Its operation is by stereolithography which originates in the same process as FDM technology by the superposition of successive layers in order to create a physical part. Why is 3D resin printing different?
Normally, the filament deposit is created by a nozzle that flows the plastic wire, resin printers rely on a laser traveling the surface of a tank of liquid resin to solidify layers one after the other. The build plate is inverted, so it goes up as the part is built.
Why is filament deposition technology limited?
This comes mainly from the precision of the outlet diameter of the nozzle (generally 0.4 mm), SLA technology will be favored by the use of a very fine laser oriented to the resin tank by a set of mirrors.
So, it is quite easy to differentiate with the naked eye a part printed with a layer thickness of 25µ on a resin printer and the same part on a filament printer. The quality of the surface finish, at the same layer thickness, will always be superior on a part printed in SLA. The resin will exhibit much less streaking than layering layers of molten plastic. This is because the extruded plastic needs to be spread out to get a good adhesion, which is not the case with technology using a laser. The processing will be much finer and more concentrated.
There is a distinct difference in the fine details which are generally altered during additive molten filament printing, when the nozzle from which permanent heat emanates will have a strong tendency to remelt the previously deposited plastic and thus create a smearing effect and sag on the pointed parts in particular.