Stereo-lithography, also known as SLA, is a 3D printing process where, prototypes are built layer by layer using light from a laser. Photo polymer resins like clear resin, standard resin, to name a few, are used to make prototypes in SLA. These resins, or ‘build materials’ as they are called are in a liquid state. The 3d printer’s build tray is submerged in a basin of photosensitive material. The depth to which a build tray is submerged depends on the strength of the laser being used, type of material and required tolerances. A part’s entire cross section is traversed by the laser as it builds up each layer.
SLA is better suited to printing parts with small and well defined features. The SLA process works with polymers & resins, not metals. When printed using SLA, parts generally yield higher dimensional tolerances a better surface finish.
SLA presents a challenge when printing larger parts, as they need support during the printing process. This becomes a major hindrance when printing parts with complex geometries.
Selective Laser Sintering or SLS, is when prototypes are 3D printed using powdered building materials. The technique uses polyamide and polystyrene powders as building materials. The materials are binded together to create desired structures. Layers of the materials are carefully on the build tray using a leveler or roller. Cross sections of the prototype are then sintered layer by layer by a laser. Just like SLA, thickness of the layers being printed depends largely on the strength of laser being used, type of material and required tolerances.
SLS can work with metals like steel, titanium and nickel in addition to polymers. Parts built using SLS are generally tougher than the ones built using SLA.
Once printed, the parts need to be cooled down. Efforts to speed up the cooling process may result in variations from the intended designs. When using metals in the SLS process, one has to take extra caution not to breathe in the fine particles that may be harmful.
Liquid photopolymers required for SLA cost around $80 to $100 per liter, whereas SLS powders cost somewhere between $300 to $600 per kilogram. SLS requires high peak energies as compared to SLA to compress metal powders, making it a costlier alternative. When it comes to surface finish, SLA parts are preferred due to their mold like finish, however, SLS is more suited to printing parts with higher tolerances.