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Stereolithography (SLA) is an additive fabrication process that builds parts in a pool of UV-curable photopolymer resin using a computer controlled laser. The laser is used to trace out and cure a cross-section of the part design on the surface of the liquid resin. The solidified layer is then lowered just below the surface of the liquid resin and the process is repeated. Each newly cured layer adheres to the layer below it. This process continues until the part is completed. SLA was the first “rapid prototyping” technology.

Pros: For concept models or patterns to be used as masters for other prototyping methods, SLA can produce parts with complex geometries and excellent surface finishes as compared to other additive processes. The cost is very competitive and the technology is available from several sources.

Cons: Prototype parts are much weaker than those made from engineering grade resins, so the parts made using SLA are typically unsuitable for functional testing. Also, since the resin is UV-curable, exposure to sunlight continues to cure the resin and parts can become brittle over time. While SLA can make parts with complex geometries, it gives no insight into the eventual manufacturability of the design. The liquid polymers can be very toxic.