## 3 Results and Discussion
### Indirect FFF 3D Printing  
This experimentation validated the first workflow shown in Figures 3 and 4. In detail, indirect FFF 3D printing was selected to manufacture the component in Figure 2b, avoiding sub-assemblies or cut-offs and reprocessing a wider range of recycled fiber-reinforced composites as new reinforcements for epoxy-based resins, in this case both rGFRPs and rCFs. As shown in Figure 6, only a few modifications were made to the original version of the 3D model. First, some features, such as the central guidance hole, were removed to eliminate possible overhangs and undercuts during the extraction of the component from the hypothetical model. In this case, a drilling process would be performed after removing the part from the mold. Moreover, some radii were added to the main sharp angles to facilitate the distribution of the material into the mold cavity, as well as a possible source of cracks and failures in the final part. Finally, the thicknesses of the whole model were checked to avoid thin cross-sections and ensure quite uniform measures, ranging between 2 and 4 mm, hence facilitating the curing process. Considering the geometry and the selected thermoset composite materials, flexible cavities were included in the design of the mold to facilitate the extraction of the parts, as  
Figure 6: 3D model of the part from the automotive case study (Modified version for indirect FFF 3D Printing). well as the complete hardening of the epoxy resin after its pouring. Furthermore, choosing silicone for manufacturing the cavities avoided their deformation during the hardening reaction of Araldite BY158, which generates exothermic heat. To validate the use of silicone in the final design, some trials with a cut-off of the part were done by designing and manufacturing a 3D-printed open mold with a silicone cavity. As from Figure 7a, the mold consisted of an external rigid part in PETG fabricated through FFF 3D printing and a flexible part obtained by pouring the GIs-PRO 30 silicone into the rigid mold. The shape of the cavity was given through a 3D-printed counter mold made of PLA, which was removed after silicone curing. The bonding between silicone and PETG was obtained by designing and fabricating mechanical interlocking areas inspired by the work of [PERSON] et al. [26]. The \"modifier\" feature of PrusaSlicer was used to change the slicing parameters of a specific section of the model to be 3D printed [45], which means the rigid part of the mold. In this case, the interfacial area was fabricated by changing the type and percentage of infill, i.e., 40% gyroid, and removing top layers, creating a controlled porosity in the mold. In this way, liquid silicone can pass through the infill part and fill the empty spaces, ensuring mechanical interlocking after its complete curing. The gyroid infill was selected to increase the volume of the rigid interlocking structure, whereas the whole structure was designed entirely in PrusaSlicer to simplify its development and manufacturing by avoiding using parametric design tools. Figure 7b,c show the parts obtained by pouring the epoxy-based thermosetting resin filled with rGFRPs and rCFs. In detail, the first epoxy-resin based formulation (Figure 6b) contains 30% wt. of shredded rGFRPs with a mean granulometry of 1 mm, whereas the second one (Figure 6c) has 20% wt. of shredded rCFs with a mean granulometry of 63 \(\upmu\)m, which were also selected according to the previous tensile tests. Even if both part batches were successfully fabricated with the same open mold, more deformations are visible in the part made of rGFRPs, which also shows a higher presence of air bubbles, especially close to the radii and the ribs. This issue may be linked to the higher thermal conductivity of CFs, which helps in dissipating the heat from the exothermic reaction from the crosslinking of the epoxy-based resin [46; 47]. After this first trial using an indirect FFF 3D printing approach, the mold of the part was designed and manufactured to produce it as a single component.