What is SLS 3D Printing?
Selective laser sintering uses powder bed fusion technology, to fuse plastic particles together. Thus creating solid parts. The powder form is heated just below melting point. This is achieved using a C02 laser, drawing a cross section of the part layer by layer. Subsequently, the platform then drops a layer and the re-coating roller spreads a new layer on top. The process is repeated until a complete part/ parts are left. This allows SLS machines to print multiple parts with no supports. Furthermore making it the 3d printing process most suited to medium scale production of end use parts.
SLS and Design
Selective laser sintering is hands down the best 3d printing option for final use prototypes. The main reasons for this are: it can print all the fine details that SLA can (only without the smooth finish). In addition, SLS prints achieve superior mechanical properties that can rival finished injection molded products.
Furthermore, This property also makes selective laser sintering the best suited 3d printing process for small scale production. This is due to being able to fill the entire build volume with seperate and even interlinking, without any loss in quality.
3D Printing Powders
The almost exclusively used type of plastic is Polyamide (PA). This is due to its low thermal conductivity. PA 12 Nylon is the most common and costs $50-60 per kilo. In Addition, PA boasts excellent chemical resistance and mechanical properties, making it perfect for low batch prodution (roughly 50-500 parts depending on the size).
PA powders come in standard black white or grey, however they can be dyed. Additionally You can enhance the properties of SLS parts. For instance: strength, toughness, heat and chemical resistance can be achieved by mixing other materials. Some examples include: aluminium and glass. Below are some general properties of each.
PA 12 Nylon
Cost Effective
High mechanical properties
Matte/ textured surface
Alumide (Aluminium filled)
Metallic look
Heat resistant
Good density to stiffness ratio
PA-GF (Glass filled)
High Stiffness
Good strength to wieght ratio
Wear resistant
Selective Laser Sintering Printer perameters
SLS 3D Printing has 3 main perameters that impact print quality. These are:
-Laser resolution or spot size
-Layer Height
-Powder particle size
A Smoother surface finish can be achieved by increasing the laser resolution and lowering the layer height. Additionally, finer powders produce a smoother finish, although they are harder to handle than their courser counterparts.
SLS Layer Adhesion
Selective laser sintering prints layer by layer, just like SLA and FDM. However, here the particles are fused in all directions. This gives SLS prints isotropic properties, and also one of the main reason that it achieves such excellent mechanical properties, when compared to other 3d printing methods.
Comparatively, adding a composite, like glass, can make the structure anisotropic, stretching out the structure. This usually results in the model being weaker in the build direction.
Dimensional Accuracy SLS
SLS 3D printing suffers from warping . Each layer shrinks slightly as it cools and pulls on the new layer after it is added. Best practice is to angle the parts to minimise larger surface area of each layer. This keeps the heat of each layer more consistent, mitigating shrinkage as much as possible. This does, however, nearly double the lead time as the whole chamber must cool before parts are removed.
Furthermore, PA has a shrink range of around 3%. However this is accounted for in the build preperation of the service providers.
SLS Post-processing
Loose Powder Removal
The Part must first be removed from the bin. Then, excess powder is removed by a mixture of brushing and blasting with compressed air. Finally parts are ‘plastic bead blasted’ to remove any powder that is stuck to the surface.
In Comparison to other printing methods, SLS has a grainy surface texture.It can however, be made smooth with paint and/or polish.
Polishing
Sintered parts are polished in ‘Vibro Machines’. These are vibrating chambers, filled with little chips that gradually erode the surface, until the ideal surface finish it achieved.
The one drawback is, it does have a small effect on parts dimensions. Also any sharp edges or corners will become rounded. Thus, if a part has intricate details or requires a tight tolerance in order to work, the part is best left ‘un-tumbled’.
