An American company called 3D Systems first developed this system. At the moment it is considered to be the most successful, probably because it was the first to wet the engineers appetite.

Schematic Diagram of the SLA process

• The STL file of the component is sliced by the devices software. Each slice is then etched onto the surface of a photosensitive UV curable resin using a "swinging" laser. Where the beam of the laser strikes the surface the resin is cured, each layer of cured resin being, typically, 0.13mm thick.
• At the end of each pass, which covers the whole surface of that layer, the platform descends allowing liquid resin to flow over that previously cured to a depth of 0.13mm. A re-coating blade passes over the surface to ensure a consistent layer thickness is achieved.
• The re-coating blade was found to be necessary since prior to this, air entrapment caused build problems.
• In those components whose features form an overhang, as in the above diagram, the software anticipates the situation and develops a series of stilts on which a platform can be formed. Without this facility there would be little control over the position of the "floating" feature.
• To avoid the model sticking to the build platform a lattice base is created thus reducing the contact area of resin to platen.
• On completion, the model is carefully removed and washed in a solvent to remove uncured resin and placed in an UV oven to ensure all resin is cured.
• The above system suffers with the problem that if an area of a particular layer did not attract a deposit of resin a cavity would been formed. As the table descends this cavity would fill with liquid resin to effectively create two layers, the lower one of which would not be reached by the laser. Depending on its use, trapped uncured resin in a prototype can cause difficulties in the post manufacture situation; e.g. if heated, expansion may effect accuracy or even destroy a mould. A recent development designed to overcome the above problem replaces the re-coating blade with a vacuum operated snorkel, which is filled at the start and end of each pass. As the filled snorkel passes over the model a controlled amount of resin is released, ensuring that the entire surface is coated. An added bonus of this adaptation is that resins of varying viscosity can be used.

Adaptation of the SLA process by adding a snorkel