SLA (Stereolithography Apparatus): SLA was the first form of rapid prototyping (RP) created by Chuck Hull (founder of 3D Systems ®) in 1986.
A laser, thanks to a photopolymerization process, solidifies a liquid resin, consisting of epoxy polymers, poured into a tank inside the 3d printer. At the end of the process the product thus obtained is put into an ultraviolet light oven to finish the polymerization process. The stereolithography ensures very precise results with excellent surface finishes (but sometimes with lower mechanical and thermal properties) and it is also used to make matrices for silicone molds. Due to the high cost of 3d printers and materials it is still one of the most expensive technologies, but it is frequently used thanks the unique surface quality and the possibility of realizing monolithic objects of large dimensions.
MICRO-SLA: Recently there has been a proliferation of cheaper SLA printers with substantially lower costs and smaller print volumes(less than 20 cm.) Excellent resolution (up to 25 microns), good surface quality and low costs are making these printers very popular in spite of the modest sizes, the long printing times and the reduced mechanical functionalities. The result is a nice and detailed model ideal for jewelery, art, medical and dental fields, etc. The ability to print even flexible, transparent, biocompatible and castable (replacing foundry waxes) materials contributes to the success of these machines.
DLP (Digital Light Processing): Polymerization of resins through light is very similar to SLA technology. DLP polymerizes a liquid resin, contained in a tank, thanks to LED or LCD projectors. This process is completed layer by layer from the bottom, with layers of few microns. DLP 3d printers can be very much cheaper but still not reach the high standards of accuracy and quality of SLA.