**3. Stereolithography**

Stereolithography (SLA) was developed in 1984 by Charles Hull and was the first available commercial AM process. SLA is a vat photopolymerization process based on the solidification of a liquid resin using a UV laser. Since the process takes place in a liquid, support structures are necessary during the building phase. These are made by the same material of the parts and are specified in the machine parameter settings [1].

Concerning **Figure 4**, the manufacturing process starts with the building platform lowered from the top of the vat by a layer thickness. Then, a recoater blade smooths the surface of the vat, and a UV laser cures the material. Then, the platform is lowered by a layer thickness, and the process is repeated until the part is completed. At the end of the process, the platform is lifted, the part is drained and removed from the platform [8]. Then, it is placed in a UV oven to complete the curing. During the solidification process,

**Figure 4.** *Stereolithography process.*

usually, the photopolymer shrinks. This shrinkage induces compression stresses on the previous layers that may cause curling and distortion. These effects can be reduced by adopting clever scan strategies, such as Star Weave or ACES [1].

Acrylates resins were the first photopolymers developed. These resins had high reactivity but produced inaccurate parts due to a significant shrinkage (5–20%) and a tendency to warp and curl. They are low viscosity resins used for visual or anatomic models, with a low accuracy but high-speed manufacturing. On the contrary, epoxy resins present high viscosity and are used for functional parts. They have slow photo speed but allow more accurate, harder, and stronger parts than the acrylate ones.

Furthermore, they exhibit low levels of shrinkage (1–2%), reducing the risk of warp and curl. Most of the commercially available resins are epoxides with acrylate content to combine the advantages of both materials [2]. Moreover, it is possible to suspend ceramic particles in a resin to obtain a ceramic–polymer composite material and improve mechanical and thermal properties.

SLA allows the manufacturing of parts with good accuracy ranges from 25 to 50 μm and smooth surfaces; the typical average surface roughness Ra is lower than 10 μm. However, the metal plating of the internal channels can be critical in terms of adhesion, uniformity of the metal coating, and long-term stability [9], limiting the applicability of SLA in the manufacturing of some RF applications (e.g., waveguide components for space).
