SLS ( Selective Laser Sintering ) is one of the earliest 3D printing technologies. Nowadays, the two most common powder bed fusion systems are plastic based, which is called SLS.
The SLS 3D printer will first heat up the printing space which is filled with nitrogen, but the temperature is below the melting point of the powder. Then the SLS 3D printer will lower the container containing the powder, after that, a powder coating roller will move to remove a layer of powder on the printing platform. Then a laser will be emitted according to the position of the cross-sectional contour of the 3D model, and the powder where it is located will be sintered. The powder will form a layer of solid contour after melting. As soon as the first layer is sintered, the working platform will drop by the height of one section layer of the 3D model, and the the powder coating roller will move again to spread a new layer. These processes will repeat until the 3D printing is complete.
After the model is printed, it is necessary to wait for the forming cylinder to cool before taking out the printed model for post-processing.
The most commonly used material for SLS is Nylon, it is a thermoplastic that is lightweight, strong and flexible. Nylon is ideal for rapid prototyping and production due to its stability to shock, chemicals, heat, UV, water and dirt.
Nylons commonly used for SLS are Nylon 11 and Nylon 12, which is also called PA 11 and PA 12. PA 11 is slightly more flexible and impact resistant while PA 12 is stronger, more abrasion resistant and biocompatible.
A typical SLS printed model has a porosity of about 30%, which gives it a unique granular surface. This also means that the SLS models can absorb water, so they can be dyed in various kind of color in hot water. But special handling is required if they need to be used in wet environment.
SLS models are prone to shrinkage and warping, as the newly sintered layer cools, its size decreased and internal stress increases, finally pull the lower layer upwards. The shrinkage is usually 3%-3.5%, so machine operators should take this into account during the preparation phase and size the design accordingly.
Oversintering occurs when radiant heat fuses unsintered powder around a feature, which can result in the loss of small features such as slots and holes.Oversintering or not depends on the size and wall thickness of feature. For example, a 0.5mm wide slot or a 1mm diameter hole will be successfully printed on a 2mm thick wall, but these features will disappear when the wall is 4mm thick or more. As a rule of thumb, slots greater than 0.8mm wide and holes greater than 2mm diameter can be printed in SLS.
Since SLS does not require support material, parts with hollow sections can be printed easily and accurately, and these sections also reduce weight and cost.
Unsintered powder needs to be removed from inside of the model through the holes, so it is recommended to add at least 2 holes with a minimum diameters of 5mm when designing.
If high strength is required, then the model must be printed completely solid. Another option is to make a hollow design without holes. The tightly packed powder will be left in the model in this way, which is able to increase its mass and provide some additional resistance to mechanical loads. An internal honeycomb mesh structure can be added to the hollow interior (similar to fill patterns used in FDM ) to further increase the strength of the assembly. This can also reduce warping.
Models produced by SLS have granular surfaces that smudges easily. But using various post-processing methods such as polishing, staining and painting, the appearance of SLS printed models can be raised to a very high standard. Their functionality can also be enhanced by adding a waterproof coating or metal plating.
In addition to making quick models, SLS 3D printing technology can also be used for small batch production. One of its benefits is that it does not require support during the printing process because the model itself is supported by the printing powder. However, it also has disadvantages, it requires a special laboratory environment due to its industrial grade requirements, and the maintenance cost will be very expensive. On the other hand, there will be graininess and laser spots on the surface of the printed model.
Compared with FDM and DLP, the popularity of SLS printing technology is not high due to its high price, complex post-processing and considerable printer size. However, with the continuous development of new materials and SLS itself, it is believed that SLS will be as popular as FDM and other technologies in the future.
