Stereolithography (SLA)

Normally a higher resolution solid part (and usually slower and more expensive)

Stereolithography (SLA), https://en.wikipedia.org/wiki/Stereolithography, is a 3D printing process where individual layers of a part are formed by a light source (a laser) curing a polymer resin.  The laser outlines each layer of the part causing the liquid polymer resin to be cured.  This process is then repeated usually with a mechanical process in between where a wiper sweeps through the resin shearing the last layer of the part and preparing the part for the next layer.

So in essence, SLA creates a part layer by layer but rather than laying a melted polymer filament a liquid resin is cured by a laser.  There are several major differences in this approach.  The first is that it is far easier to create solid parts via SLA than for FDM since it is much easier for the laser to cure a large resin area rather than lay filament lines in very close proximity in a single layer.  In fact, it can be quite difficult to make a SLA part hollow since liquid resin could become trapped in the solid part (this same phenomena also happens in powder based prints which is more advanced technique).  Another major difference is SLA parts in general require a far larger number of supports and the wiping motion of the SLA printer and shearing forces experienced require that the part be oriented in a manner than it is not very intuitive initially.  Lastly, SLA printers in general offer far greater resolution and surface detail than typical FDM parts.  For these reasons, SLA parts at USNA are often more practical parts than FDM parts although there are advantages to both methods.

Most current SLA printers, including all at MakerSpace USNA, are specifically inverted SLA printers.  This simply means that the part is printed from the bottom to the top of the part in an inverted fashion, i.e. the part is literally pulled out of the liquid resin.  This has no practical affect on the process but does look unusual at first since the bottom of the part will be at the top of the printer and the entire part looks upside down.  The reason for this is very practical and makes sense.  If the part were not inverted, that would mean the part would remain in the resin pool and the part would have to go down (vice up) to prepare for the next layers.  This process is achievable but is far less practical.

Just like there are different FDM filaments, there are and can be very different liquid resins.  MakerSpace USNA has a wide variety of resins including resins that produced highly flexible or elastic parts to resins that are designed to produce stronger or tougher parts. 

In simple terms, SLA is a slightly more expensive process and is somewhat slower than most FDM processes, but the final part is often more desirable.  For this reason parts at MakerSpace USNA are often prototyped with FDM printers and then printed via SLA for final production parts.

One of the most common uses of SLA printers is orthodontics!  All of the major online orthodontic solutions uses CAD scans of your teeth to then create retainers that are simply printed using special medical grade resins.