## 3 Results and Discussion
### 3D Printing  
With a dimensional error of about 0.3 mm, as the result of the entire process chain from reverse engineering to printing, the concept model still allows verifying the applicability on the pump axis and in the diffuser seat. This is crucial to make the designer able to carry out the appropriate changes before the final printing of the functional model in SLM. The printing process was 15 h long. Figure 10 shows the concept model during and after printing. The functional model was printed through the SLM technology. The geometry of the impeller required the use of a lot of support material (as shown in Figure 8) which, requiring work to remove, made printing quite expensive (more than \(\upc1200.00\) before taxes). However, the cost can be justified by the flexibility in changing the pitch angle of the blades and by the drastic reduction in costs related to the logistics of the pieces produced, making the solution in additive manufacturing interesting. Figure 11 shows the printed impeller. Comparing the main dimensions of the 3D printed functional component with the original one made of stainless-steel sheets, negligible differences have been shown thanks to the ability to predict the volume shrinkage during SLM (around 2%), and therefore foreseeing an appropriate increased volume to be printed. It is evident from Figure 11 that the functional model is characterized by a rough surface, which is around 8.5 \(\upmu\)m (measured by using the mechanical profilometer Sloan  
Figure 11: Functional model of the impeller printed through the SLM technology by the service ZARE S.r.l. : (**a**) 3D view, (**b**) bottom view, and (**c**) top view. Figure 10: Concept model printing of the impeller of a submersible electric pump through FDM technology by using PET as base material: (**a**) during and (**b**) after. Dektak 3030), against the typical values of laminated steel sheets less than 0.8 \(\upmu\)m [47]. This can be critical for the chosen component, which should have a reduced surface roughness in order to guarantee the highest efficiency of the pump [48; 49]. In fact, the enhanced surface roughness of SLM printed products is one of the main drawbacks of the technology [50; 51; 52]. Therefore, there is a need for a post-processing treatment aimed at obtaining the required surface roughness for the specific application, leading to a further increase in the final cost of the component. On the other hand, the typical mechanical characteristics of AISI 316L stainless steel pieces produced by SLM appear to be improved if compared with annealed samples, both in terms of hardness and tensile strength, but with lower ductility [53; 54]. Table 8 reports the results of the geometrical accuracy. It suggests that the printing process follows the geometrical constraints of \(\pm\)0.1 mm on the blade tolerances with respect to both the original and the CAD models. In fact, the absolute error, defined as the difference between the measured (average) and the target (from CAD model) values of the various thicknesses, is 1% for the blades and the upper flared part, while around 3% for the lower flared part. The greater deviation observed for the upper flared part with respect to the lower one (i.e., with standard deviations of 0.043 vs. 0.005, respectively) is due to the post-processing operations needed for the removal of the support structures. However, such a discrepancy can be reduced by means of a further finishing process ensuring a more constant thickness for all of the impeller's sections. Moreover, the SLM process allows reproducing the blades with an improved repeatability if compared with the traditional cutting and bending of sheet laminates used to produce the original impeller. In fact, the standard deviation obtained for the six blades' thicknesses is around 0.030 for the original impeller (Table 2) and against around 0.018 for the 3D printed one.