## 3 Results and Discussion
### UV-Assisted Direct Ink Writing (UV-DIW)  
UV-Assisted DIW was selected as a manufacturing process to use in the second workflow from Figure 2. In this case, the selection was made to avoid the use of molds or tools, trying to reduce the costs and timings related to their manufacturing. First trials were performed by using both rGFRPs and rCFs as reinforcements for photo- and thermally-curable acrylic resins. However, the final parts were fabricated only with rGFRPs. As a matter of fact, the presence of rCFs in the 3D printable ink interferes with the absorption of the UV radiation for the activation of the photo-initiator in the acrylic-based matrix system [21,22]. Hence, only a few layers were successfully completed without collapsing during the first 3D printing tests with rCF-based inks. As visible from Figure 10, the original model was strongly modified to match the manufacturing constraints of UV-DIW. First, the use of supports should be avoided, since they may be difficult to remove from the final shape, requiring more post-processing steps. Furthermore, the volumes and dimensions of the parts should be optimized to avoid long printing times, i.e., >8-10 h, obtaining functional parts for technical applications [16]. In UV-DIW, the printing speed is affected by the UV conversion and the cross-section geometries, resulting in higher times and slower speeds when the UV-irradiation time should be increased to completely crosslink the ink [21]. As a result, the original model was cut into six different parts, avoiding overhangs and tilted surfaces to be 3D printed. This modification helped in optimizing the printing parameters of the different sub-parts. For instance, the printing speed of the perimeters was increased from 10 to 15 mm/s for the front part of the connector, whereas the lowest speed was set to fabricate the three squared sub-parts of the back part (Figure 10). Some additional features were added to the parts to  
Figure 9: Connector from indirect 3D printing: (**a**) cast part in rGFRPs (30% wt., 200 \(\upmu\)m, top) and rCFRPs (30% wt, 200 \(\upmu\)m, bottom); (**b**) cast part in rCFRPs; (**c**) detail of the cast part in rCFRPs. facilitate the assembly of the final component, i.e., mechanical interlockings. In this case, the sharp angles and wall thicknesses were not modified, since their presence did not affect the quality of the final part. The 3D printing process of one sub-part made in rGFRPs is visible in Figure 11a. The selected 3D printable ink contains the 55% wt. of shredded rGFRPs with a mean granulometry lower than 80 \(\upmu\)m, which previously was successfully used to manufacture concept applications and technical parts [16,20]. Moreover, the mechanical properties of the final objects meet the requirements of the selected component for the automotive sector, especially stiffness, reaching an elastic modulus of \(\sim\)5.5 GPa [20,21]. Fiber granulometry was also selected to prevent clogging, considering the nozzle diameter. From the literature, only 3D printable ink containing particles with a maximum length of 160 \(\upmu\)m can be extruded by using a nozzle diameter of 1 mm. The 3D printed part (Figure 11b) shows good accuracy of the features from the 3D model, including the ribs, as well as no areas with significant deformations or under-extrusions. As for the previous experimentation, reaching a consistent extrusion flow helps in reducing the possible points of cracks in the event of an accident during the product lifespan. After their fabrication, the six sub-parts can be assembled by using the mechanical interlocking and the same epoxy adhesive used for fixing the connector part to the aluminum profile and the battery floor pan of the car structure. However, additional post-processing is required to decrease the roughness of the parts, facilitating a better match with their location in the aluminum profile.