Fabrication and Its Microstructural, Thermal and Rheological Properties
Surfactant E (feedstock FE) slightly advances the start of the first mass fall corresponding to POM. More remarkably, in the feedstocks FA-MM and FE-MM, melamine decomposes at around 320 \({}^{\circ}\)C and this produces the thermal degradation of POM shifting to higher temperatures, not affecting HDPE profile. The mass change reaches lower values (82.94 and 83.52 wt.%, respectively) since the melamine content contributes to the overall amount decomposed in these feedstocks, while keeping the same binder ratio as in FA and FE. Once the melamine is part of feedstocks FA-MM and FE-MM as an additive, it will be kept inactive during the injection or 3D printing process of the parts, playing its role only during the debinding stage. Nevertheless, melamine is expected to affect the flow properties of the new feedstocks due to the solid-molten interactions occurring (melting of melamine is not occurring at the injection temperature (190 \({}^{\circ}\)C), as confirmed by DSC in Figure 1, MM). For this reason, the rheological study performed in all the feedstocks aims to understand and predict the behavior of the modified materials in the MIM or AM process. First, viscosity measurements and rheological parameter calculations were performed using a capillary rheometer (Dynisco LCR7000, Franklin, MA, USA) at temperatures of 190, 200 and 210 \({}^{\circ}\)C, passing the molten feedstock through a capillary with a length/diameter index of 30 and in the high shear rate range (10-7000 s\({}^{-1}\)). The apparent viscosity of feedstocks with both surfactants A and E containing the melamine (176 Pa\(\cdot\)s for both FA-MM and FE-MM) is lower than that of the pure feedstocks (342 Pa\(\cdot\)s for FA and 433 Pa\(\cdot\)s for FE feedstocks), values measured at 190 \({}^{\circ}\)C and 1000 s\({}^{-1}\). Thus, the yield stress parameter is also lower under the melamine effect (defined as the effort required to start flowing), as can be seen in Figure 3. Both facts might be explained by a better polymer chain alignment in the shear direction, which is enhanced by the supramolecular interactions created between the functional groups of the binder components, the surfactant and the melamine and by the extrusion that occurs in the capillary rheometer, helping the flow process [14]. Another important parameter is the activation energy (Figure 4), which refers to the influence of temperature on the feedstock; the lower the value, the more stable the material remains during the injection or the extrusion process in the 3D printing machine. According to the results, melamine does not have a significant influence on the final value when surfactant A is employed (around 10 kJ/mol in FA and FA-MM). However, the  
Figure 3: Thermogravimetric analysis (TGA) of the feedstocks fabricated (mass change in wt.%). activation energy notably increases with surfactant E in FE feedstock, close to the threshold value considered in PIM technology (40 kJ/mol) [12]. Therefore, surfactant E introduces a higher sensitivity to temperature in feedstock FE compared to conventional FA. Since the yield stress values in this material are high too, it is deduced that the admissible solid loading (metallic powder concentration in the binder) of the FE system is lower compared to FA. More remarkable is the fact that, if melamine and surfactant E are present (FE-MM), both activation energy and yield stress dramatically decrease. It can be concluded that the combination of melamine and surfactant E leads to an increase in the solid loading admissible in the mixture, which is an important parameter in the feedstocks to ensure good sinterability. This can be explained by better binder-powder interactions due to melamine acting as a second surfactant. Secondly, an oscillatory rheometer (TA Instruments, New Castle, DE, USA, Discovery HR20) was employed to study the behavior of the feedstocks in the range 190-210 \({}^{\circ}\)C and at lower shear rates (down to 10\({}^{-7}\) s\({}^{-1}\)), selecting parallel plates with 25 mm diameter.