## 1 Introduction
If this aspect is negligible in prototyping, certainly it is not so for large-scale production. Instead, at the quality level of the finished object, due to the overlapping of layers typical of AM processes, it is possible that the presence of \"hidden\" defects between one layer and another, which are more difficult to verify than for a solid object, are first cast and then finished as in classical production [23; 24]. Nowadays, additive manufacturing systems are based on different technologies. For example, fused deposition modeling (FDM) uses thermoplastic materials (such as PLA, ABS, nylon, PET, ultem, polycarbonates) melted at high temperatures through an extruder and then deposited on a printing plate, creating objects of complex geometry with an accurate level of precision [25]. The selective laser sintering (SLS) process instead uses a laser beam to selectively sinter portions of a powder bed within a compact layer. This technique allows the use not only of plastic material but also of ceramic, glass, and metal powders [26]. Once a cross-section of powder is sintered, the printing platform is lowered to allow the deposition of a subsequent layer of powder. The process is repeated until the production of the model is complete. The selective laser melting (SLM) technique instead allows the use of different materials (such as aluminum alloys, titanium, chromium, cobalt, steel, and nickel) fused from a bed of powder through a high power laser, obtaining prototypes and components such as inserts for injection molds [27]. The products have excellent mechanical characteristics and a high finishing degree, such as to be used also in the goldsmith sector and in the production of medical prostheses. Finally, the multi jet fusion (MJF) process, developed by Hewlett-Packard (HP), represents the last evolution of AM technologies. It is based on the use of multiple material jet nozzles, which deposit a fusion agent to selectively melt the particles [28]. This technology allows one to produce very thin successive layers and to obtain a finished object with a high density, low porosity, and a very high level of aesthetic finish. The presence of more nozzles also allows a significant increase in productivity, compared to other 3D printing systems [29]. Currently, the application of AM processes is still an unexplored area in many industrial sectors, including that of the production of impellers for electric pumps, proposed in this study, in which its modeling is analyzed using reverse engineering techniques [30]. This operating machine is composed of an electric motor, generally asynchronous, connected through a joint to a pump (axial, centrifugal, or radial). Its task is to transform electrical energy into potential energy, available in the form of pressure in the liquid. Most of the impellers on the market are made using precision casting processes, but it is estimated that in the coming years there will be an increasing use of additive manufacturing, similar to what happened to the production of components for the aerospace industry (subsidized by heavy investments in major airlines companies) [31]. The integration of reverse engineering (RE) techniques to AM processes has been already successfully studied in many sectors, such as the industrial sector, and is involved in the maintenance or replacement of machine elements, such as turbine blades [32], and the development of specific surgical implants for patients [33], until the application in the production of small-scale cultural artifacts [34]. The integration of additive manufacturing technology into the high-performance stainless steel submersible pump market would allow the development of impeller blade profiles with a greater hydraulic efficiency [35], thanks to a greater design freedom, with the possibility of easily creating complex shapes and using ultra-light and ultra-resistant materials, such as titanium or aluminum alloys, to reduce masses, thicknesses, and wear.