## 2 Materials and Methods
### Indirect FFF 3D Printing  
The term Indirect 3D printing refers to the use of 3D printing processes to manufacture the tooling used in conventional manufacturing to obtain a component or a part, i.e., molds, inserts and utensils [23]. This workflow comes from the Rapid Tooling techniques and may help in fostering quick, customizable, and less expensive manufacturing of tools for prototypes, pre-series, or small batches of production, ranging from small- to large-scale products [24; 25]. In this work, customizable mold silicone cavities were designed and manufactured to produce the parts from the automotive case study. The molds were designed using the CAD software Fusion 360 (Autodesk, San Rafael, CA, USA), and the slicing software PrusaSlicer was used to create the code files to be printed (Prusa Research, Prague, Czech Republic). The same CAD software was also used to modify the STEP files of the component used for the experimentation, better described in Results and Discussion. The rigid parts of the counter mold silicone were printed with the low-cost desktop-size FFF 3D printer Prusa i3 MK3S equipped with a 0.6 mm diameter nozzle (Prusa Research, Prague, Czech Republic). The 3D printing parameters of the molds and counter molds are summarized in Table 1. In detail, the parameters of the molds, in particular infill, were chosen to create mechanical interlockings for the silicone cavities to be poured, as better explained in the next section [26]. The parameters of the counter molds were selected to reduce the possibility of leakages during the curing phase of the silicone. The silicone cavities were manufactured by pouring the Gls-PRO 30 silicone into the assembled molds and counter molds, which were removed after 24 h of curing at room temperature. The molds were then used to fabricate the component parts with the epoxy-based thermosetting resin filled with shredded rGFRPs or rCFs. First, Araldite BY158 and Aradur 21 in the weight ratio of 100/28 were mixed to obtain the epoxy-resin based matrix with a gel time of 80 min, corresponding to the maximum processing time of the material formulations before starting to harden. The shredded rGFRPs or rCFs were then added to the matrix and mixed for 15 min with a mechanical stirre at 50 rpm. In detail, 30% wt. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Part** & **Parameters** & **Units** & **Values** \\ \hline \multirow{8}{*}{Rigid molds (PETG)} & Perimeters & // & 2 \\  & Top/bottom layers & // & 4 \\  & Infill & \% & 20 \\  & Infill (interlocking—bonding) & \% & 40 \\  & Speed & mm/s & 45 \\  & Layer height & mm & 0.3 \\  & Nozzle diameter & mm & 0.6 \\  & Temperature (nozzle) & \({}^{\circ}\)C & 235 \\  & Temperature (bed) & \({}^{\circ}\)C & 85 \\ \hline \multirow{8}{*}{Silicone counter molds (PLA)} & Perimeters & // & 2 \\  & Top/bottom layers & // & 3 \\ \cline{1-1}  & Infill & \% & 20 \\ \cline{1-1}  & Speed & mm/s & 70 \\ \cline{1-1}  & Layer height & mm & 0.3 \\ \cline{1-1}  & Nozzle diameter & mm & 0.6 \\ \cline{1-1}  & Temperature (nozzle) & \({}^{\circ}\)C & 215 \\ \cline{1-1}  & Temperature (bed) & \({}^{\circ}\)C & 60 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: 3D Printing parameters (3D printed mold parts). of rGFRPs (200 \(\upmu\)m or 1 mm) and 20 or 30% wt. of rCFs (63 \(\upmu\)m or 200 \(\upmu\)m) were added to the epoxy-based system, obtaining four different formulations to test: 30% wt. rGFRPs (200 \(\upmu\)m), 30% wt. rGFRPs (1 mm), 20% wt. rCFs (63 \(\upmu\)m), and 30% wt. rCFs (200 \(\upmu\)m). After mixing, these mixtures underwent a low vacuum level for 15 min to reduce the presence of air bubbles. After processing, the molds were kept for 24 h at room temperature. Parts were then removed from the mold and underwent a post-curing of 1 h at 100 \({}^{\circ}\)C to complete crosslinking of the thermal-curable epoxy-based matrix. Post-processing was then performed to remove the extra material corresponding to the pouring channels.