## 2 Materials and Methods
Figure 2: Schematic diagram of the specimen, with positive and negative square test patterns. In the specific case of this study, a factorial design of experiment (DOE), with three factors, was carried out to evaluate the geometric characteristics of the different parts obtained. The three factors taken into account for this experiment are: exposure time, direction of the geometry, and machine used. In the case of exposure time, three different levels have been considered (1, 2, and 3). These levels correspond to different exposure times that vary according to the technology used, based on the time recommended by the manufacturer. In the DLP process, the exposure times are 1.5, 2, and 2.5 s, and for LCD, the exposure times are 6, 7, and 8 s (Table 2). On the other hand, the geometry direction factor consists of two levels: female, defined as a gap, or male, defined as an axis (Table 2). The last factor that has been assessed in the experiment is the technology used in the manufacture, which, as mentioned above, would be DLP or LCD. Furthermore, in order to obtain sufficient data to carry out the statistical study and to evaluate the repeatability of the manufacturing process, 6 repetitions of each case have been carried out, resulting in 72 repetitions for the designed geometry. In order to observe the influence of the factors mentioned in this study, in the specific case of this experiment, the perimeter has been taken as the response variable, since the shape of the pieces is designed so that they can fit together hollow-axis, and this depends, to a large extent, on the perimeter measured in each of the pieces. The third type of the pieces consist of a pyramid with a cylindrical base where the different steps have been designed with the same height, but different sizes (Figure 3). All of the previous studies focused mainly on the analysis of the geometric characteristics of the parts, oriented towards the horizontal resolution of the manufacturing equipment used. However, it is also important to analyse the vertical resolution of the machines. For this reason, the part has been designed in the form of a stepped pyramid, with a circular base (Figure 3). Each pyramid consists of 16 steps, with a height of 200 \(\upmu\)m for each step, i.e., the total height of the pyramid, with respect to the base, is 3.2 mm. On the other hand, the diameter of the base ranges from 3 mm to 100 \(\upmu\)m.  
The study consisted of evaluating the heights of all the steps of each piece obtained. Three repetitions of each piece were carried out for each technology. In order to evaluate the differences between the two technologies, the heights of the different steps were measured to see if they were close to the design values, establishing the standard deviation of the data. \begin{table}
\begin{tabular}{c c c c} \hline
**Experiment Number** & **Exposure Time (s)** & **Geometry** & **Technology** \\ \hline
1 & 1 (1.5) & Shaft (Positive) & DLP \\ \hline
2 & 1 (1.5) & Hole (Negative) & DLP \\ \hline
3 & 2 (2) & Shaft (Positive) & DLP \\ \hline
4 & 2 (2) & Hole (Negative) & DLP \\ \hline
5 & 3 (2.5) & Shaft (Positive) & DLP \\ \hline
6 & 3 (2.5) & Hole (Negative) & DLP \\ \hline
7 & 1 (6) & Shaft (Positive) & LCD \\ \hline
8 & 1 (6) & Hole (Negative) & LCD \\ \hline
9 & 2 (7) & Shaft (Positive) & LCD \\ \hline
10 & 2 (7) & Hole (Negative) & LCD \\ \hline
11 & 3 (8) & Shaft (Positive) & LCD \\ \hline
12 & 3 (8) & Hole (Negative) & LCD \\ \hline \end{tabular}
\end{table} 
Table 2: Design of Experiment.