Selective Laser Sintering (SLS) 3D printers use CO2 lasers and thermoplastic polymer powders to make parts. Due to the high power laser, it is generally considered to be more complex than FDM and SLA.
The SLS 3D printer consists of a powder silo, a build platform, a powder coater, a CO2 laser, a galvanometer, a set of heaters and a powder feeder.
In general, the printing process begins with filling a powder silo with a quantity of polymer powder. The box is then placed in the machine and the heating phase begins. Prior to printing, the heater is used to bring the powder to a temperature just below its melting point.
The actual printing begins with a recoater depositing a single layer of powder onto the build platform. This is where the CO2 laser comes into play. The role of the laser is to selectively induce fusion between the particles to form a solid at a particular location.
An ammeter is used to navigate the laser beam to a specific point on the build platform. If you don't know what an ammeter is, you can think of it as a small mirror.
The laser moves along the "point-to-point" pattern, curing the entire cross-sectional area of the layer. After the layer is completed, the recoater deposits a new layer of powder and builds the platform down a layer of height. The process is then repeated until the part is complete.