Stereolithography and selective laser sintering are two very different forms of 3D printing/additive manufacturing technology. They have little in common other than building parts a layer at a time.
First, these two systems build with different materials. Selective laser sintering, also known as SLS, sinters powder material (often a plastic in powder form) to form an object. Sintering is a process that involves melting the powder using the heat from a laser beam to form a solid mass. The laser can be a high-powered carbon-dioxide laser, a pulse laser, or a similar high-powered laser. The melting, though, stops before the powder reaches a state of liquefaction. The melting is enough to bind to a previous layer of sintered powder.
In SLS, once a layer has been sintered, the machine spreads a new layer of powder over the previous one, and the laser continues to trace the form, creating a new layer of sintered material. This process is repeated, layer by layer, until the part is built.
Once the layer-by-layer build is complete, technicians remove the object from the non-sintered powder. The non-sintered powder can be used in the next build.
Stereolithography (SL) uses a photosensitive liquid material (a resin) that is usually housed in a vat. SL systems use a different type of laser, one that delivers UV light to trace a pattern on a thin layer of liquid photopolymer resin. As the laser touches the resin, the resin solidifies. Usually, this process is done a layer at a time. Recent innovations have found new ways to more quickly solidify the photopolymer resin.
The resolutions you can obtain with these two processes are generally dependent on the width of the laser beam and/or the layer thickness of the powder. Remember, both systems use different types of lasers so the wavelengths of the lasers differ as well. One point to note is that the UV focus spot can be smaller than the infrared laser of an SLS printer.
The materials you would use with SL technology are not the same as those you would use with SLS technology. That said, SL and SLS can affect a chosen material’s mechanical properties. The resins used in SL can be more rigid and brittle than the materials used in SLS. It is best to consult with the vendor of the additive technology to make the right choice for your application.
The surface finish of parts made by either of these processes is different. SLS parts tend to have a rough surface finish that may require post processing. SL parts tend to have a smoother finish.
Even though 3D printing technology can handle just about any geometry, some of those shapes may need support throughout the printing process. With SLS technology, the need for supports is uncommon because the bed of powder usually delivers most of the support you need. Depending on the geometry being printed with SL technology, you might need to include support structures.
As mentioned earlier, these two technologies deliver different finishes to the final part. Depending on the purpose of the final part, post processing may be needed. Polishing is one post processing option for SLS parts and can take the form of sanding or even machining to achieve the desired finish.
SL parts usually need minimal finishing, but coatings can be applied for specific finishes or to help strengthen the part.
Finally, there are differences in cost. Most SL printers are smaller (to not waste resin material) and therefore less expensive. SLS machines can be large and therefore cost more. SLS can also work with metal powders, so that will add to the total cost of ownership as metal will require extra safety precautions. With both technologies, the cost of materials covers a wide range.
Each technology has its pros and cons and it will depend on the project requirements which option is best suitable for the job.