Boss Design for 3D Printing
A boss is a printed cylindrical post, usually built around a hole, that gives a screw or a pin something thicker to bite into than the wall around it.
Does an FDM boss need a thinner wall, the way a molded boss does?
Stratasys Direct's FDM Design Guidelines state that it is not necessary to reduce the wall thickness of a boss, rib or gusset in FDM parts. The guide adds that gussets or ribs should support the boss instead, that draft is unnecessary, and that a fillet is optional but reduces stress concentration.
That is the opposite instinct from the injection-molding habit covered further down this page, where a boss wall is deliberately thinned to stop a sink mark from showing on the surface above it.
How tall can an FDM boss or post be before it stops printing cleanly?
Markforged's Composites Design Guide says to avoid printing posts with a height more than five times their diameter, and gives a minimum post diameter of 1.6 mm in X and Y and 2.0 mm in Z. Those figures are for Onyx material on 100 micron layers, not a universal FDM constant.
| Axis | Minimum post diameter (mm) | Provenance |
|---|---|---|
| X | 1.6 | Markforged Composites Design Guide v1.4 |
| Y | 1.6 | Markforged Composites Design Guide v1.4 |
| Z | 2.0 | Markforged Composites Design Guide v1.4 |
How do injection-molding boss rules compare, and do they apply to FDM?
Injection-molding boss guidance solves a different problem than FDM: a sink mark and mold ejection, not layer adhesion. Protolabs states a molded boss wall should run 40 to 60 percent of the part's nominal wall thickness, and carrying that thinning habit into an FDM boss removes material for no reason and weakens the part.
The row below is a molded-parts rule from a manufacturing bureau, not FDM guidance. The wall thickness answer at the top of this page is the one that applies to a printed boss.
| Feature | Injection-molding figure (percent of nominal wall) | Provenance |
|---|---|---|
| Boss wall thickness | 40 to 60 percent | Protolabs, plastic boss design page |
What this page does not tell you
- This page compares FDM boss guidance with an injection-molding boss rule used elsewhere in manufacturing. The injection-molding figure does not apply to a printed part and is shown only so a reader does not carry it over by habit.
- Boss strength also depends on which perimeter count and infill wrap around it, and neither guide below publishes a number for that interaction.
- None of these figures were measured on the printers that will make your part.
When to print a test coupon instead
Print a small boss or post test coupon at the diameter and height you are considering before committing to a full run, especially when a screw or a pin will thread into it repeatedly. Send the file through a review request and tell us which boss dimensions to check. A person reviews your request and follows up by email.
Sources
- FDM Design Guidelines (2017 edition), Stratasys Direct
States the recommended minimum wall thickness is generally 4x the layer or slice thickness, with a table mapping slice thickness 0.005 to 0.020 in to minimum wall 0.020 to 0.100 in, and a case-by-case absolute floor of 0.008 in. This 2017 edition also distinguishes a single-contour wall (2x slice thickness) from the general 4x recommendation.
- Composites Design Guide v1.4, Markforged
States a minimum part dimension of 1.6 mm in X, 1.6 mm in Y and 0.8 mm in Z, on Onyx material with 100 micron layers.
- Plastic Boss Design on Molded Parts, Protolabs
States a molded boss wall thickness of 40 to 60 percent of the part's nominal wall thickness, for injection-molded parts.
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Published 5 September 2026.