Selective laser sintering (SLS) 3D printers make use of a CO2 laser and a thermoplastic polymer powder to build parts. Because of the high power laser, it’s generally considered more complicated than both FDM and SLA.

SLS 3D printers consist of a powder bin, a build platform, a powder recoater, a CO2 laser, a set of galvanometers, a set of heaters, and a powder feeder.

Generally speaking, the printing process begins with filling the powder bin with a certain amount of polymer powder. The bin is then placed into the machine, where the heating stage begins. Before printing, heaters are used to get the powder to a temperature just below its melting point.

The actual printing starts with the re-coater depositing a single layer of powder onto the build platform. That’s where the CO2 laser comes into play. The laser’s role is to selectively induce fusion between particles to form a solid at specific locations.

Galvanometers are used to navigate the laser beam to a specific point on the build platform. If you don’t know what galvanometers are, think of them as tiny mirrors.

The laser moves along a “point-to-point” pattern, solidifying the whole cross-sectional area of a layer. After the layer is complete, the re-coater deposits a new layer of powder and the build platform moves down one layer in height. The process is then repeated until the part is complete.

Process of Selective Laser Sintering

Like all methods of 3D printing, an object printed with an SLS machine starts as a computer-aided design (CAD) file. CAD files are converted to .STL format, which can be understood by a 3D printing apparatus.

Objects printed with SLS are made with powder materials, most commonly plastics, such as nylon, which are dispersed in a thin layer on top of the build platform inside an SLS machine.

A laser, which is controlled by a computer that tells it what object to "print," pulses down on the platform, tracing a cross-section of the object onto the powder.

The laser heats the powder either to just below its boiling point (sintering) or above its boiling point (melting), which fuses the particles in the powder together into a solid form.

Once the initial layer is formed, the platform of the SLS machine drops usually by less than 0.1mm exposing a new layer of powder for the laser to trace and fuse together. This process continues again and again until the entire object has been printed.

When the object is fully formed, it is left to cool in the machine before being removed.

Unlike other methods of 3D printing, SLS requires very little additional tooling once an object is printed, meaning that objects don't usually have to be sanded or otherwise altered once they come out of the SLS machine.

SLS doesn't require the use of additional supports to hold an object together while it is being printed. Such supports are often necessary with other 3D printing methods, such as stereolithography or fused deposition modeling, making these methods more time-consuming than SLS.

SLS machines can print objects in a variety of materials, such as plastics, glass, ceramics and even metal (which is a related process known as direct metal laser sintering). This makes it a popular process for creating both prototypes as well as final products.

SLS has proved to be particularly useful for industries that need only a small quantity of objects printed in high quality materials. One example of this is the aerospace industry, in which SLS is used to build prototypes for airplane parts.

Because airplanes are built in small quantities and remain in service for many years, it isn't cost-effective for companies to produce physical molds for airplane parts. These molds would be too expensive to make and would then need to be stored for long periods of time without being damaged or corroded.

Using SLS, companies can create prototypes that are stored digitally as .STL files, which they can redesign or reprint as needed. Because SLS machines can print in a range of high-quality materials, from flexible plastic to food-grade ceramic, SLS is also a popular method for 3D printing customized products, such as hearing aids, dental retainers and prosthetics.

Advantage

• The biggest advantage of SLS is that there’s no need for additional support material, regardless of the part’s geometry.
• As parts are built inside the powder bin, the non-sintered powder acts as a support material to the printed part. After the part has been printed, it’s left with no markings of the support material on its surface. The powder need only be brushed away.
• Because parts are built by a laser, which solidifies a crosssectional area of each part, multiple parts can easily be printed at once. This makes SLS ideal for small manufacturing runs. The goal with SLS printers is to fill the build area with as many prints as possible so as to reduce the non-sintered waste powder.

Disadvantage

• Unlike FDM, parts printed with an SLS printer can’t be used right after the print process is complete. That’s because parts must cool down, which can take a long time.
• The most significant disadvantage of SLS requires high-end technology and a lot of power. As such, it’s not quite ready to hit the consumer market.