Stereo lithography, or SLA, is a common method of rapid prototyping plastic parts. It allows the designer to specify fairly complex geometries which can be reproduced with high accuracy.

SLA uses a CNC-driven laser to partially cure a pattern into a liquid photocurable resin. The part's geometry is formed by gradually building up layers of the cured resin on a vertically movable stage (z-stage). As each layer (usually a fraction of a milimeter thick) is hardened by the laser, the stage drops down a step into the resin bath. This covers the top of the part in more resin, which is then cured by the laser. The process is repeated until all layers have been deposited and the part can be removed from the bath.

At this point, the form is washed to clean off any excess resin and placed into an ultraviolet oven to finalize the curing process.

The process is a lot like a CAT Scan in reverse. Since it has to build up the part a slice at a time, it is not a fast procedure, and so is only suited for prototypes or very limited production items. The choice of resins also limits the material properties of the final form.