Why Does an SLS or MJF Nylon Part Sometimes Wear Unevenly Across Identical Geometry Printed in Different Orientations?

Both processes build parts layer by layer, and the bond between layers is never quite as strong as the material within a layer. A wear surface parallel to the layer lines sees material pull apart along that weaker plane. The same surface built perpendicular to the layers sees the full in-plane strength of the sintered or fused nylon instead.

The anisotropy is real, just smaller than people expect

Powder-bed processes like SLS and MJF are more isotropic than filament printing, because each layer fuses across its whole area rather than along discrete deposited beads. The surrounding unfused powder also supports the part during the build in a way FDM's open air doesn't. But the layer-to-layer bond still isn't identical to in-layer strength, and a wear surface loaded by repeated sliding or rubbing finds that difference eventually.

Print a bushing or gear standing up versus lying flat and the wear surface's relationship to the layer stack changes. One orientation loads the interlayer bond in shear as the part wears. The other loads it mostly in compression along the layer plane. Given enough cycles, the shear-loaded orientation wears faster.

What actually drives how much it matters

Wall thickness and feature size change the size of the effect. A thick wear surface has enough cross-section that orientation barely registers. A thin tooth on a small gear, or a snap-fit tab flexing over and over, has far less margin. That's where orientation decides whether the part lasts.

Powder bed temperature and cooling rate affect how completely each layer fuses to the next. That varies somewhat by machine and by where in the build chamber a part sits.

Two nominally identical parts from the same job can show slightly different wear even at the same orientation. It's a small effect next to orientation, but it's part of why testing one sample from a run doesn't tell you everything about how the rest will behave.

What to actually do

Orient the part so the wear surface's sliding direction crosses the layer lines rather than running parallel to them.

For a gear, that usually means building with the tooth face perpendicular to the layer stack instead of flat on the platen, so the contact surface isn't relying on interlayer bond strength during meshing loads.

If wear life matters and geometry allows it, a slightly thicker wall or a bigger tooth root radius buys margin against the anisotropy directly, which is cheaper than iterating on orientation alone. DigiForge runs MJF and SLS in several nylon grades, including glass-filled and PA12 variants that hold up better under repeated wear than base nylon. Which one is worth the upgrade depends on the specific load the part actually sees.