Nylon Moisture Absorption in SLS and MJF PA12 Parts

Nylon moisture absorption is the first thing people worry about when they move a part from FDM to SLS or MJF.

How Much Moisture Does an SLS or MJF Nylon Part Absorb, and Does It Change the Part's Size or Strength?

A lot less than nylon's reputation suggests, because most SLS and MJF parts are PA12, not Nylon 6. One supplier's PA12 data lists about 1.4% water at saturation against 8.5–10% for Nylon 6, so the part gets a little more flexible in humid air but stays close to size.

The question came from an r/3Dprinting thread asking how SLS prints hold up in a wet environment. Replies pointed out that SLS is a slightly porous process, so some uptake will happen, but PA12 absorbs "way less than PA6," and the main effect is that prints get a bit more flexible.

Why PA12 isn't Nylon 6

Nylon's bad name for soaking up water mostly comes from Nylon 6. Plastock's Nylon 6 data lists 1.3–1.8% water absorption in 24 hours and 8.5–10% at saturation. Its PA12 sheet lists 0.1% in 24 hours and 1.4% at saturation.

Those numbers are for solid stock tested to ISO 62. A printed powder grade won't match them exactly, so pull the data sheet for the specific powder if the number matters to your design.

What nylon moisture absorption does in service

Stiffness is where you'll notice it. A part fresh out of the build is stiffer than the same part after a few weeks in humid air. If you design a snap or clip around dry properties, it will feel softer later. Plastock flags this for Nylon 6 directly: its listed values are dry-as-molded, and performance shifts once the material reaches equilibrium moisture.

Size changes less. Plastock describes PA12's low uptake as the reason parts stay dimensionally stable in humid or wet environments. Long sliding fits and press fits still deserve some margin.

The same thread mentioned vapor polishing. It closes the surface and makes the part more water resistant, but it doesn't turn nylon into a waterproof material.

Designing for it

Use conditioned properties if the part lives outdoors or near water, and leave clearance on fits that have to work in both a dry winter shop and a humid summer. UV and heat are separate problems from moisture. We covered those in MJF PA12 vs SLS dyed nylon for outdoor brackets. For the process side, see MJF vs SLS for a functional nylon part.

At DigiForge

Our materials page lists MJF Nylon, SLS Nylon, SLS Nylon PA12, MJF Glass-Filled Nylon, MJF Nylon 11, SLS PP GB Glass Filled and Thermoplastic Polyurethane (TPU). Listed printing tolerance is ±0.3 mm or ±0.3%, whichever is greater. If the part will see water or high humidity, say so with your upload. If you want it sealed or smoothed, ask us when you quote.