Under Formlabs’ “SLS Powders Explained,” a viewer asked, “Is it possible to make the particles smaller so you can get higher detail and the parts don’t look like sandcastles?” One reply blamed the laser. Another said the sandy look is mostly partially sintered powder on the surface, which blasting can remove. Both touch on why SLS particle size is what it is.
Why isn’t SLS particle size smaller?
Because very fine powder stops spreading well. A 2024 open-access review of polymer SLS powders in KONA explains that smaller particles are better in theory for surface quality and accuracy. But for fine powders, van der Waals forces outweigh gravity, so particles clump and lay down uneven layers. The review says a range of 45 to 90 μm is usually preferred, though some authors put it at 20 to 80 μm.
It also cites tests where finer wasn’t better. A PA12 powder averaging about 42 μm packed less densely than a standard 59 μm powder. And a powder with a large number of particles under 20 μm failed processability testing.
How big are commercial powders?
Right in that window. HP’s datasheet lists a particle size of 60 μm for HP 3D High Reusability PA 12.
So where does the sandcastle look come from?
The second reply was close. HP Labs researchers, writing about MJF parts, describe the gray surface as partially fused powder particles that leave a rough, light-scattering skin. HP’s white paper on tumbling says the roughness of powder-printed parts comes from the particle size distribution, with small particles attached to the surface. Its raw MJF test parts measured an average Ra of 10.74 μm.
What actually makes parts smoother?
Post-processing, not finer powder. In HP’s tumbling tests, ceramic media brought Ra down to a predicted 1.5 μm after 24 hours, and plastic media to about 4.4 μm. If a customer wants fine detail and a smooth skin, quote the finish step rather than hunting for a finer powder that may not spread.
Related on the Solvi blog: SLS Fine Detail: How Small Can Features Go?; SLS Orange Peel: Why It Happens and How to Fix It; Incoming SLS Powder: Check It Before It Prints.
Sources: YouTube: SLS Powders Explained (Formlabs), viewer question and replies; KONA Powder and Particle Journal: Role of powder properties and flowability in polymer SLS, a review (open access); HP 3D High Reusability PA 12 datasheet (PDF); HP Labs: Improving aesthetics through post-processing for 3D printed parts (PDF); HP white paper: Smoothing MJF parts with a vibratory finish (PDF).