## 2 Related Works
### Obtaining the Physical Prototype  
Today, the way of understanding reality in our society is constantly evolving given the ease of access to the digital tools that we currently have. In addition, the use of new technologies has been increased, given the situation that COVID-19 has generated, producing a great change when it comes to relating to each other [29; 30; 31; 32] and with the heritage that surrounds us [33; 34; 35; 36]. The need arises to bring heritage closer to any part of the world in the simplest way possible; therefore, the need for digital twins has become relevant and, going a little further, combining digital twins with the most avant-garde technologies related to 3D printing, we can obtain physical prototypes that allow us to discover the cultural and architectural heritage in a different and cheaper way [11; 25; 37; 38]. We can say that there are three large families of 3D printing technologies. In the first place, we would say that we have the family of addition (additive manufacturing), that is, the one that encompasses all the technologies that replicate reality by adding material, where nothing existed. Based on layers, we can generate our physical twin; in a certain way, we could compare it to the Greek technique of casting with lost wax [26; 39]. Secondly, we have subtraction technologies (subtractive manufacturing); these technologies, from blocks of raw material, by means of milling cutters or other elements, remove layers of material and give rise to our physical twin as if it were a classic stone sculpture (normally we will talk about CNC machines here). Lastly, we would talk about cutting technologies (manufacturing by cutting): these technologies, based on sheets of material and cutting templates (DWG, DXF, etc. ), will generate pieces that we will assemble; normally, these are laser cutting machines, although we could generate similar parts with CNC printers. In Figure 5, we can see different examples developed by the research team in collaboration with the University of the Americas of Puebla, Mexico. Figure 4: In the image: (**Left**): the author flying the drone. (**Right**): the author positioning the laser scanner. In the case of this research, it has been decided to use an addition technology, more specifically, VAT polymerization, specifically laser SLA (selective exposure to light by laser). A brief summary of the different additive manufacturing technologies has been prepared below:  
First of all, if we see Figure 6a, we have the material extrusion technology FDM (fused deposition modelling). This technology stands out for its optimal surface finishes, a wide range of colors and materials, and a low cost. On the contrary, we have low resolution and, in many cases, fragility of the pieces. Secondly, in Figure 6b, we have outlined the operation of the technology known as material jetting (MJ). This procedure allows the creation of objects by combining different materials and colors. It is based on photopolymeric liquid resin that is heated to 30-60 \({}^{\circ}\)C, thus obtaining the right viscosity for printing. The print head, which is very similar to that of an inkjet printer, distributes the photopolymer resin on the platform. The curing of this resin is performed with UV light. The projector of this UV light is integrated into the print head and will form different superimposed layers. Like the SLA process that we will see later, this technology is based on the principle of photopolymerization. Among its characteristics is an optimal surface finish, a full range of colors, and a multitude of materials with different physical and mechanical properties, but it has a high cost. Thirdly, in Figure 6c, within the additive printing technologies, we highlight the selective laser sintering (SLS) technology, which is a rapid prototyping technique in which a layer of powder is deposited (a few tenths millimeter) in a vat that has been heated to a temperature slightly below the melting point of the powder. Next, a high-power laser causes the deposited particles to fuse and solidify.