## 2 Materials and Methods  
The dummy method in SPIF consists of the simultaneous deforming of two overlapping sheets [38]. This method has been used to improve the surface finish [29] and to deform previously welded sheets by means of friction stir welding [30]. In this case, the dummy method was used to manufacture non-stick molds from sheets pre-coated with PTFE resin (Figure 1). The molds were made with aluminum-magnesium EN AW 5754-H32 sheets, the dimensions being 210 \(\times\) 210 mm and 1.2 mm thick. On one side, a circular pattern was printed to control the deforming undergone at each point [39]. The other side was covered with a resin-rich PTFE. To protect this re-covering during the incremental deforming, a PVC dummy sheet of 210 \(\times\) 210 mm and 3.0 mm thick was used. The design chosen for the molds was a truncated pyramidal geometrical shape, as it is of industrial interest and facilitates the geometrical characteristics. Several tests were carried out in order to determine the maximum depth and angle that the sheets were able to reach. The final dimensions are shown in Figure 2.  
The pieces were drawn using SolidWorks parametric software. Once drawn, the models were exported to the MasterCAM program, where the strategy was defined, and the technological parameters of the process were determined. This being done, the MasterCAM automatically generated the NC programs. The tool path strategy used was contour-parallel. Nine tests were carried out, with different pitch and feed-rate values. For the pitch, the values 0.3, 0.6 and 1.0 mm were used; for the feed-rate, the values were 1800, 2000 and 2200 mm/min. The spindle speed in all cases was set at 750 RPM. The geometrical parameters and technologies were selected according to database collected from the literature (Table 2) and after carrying out several sensitivity tests (Table 3). Breakage of any of the materials was avoided (PVC, PTFE, EN-AW 5754), since the aim was to attain healthy molds that could be studied geometrically. Figure 1: Schematic representation of SPIF process. Figure 2: Dimensions of the mold used in the tests (unit: mm). It is important to bear in mind that during the process, there is a simultaneous deformation of two sheets with very different mechanical properties and behaviors. The dummy sheet (PVC) is that which is in contact with the punch, so that, although the intention is to deform the aluminum, it is necessary to work with values that are compatible with the PVC (Table 2). The tests were carried out at a model QP2026-L Chevalier machining center, equipped with a Fanuc numerical control. The tool used for the forming was a 14.68 mm diameter punch, made up of two pieces: an EN AW 2024 aluminum body and a stainless steel spherical tip. SAE 30 mineral oil was used for lubrication, following the recommendations of [PERSON] et al. [PERSON] et al. (2016). Experiment set-up is shown in Figure 3.  
Once the pieces were manufactured, different geometrical parameters were measured (Figure 4): minimum thickness, depth, height, initial angle, elastic recovery, and area of deviation between theoretical and real profile. \begin{table}
\begin{tabular}{c c c c c} \hline
**Authors** & **Material** & **Feed-Rate (mm/min)** & **Spindle Speed (RPM)** & **Pitch (mm)** \\ \hline [PERSON] et al. [PERSON] et al. (2000) & PVC & 1500 & Free & 0.5 \\ [PERSON], [PERSON] \& [PERSON] (2011) & PVC & 1000 & Free & 0.5 \\ [PERSON] et al. [PERSON] et al. (2011) & PVC & 1500 & Free & 0.5 \\ [PERSON], [PERSON] \& [PERSON] et al. (2012) & PVC & 2000 & 2000 & 0.5/1.0 \\ [PERSON] et al. [PERSON] et al. (2013) & PVC & 1500 & 500 & 0.5 \\ [PERSON] et al. [PERSON] et al. (2014) & EN-AW 5754 & 2000/8000 & 2500 & 0.5 \\ [PERSON] et al. [PERSON] et al. (2015) & EN-AW 5754 & 2000 & – & 0.5 \\ [PERSON] et al. [PERSON] et al. (2019) & EN-AW 5754 & 2000 & 200 & 1.0 \\ [PERSON] \& [PERSON] and [PERSON] (2014) & EN-AW 5754 & 3000/7500 & 1000/2000 & 0.2 \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters used in literature for SPIF of PVC and EN-AW 5754.