SLS v/s FFF
Basic know-how regarding material and technology pertaining to the part requirement and application will go a long way in achieving aesthetically good looking and functionally competent prototypes.
The basic difference between Selective Laser Sintering (SLS) and Fused Filament Fabrication(FFF) is in the way the parts are fabricated. FFF is a filament based technology, whereas SLS is a powder-based laser sintering technology. In FFF, the material enters the extruder in the form of a filament where it is heated to a temperature of about 200˚C. The nozzle then deposits this semi-liquid material on the build platform. After each layer of material that is deposited, the platform goes down by an amount equivalent to one layer thickness, to pave way for a new layer of material. The entire part is fabricated bottom-up.
In SLS, there’s a bed of powder material. The laser scans and sinters the powder depending on the part geometry. Then the platform goes down by an amount equivalent to one layer thickness. The recoater then moves the new layer of material in its place. The laser scans and sinters the powder, and the platform goes down again and this process continues till the entire part is fabricated bottom-up.
In SLS technology, you can fabricate multiple pieces in one single build by nesting the parts throughout the bed volume.
For any overhanging feature, or an unsupported part geometry, support structures will be generated in FFF technology. The removal of supports leaves burr marks on the surface, thus additional cost is incurred in post-processing the parts. But in SLS technology, since the unsintered powder itself acts as support, there is no support structure generation. This unsintered powder can later be cleaned away leaving behind a final part geometry.
Due to absence of support structures in SLS, the surface quality isn’t compromised. Thus, moving part assemblies and snap fit geometries are better suited in SLS technology as compared to FFF technology. Know more about why SLS is more suited for moving part assemblies.
The layer thickness is FFF technology, on an average across the industry, is 180µm, whereas that in SLS technology is 100µm. The lesser the layer thickness, the finer the surface finish but more time is required for part fabrication, and vice versa.
SLS is scalable, leading to a shorter lead-time for multiple part quantities.
FFF parts have a rough surface finish and distinct layer marks on the surface. Whereas SLS comes with an inherent grainy surface finish owing to powdery material with little or no visible layer marks, unless the geometry has curvature or features at an angle.
It would be unjust to compare the speed of SLS and FFF processes since one is a laser sintering technology and the other is a filament based technology. Nevertheless, an important point to consider is that, in SLS technology, you can fabricate multiple pieces in one single build by nesting the parts throughout the bed volume. Whereas in FFF technology, you can only fabricate one part at a time. Thus SLS is scalable, leading to a shorter lead-time for multiple part quantities.
Eg: If there’s a part geometry that takes 10 hours to fabricate in FFF, then to fabricate 100 similar quantities, it will take 1000 hours. The time required to manufacture the part is constant, irrespective of the number of quantities required.
But in SLS, let’s say if the same part takes 20 hours to fabricate (On an average a SLS machine would run for approx. 20hrs) then 100 parts can be fabricated in the same build in the same 20 hrs. Because in SLS technology you can nest multiple parts together and sinter them in one single build. There’s a clear lead-time advantage in SLS when you want to fabricate multiple pieces.
More dimensionally accurate parts are fabricated in SLS technology as compared to FFF.
FFF is optimal when the requirement is form visualization and validation. But for functional part geometries where fitment and testing needs to be done, FFF isn’t best suited as support structures, rough surface finish and poor dimensional accuracy hamper the intended function of the part geometry. In such a scenario SLS technology is the ideal fit. Also, the accuracy in FFF is around (+/-)300–400µm. Whereas that in SLS technology is approx. (+/-)200µm. Thus more dimensionally accurate parts are fabricated in SLS technology as compared to FFF.
Both, FFF and SLS have their pros and cons and they cater to their own markets and applications. Thus, there is no contention of one being better than the other, but a basic know-how regarding material and technology pertaining to the part requirement and application will go a long way in achieving aesthetically good looking and functionally competent prototypes.
