Designing FDM parts for nylon including moisture behaviour
PA12 and PA6 give excellent toughness and wear resistance but move dimensionally with moisture. Here is what that means for design.
Nylon, meaning PA12 and PA6 in FDM, is chosen for its wear resistance, fatigue resistance and toughness at low temperatures, properties that make it a common substitute for machined or moulded functional parts such as gears, snap fits and living hinges. It is a semi-crystalline material, which means less shrinkage variation while cooling than ABS or ASA, but it introduces a second dimensional effect that those materials do not have: moisture uptake from the surrounding air.
Nylon absorbs moisture from ambient air over days to weeks, and as it does, the polymer swells slightly and its mechanical properties shift, generally becoming tougher and less stiff as it moves from a dry, as-printed state toward equilibrium with room humidity. A part measured immediately after printing and again a month later will not be exactly the same size, and a design that ignores this can end up out of tolerance in service even though it printed correctly.
Designing for moisture movement, not just print shrinkage
For tight fitting assemblies, the practical approach is to decide up front which state the part will spend most of its service life in, dry or humidity equilibrated, and size critical features for that state rather than for the as-printed dimension. A bearing seat or sliding fit designed only around the freshly printed size can bind once the part swells in a humid environment, or develop excess play if the mating part does not move with it.
- Size sliding and press fits for the equilibrium condition the part will actually see in use, not the as-printed state
- Add slightly more clearance on nylon fits than the standard 0.2 to 0.4 mm used for PLA or PETG
- Store finished parts and unopened filament in sealed bags with desiccant before use if dimensional stability matters
- Where dimensional stability is critical, consider PA12-CF or PPA-CF, which absorb less moisture than unfilled nylon
- Avoid designs that rely on a nylon part staying exactly one size over months of service in variable humidity
Mechanical design features that suit nylon
Nylon's toughness and fatigue resistance make it a strong candidate for living hinges, snap fits and gears, parts that flex or engage repeatedly without cracking. These features should be designed with generous fillets at the flex point and a wall thickness tuned to the specific flex requirement, since nylon is more forgiving of repeated cycling than PLA or PETG but still fails eventually if the local strain is too high.
For gears and wear surfaces, nylon's low friction coefficient against itself and against metal is a genuine advantage over ABS or PETG, but the surface finish from FDM layer lines still needs to be considered. Light post machining or a controlled orientation that puts the wear surface parallel to layers rather than across them extends service life considerably.
| Property | PA12 (unfilled) | PA12-CF |
|---|---|---|
| Moisture uptake | Moderate to high over weeks | Lower, more dimensionally stable |
| Stiffness | Moderate | Higher, more brittle at impact |
| Surface finish | Smoother, better for cosmetic parts | Visible fibre texture |
| Best use | Snap fits, hinges, general functional parts | Rigid jigs and fixtures, dimensionally critical parts |
- Problem
- Bearing seat was sized to the as-printed diameter and developed excess play after two weeks in a humid production environment.
- Change
- Recalculated the bore for the moisture-equilibrated dimension and switched to PA12-CF for that housing revision.
- Result
- Fit remained within specification across a six week storage and use trial.
Sequence and storage on the shop floor
If parts need to be measured or assembled at a defined dimension, agree on the timing: measuring immediately after printing gives a different number than measuring after two weeks on a shelf. For assemblies with tight nylon-to-nylon or nylon-to-metal fits, it is common practice to condition parts for a set period before final inspection so the measurement reflects the state the part will actually be in during service.
Frequently asked questions
- How much does a PA12 part grow after printing?
- Growth depends on ambient humidity and part thickness, but a fair rule of thumb is up to a few tenths of a percent linear over several weeks in typical indoor conditions, more in humid environments. For dimensionally critical fits, size for the equilibrium condition rather than the freshly printed size.
- Should we specify PA12 or PA6 for a wear part?
- PA12 generally has lower moisture uptake and better dimensional stability, while PA6 offers somewhat higher stiffness and heat resistance but absorbs moisture faster. For most wear parts where dimensional stability matters, PA12 is the safer default.
- Does PA12-CF eliminate the moisture problem entirely?
- No, it reduces moisture uptake and the resulting dimensional shift compared with unfilled PA12, but it does not eliminate the effect. It is a meaningful improvement for dimensionally sensitive parts, not a complete fix.
- Can nylon parts be used immediately after printing for a fit check?
- They can, but be aware the fit may shift slightly as the part equilibrates with ambient humidity over the following days to weeks. For a final go or no-go decision on a tight fit, it is worth checking again after conditioning rather than relying only on the first measurement.
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