Which nylon powder (SLS and MJF) print problems can you fix in the design?

Five nylon powder printing problems start in the file, not the printer: hollow parts full of trapped powder, large flat panels that warp, print-in-place parts that fuse together, living hinges that crack and fine detail lost in the grain. They apply to both selective laser sintering (SLS) and Multi Jet Fusion (MJF). Both melt PA12 or PA11 nylon powder layer by layer, in grey, black or white. The loose powder holds the part up as it prints. So there are no supports and no overhang limit to design around.

Below is each problem, why it happens and how to change the model. Or have our studio redraw it for you. The numbers in one place:

A rough or patchy surface and other printer-side problems are covered in common problems with 3D printed models. The SLS and MJF design guidelines sit beside every other process in 3D printing design rules for every process.

Hollow parts full of trapped powder

What you see: a hollow part comes back as heavy as a solid one, because its inside is still packed with loose powder.

Why: the part prints buried in powder, so every cavity fills with loose powder that never melted into the part (unsintered powder). A closed cavity gives that powder no way out. It stays there for good.

Fix it in the design: give a hollow part a 2.0 mm shell and at least two escape holes of 5 mm, at opposite ends, so air goes in one and powder pours out of the other. Draw them where they will not show, such as the underside or a hidden face. Nylon is priced by weight (see pricing), so a hollow part that empties properly is lighter and cheaper than a solid one.

Hollow part? Upload it: a person checks where the holes go before it prints

Large flat panels that warp

What you see: the ends or corners of a large flat panel curl up, so a lid rocks or a cover won’t sit flat.

Why: a big flat area cools unevenly inside the powder, and the larger and thinner it is, the more it curls. A thick lump next to a thin skin makes it worse, because the two cool at different rates.

Fix it in the design: make large flat panels 2.5 mm thick, add ribs across the back, and keep the thickness even. Turning the edges down, so the panel becomes a shallow tray rather than a flat sheet, stiffens it too. Tell us how big the panel is and what it covers, and we check the wall and the ribs when we look at the file.

Big flat panel? Upload it: a person checks the wall and ribs before it prints

Print-in-place parts fused together

What you see: a hinge, chain or set of gears printed already assembled (print in place) comes out as one solid lump that will not turn.

Why: powder keeps the pieces apart while they print, but only if the gap between them is wide enough. In too narrow a gap the powder fuses to both faces, or packs in so tightly it can’t be cleaned out.

Fix it in the design: leave 0.35 mm on each side between moving pieces. On a large face (bigger than 20 mm², about the size of a 4.5 mm square), leave 0.5 mm. The hole is wider than the part by twice the gap, so a 10 mm pin needs a hole 10.7 mm across. Separate parts that push together need 0.25 mm on each side. A press fit depends on the part and the load, so tell us what must fit and we confirm the figure. Fits for every process, and how to split a part into pieces that join, are in 3D printing tolerances and clearances.

Parts that must fit? Tell us which dimensions matter

Living hinges that crack

What you see: a hinge made of the part itself (a living hinge) cracks along the fold, either the first time it is bent or after a while in use.

Why: a hinge that is too thick stretches its outer face further than the nylon will go. Nylon is tough and bends a long way, but even a well-drawn living hinge tires after tens of bends, not thousands.

Fix it in the design:

• Hinge: 0.5 mm thick and 5 to 10 mm long.
• Direction: the hinge must not bend across its layers. We set the orientation, so tell us which way the part opens.
• Lifetime: a lid opened a few times in its life suits a living hinge. A lid opened all day does not, so use a pin hinge printed in place instead, with the gaps from print-in-place parts fused together.
• Snap arms: 2.0 mm thick or more. Test-fit one before you order a batch: a clip that works on screen still needs trying.

Hinge or clip? Upload it: a person flags any thin spots before it prints

Fine detail and text lost in the grain

What you see: hairline lettering, a fine texture or a thin pin comes out blurred, broken or missing.

Why: nylon powder leaves a matte, slightly grainy surface, coarse enough to swallow hairline detail. A very thin feature also has too little material to survive having the powder cleaned off it.

Fix it in the design:

• Text: a stroke of 1.0 mm, raised or recessed by 0.5 mm, in letters 5 mm tall or more.
• Other detail: raised or recessed detail down to 0.5 mm, or 1.0 mm if it has to be read.
• Walls: 1.0 mm, 1.5 mm for a thin wall standing on its own, and 2.0 mm for anything handled.
• Holes and pins: 1.5 mm or more.

Logos have their own rules in how to put a logo on a 3D print.

Upload it: a person checks every wall and letter before it prints

Size limit and tolerance

The largest single nylon piece is 670 x 360 x 550 mm. A bigger part is printed in sections and joined. Design the joints in from the start rather than cutting a finished model apart: how big a 3D print can be shows how.

As printed, expect ±0.3 mm up to 100 mm, then ±0.3% beyond that, with height at ±0.5 mm. So a 200 mm part holds ±0.6 mm. Results depend on the printer, the material and how the part is oriented. If a fit has to be tighter, tell us which face and we confirm what can be held.