Print-in-Place Clearance for SLS and MJF Assemblies
Print-in-place clearance in powder-bed nylon is bigger than FDM habits suggest: HP calls for about 0.7 mm between moving faces, an SLS guide 0.5 mm.
Hinges that come out of the printer already assembled are one of the neat tricks of powder-bed printing. Get the print-in-place clearance wrong, though, and you've got a solid lump.
How Much Gap Do Printed-in-Place Moving Parts Need?
More than FDM users expect. For MJF, HP suggests about 0.7 mm between moving faces. An SLS design guide from the University of Zurich gives 0.5 mm. Thin-walled parts can sometimes go smaller.
An r/3Dprinting user asked what clearance to use for printed hinges and bearings. Answers ranged from about 0.13 mm (0.005") to 0.3 mm. Those are FDM numbers, and they're far too tight for powder-bed nylon. The most useful reply said to print three or four sample hinges and test them in more than one orientation.
Print-in-place clearance from HP and SLS guides
HP's design handbook says spacing between faces of parts printed as assemblies should be at least 0.7 mm as a general rule. If walls are thicker than 30 mm, it says to go larger. If walls are thinner than 3 mm, it says clearance can drop to 0.3 mm, depending on the design and some iteration with the printer. Separate parts that get assembled after printing need at least 0.4 mm.
The University of Zurich's SLS design guideline lists 0.5 mm for moving parts made inside each other and 0.2 mm if they're made separately.
Why so much? HP's handbook describes thermal bleeding, where excess heat makes nearby powder stick to the part surface. In a narrow gap, that stuck powder can bridge the two faces.
Making the joint free up
The loose powder in the gap has to come out after the build, so don't seal a pin inside a fully closed bearing. Then test a coupon in the same orientation as the real part, because Z behaves differently from XY. See why MJF Z tolerance runs looser and why push-fits vary between identical parts. If the joint only needs to flex, a living hinge in SLS or MJF may beat a pin.
Before you upload
Our 3D printing capability page lists MJF Nylon and SLS Nylon among its materials. Tell us which faces have to move. For the clearance we'd suggest on your part, ask us when you quote.
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