# Additive Manufacturing of Ceramic Reference Spheres by Stereolithography (SLA)
## 3 Experimentation  
To carry out the experiment, a total of 24 pieces were manufactured (Table 2) in two different series, with 12 units in each. All the artifacts are spherical caps with a nominal diameter of 20 mm, each featuring different support geometries. These designs were intended to allow the artifacts to be placed on interchangeable supports for use as reference elements or targets in metrological applications. To enrich the study and enable an analysis of the influence of the manufacturing process, four design variables were considered:  
* Vertical orientation (VO), which can be either Up (U) if the sphere has the pole up or Down (D) if the pole is down. * Interior structure (IS), with three options: without internal support (E), with internal support (F), and with a lattice-type infill (L), see Figure 4. In the case of the solid support without contact, the gap part/support was 0.3 mm. * External geometry of the artifact (EG), with two basic shapes considered: mushroom (M) and bullet (B). See Figure 5.
* Wall thickness (WT), either 1 or 2 mm. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Artifacts \(\mathcal{O}\) 20 mm**} \\ \cline{2-5}  & & **WT 1 = 1 mm** & \multicolumn{2}{c}{**WT 1 = 2 mm**} \\ \cline{3-5}  & **Mushroom 3** & **Bullet 3** & **Mushroom 3** & **Bullet 3** \\ \hline \multirow{3}{*}{UP 2} & Reticular filling (L) & - & - & 3 & - \\  & With inner support (F) & 3 & 3 & 3 & 3 \\  & Without inner support (E) & 3 & - & 3 & - \\ \hline DOWN 2 & Without inner support (E) & - & - & 3 & - \\ \hline \end{tabular} \({}^{1}\) Wall Thickness; 2 Artifact orientation; 3 External geometry.  
\end{table} 
Table 2: Artifacts manufactured according to the design variables. Figure 4: Different types of filling. There was one manufacturing variable:  
Position on the printing tray, rear (1), center (2), and front (3) (see Figures 2 and 3c). Furthermore, thinking about the subsequent placement of the artifacts in another base device, the hemispheres were designed with two different shapes: Bullet type (B) (Figure 5a) and Mushroom type (M) (Figure 5b). However, during the printing process of the first series, three units were discarded from the metrological analysis due to visible defects in the \"green\" state, the state before debinding (mushroom-type pieces without internal support, down position, and 2 mm wall thickness) (Figure 6a). Additionally, another three pieces (mushroom-type with lattice internal structure and 2 mm wall thickness) were discarded because they showed shape deviations greater than 0.3 mm after undergoing the complete process. These deformations were caused by the stresses produced by the lattice structure during the sintering phase (Figure 6b). Therefore, the subsequent metrological analysis considered only the remaining 18 artifacts. As previously mentioned, the printing process was carried out using Vat Photopolymerization by Ultraviolet Laser for Ceramics (VPP-UVL/C) in two series, each lasting approximately 18 h, using 3 DMIX ALUMINA AL-F02 alumina paste supplied by the printer's provider (3 DCeram, Bonnac-la-Cote, France). This phase of the additive manufacturing process is the most sensitive regarding the potential to damage the pieces during handling, as the pieces do not yet have their final mechanical properties. In fact, once the print bed is removed from the printer, the pieces are not in sight (Figure 7a) as they are covered by the uncured ceramic paste, making their handling even more difficult. Cleaning to remove all traces of ceramic paste is meticulously carried out under an extraction hood (Figure 7b) to prevent the operator from inhaling gases emitted by the cleaning solution recommended by the manufacturer. This phase took 8 h, performed by a technicia  
Figure 5: Different types of shape. (**a**) Bullet type with inner support. (**b**) Mushroom type with inner support. Figure 6: (**a**) Mushroom-type artifacts, down orientation, E support and WT = 2 mm. (**b**) Mushroom-type artifact, Up orientation, L support and WT = 2 mm.