SLS 3D Printing - Selective Laser Sintering

SLS or powder bed fusion, uses a heat source to fuse powder particles together, to produce a solid part.

SLS or powder bed fusion, uses a heat source to fuse powder particles together, to produce a solid part.

SLS or powder bed fusion, uses a heat source to fuse powder particles together, to produce a solid part.

SLS 3D Printing - Pros

No supports
Great tolerances
Good for batch production

No supports
Great tolerances
Good for batch production

No supports
Great tolerances
High 3D print batch production

Visible build lines
Supports needed
Post processing often neccesary

Visible build lines
Supports needed
Post processing often neccesary

Porous surface finish
Medium lead time

SLS 3D Printing - Cons

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What is Selective Laser Sintering?

 

Selective laser sintering is the process of using powder bed fusion technology with polymer powder to produce parts. The powder is heated to just below melting point. Subsequently, a re-coating roller spreads a (on average) 0.1mm layer over the build platform. After this, a C02 laser, selectively sinters a cross section of the part. The platform then drops by one layer of thickness. This process is repeated until the part is complete. Furthermore, the powder is left around each layer, which allows SLS to print with no supports.

SLS Materials

 

Low thermal conductivity materials work best for SLS 3D printing. This is because, they have more stable characteristics during the sintering phase. Polyamide (PA) is the thermosplastic, almost exclusively used in the process. Because, it boasts long-term stability and good chemical resistance. Nylon is the most common . Standard PA 12 nylon costs $50-$60 per kilo. Although SLS powders do not come in a range of colors, they can be dyed.

 

You can enhance mechanical properties, like heat and chemical resistance, powders can be mixed with other materials. For instance: aluminium, glass or carbon.

SLS postprocessing

Common SLS powder properties

Nylon 12

 

Cost Effective

 

Good mechanical properties

 

Matte suface finish

Polyalumide

 

High stiffness

 

Good strength-weight ratio

 

Wear resistance

SLS 3D Printing Characteristics

Selective laser sintering illustration

SLS Printer perameters

 

Selective laser sintering has range of perameters that can impact print quality. These include:

 

Laser spot size

 

Layer height

 

Powder particle size

 

Lowering laser size and layer height will improve surface finish and accuracy. However, this will impact lead times. Finer powders will result in a smoother surface finish. Although, they are harder to handle. In contrast, coarser powders are simpler to handle, yet leave a rougher finish.

SLS layer adhesion

 

SLS prints layer by layer, like all 3d printing. However, the powder particles, in selective laser sintering, are fused together in all directions, due to the bed being pre-heated. Therefore giving the models isotropic properties. This gives SLS prints superior durability and flexibility, compared to FDM and SLA.

 

In contrast, adding a composite (e.g glass or carbon) can make the structure anistropic, stretching out the particle structure. This makes the model weaker. Usually in the build direction.

Ilustration of isotropic mechanics
SLS dimensional accuracy

 

The laser sintering process, is susceptable to warping, much like FDM and SLA. Each layer fuses to the layer below, as it cools. The cooling causes the new layer to shrink, which pulls on the layer underneath. It is best to angle the parts to lower the surface area of each layer. Consequently keeping the heat of each layer more consistent, keeping warping to a minimum. SLS machines reduce shrinkage through heated build chambers. These keep the temperatures of powder and fused polymers close together. However, as a result the chamber must be allowed to cool before removing parts. Sometimes, the cooling period can take half the build time.

 

SLS post-processing

Loose powder removal

 

FirstlySLS parts must be removed by hand, from powder bin. After this, the remaining dust is removed with compressed air. Finally, plastic bead blasting will remove any unsintered powder, still stuck to the surface.

 

SLS parts bare a rough surface, in comparison to FDM and SLA.  The surface finish is initially grainy. However, can be made far smoother with polish or paint.

Removing an SLS part from the powder bin by hand.
SLS example pre and post polishing

SLS Polishing

 

Parts are polished in vibro machines, that are full of little chips that slowly erode the surface, until polished. However, this does have a small effect on a parts dimensions. In addition, some sharp edges become rounded. For this reason, parts with, intricate or fine details should not be tumbled.

SLS strengths and weaknesses

SLS 3d printing boasts many strengths. Firstly, the process does not require supports, which gives a very consistent surface finish. Secondly, It is great for producing durable end use parts. Thirdly, it has great layer adhesion. And finally, SLS produces high levels of intricacy and fine detail.

The cost of the machines is the main weakness, in SLS printing. Good industrial machines cost around £200000. Furthermore, they require highly skilled operators. The other main disadvantage, is the lead time. In particular, the heating and cooling, of the powder bin. which can take the same, or more than the printing.

SLS image gallery