## 3 Results and Discussion
### Tensile Tests  
Five batches of specimens were tested to study the main tensile properties of the formulations used in this work. Four of them are linked to Indirect FFF 3D printing,  
Figure 4: Scheme of the two processes of this work: Indirect FFF 3D Printing and UV-assisted DIW. Figure 3: Scheme of the workflow with recycled composites and 3D printing. Figure 2: Selected component from the automotive case study: (**a**) assembly of the connector part in the car structure; and (**b**) 3D model of the part (Original version). whereas the last one corresponds to the UV-DIW formulation used for the automotive application and previously tested [20]. The values from the tensile tests are summarized in Table 3.  
In general, a brittle failure of the samples was observed for each batch. According to Table 3 and Figure 5, higher values of tensile strength and elongation at break were obtained by testing the samples from indirect FFF 3D printing. Despite the higher filler content of the UV-DIW formulation, i.e., 55% wt., the tensile strength of specimens from indirect FFF shows higher values when considering the same kind of recycled fibers, which means rGFRPs (Figure 5a). As a matter of fact, the first (30% wt. rGFRPs 1 mm) and second batches (30% wt. rGFRPs 200 \(\upmu\)m) reached values between \(\sim\)40 and \(\sim\)46 MPa, whereas the tensile strength of UV-DIW batch is \(\sim\)33 MPa. This difference can be linked to the higher void content from the increased weight percentage of fibers, especially with higher granulometries [40; 41]. Furthermore, additional voids may be added from the UV-DIW process itself, i.e., air bubbles in the 3D printable ink during the in-situ photo-polymerization [20; 21]. Similar values were obtained by comparing rGFRPs and rCFs from indirect FFF, with higher accuracy of results by decreasing the particle size of the filler. This fact may be due to the high variability of the granulometry, which also derives from mechanically recycled fiber-reinforced polymers [20; 21; 22]. Almost a two-fold increase in the elongation at break values was reached with rGFRPs processed by indirect FFF compared to the UV-DIW process, reaching \(\sim\)1.7% with the second batch of samples (Figure 5b). A similar value was also obtained from the batch of samples made with 20% wt. of rCFs (63 \(\upmu\)m). Increasing the filler weight percentage and particle size, the value significantly decreased for both rGFRPs and rCFs batches. Figure 5: Comparison of the experimental values from the tensile tests: (**a**) Tensile strength, (**b**) elongation at break, and (**c**) elastic modulus of the materials compared in this work (values of UV-DIW from [20]). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**AM** & **Formulation** & **Tensile Strength** & **Elongation** & **Elastic Modulus** \\
**Process** & **Formulation** & **(MPa)** & **at Break (\%)** & **(GPa)** \\ \hline Indirect & 30\% wt. FGFRPs & 46.3 \(\pm\) 2.7 & 1.3 \(\pm\) 0.1 & 4.8 \(\pm\) 0.7 \\ FFF & (1 mm) & 40.5 \(\pm\) 1.9 & 1.7 \(\pm\) 0.2 & 3.6 \(\pm\) 0.4 \\ Indirect & 30\% wt. rCFs & 44.8 \(\pm\) 8.5 & 1.1 \(\pm\) 0.2 & 5.8 \(\pm\) 1.2 \\ FFF & (200 \(\upmu\)m) & & & \\ Indirect & 20\% wt. rCFs & 46.6 \(\pm\) 3.1 & 1.8 \(\pm\) 0.2 & 4.2 \(\pm\) 0.2 \\ V-DIW & 55\% wt. rGFRP & 33.6 \(\pm\) 5.9 & 0.9 \(\pm\) 0.2 & 5.5 \(\pm\) 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental tensile strength, elongation at break and elastic modulus values from the tensile tests for the formulations used in this work. The batches made with rGFRPs using Indirect FFF show an increase in the elastic modulus with higher particle sizes (Figure 5c). Similarly, the stiffness of rCFs batches increased by increasing the particle size and filler weight percentage, reaching ~5.8 GPa. A similar value was also obtained by using rGFRPs and UV-DIW, thanks to a higher content of filler and the preferential alignment during extrusion [42; 43; 44].