PA66 Moisture Absorption and Dimensional Stability

PA66 is one of the workhorse engineering plastics: strong, stiff, tough, wear-resistant and heat-resistant. It is also hygroscopic, meaning it absorbs water from the air. A molded PA66 part is never dimensionally stable until it has reached moisture equilibrium with its service environment, and the change can be large — often bigger than the molding tolerance itself.

Figure 1. PA66 moisture states from dry to saturated with design rules for dimensional stability.

Designers who ignore this ship parts that fit on the bench and jam in the field. This guide explains the mechanism, quantifies the effect, and shows how to design, dry and condition PA66 parts so their dimensions hold.

Why PA66 absorbs water

Nylon's amide groups form hydrogen bonds with water molecules. Water diffuses into the amorphous regions of the polymer, where it acts as a plasticizer: it pushes the chains apart, swelling the part and simultaneously lowering stiffness and strength.

Two consequences follow:

• Dimensions grow. Absorbed water increases volume; for unfilled PA66, saturation in a humid environment can add roughly 2–3% in linear dimension, and even normal ambient conditioning adds on the order of 1–2%.
• Properties change. Dry-as-molded PA66 is stiff and brittle; conditioned PA66 is tougher but softer and less strong.

Glass-fiber reinforcement reduces the relative swell because the fibers do not absorb water, but it does not eliminate it. A 30% glass-filled PA66 still moves meaningfully with moisture, and it also suffers more anisotropy because fiber orientation adds a directional component.

Quantifying the effect

• Dry as molded (DAM) — essentially 0% moisture; the stiffest and most brittle state, and the state most drawings are (wrongly) dimensioned to.
• Conditioned / equilibrium — typically around 2.5% moisture at 50% relative humidity and 23 °C; the state most indoor parts eventually reach.
• Saturated — up to about 8–9% moisture when immersed; the extreme case.

The practical rule: assume a part will grow after molding as it picks up moisture, and dimension the drawing to the service condition, not to the dry state. If the drawing is to the dry state, the part will be undersized once conditioned.

Moisture also interacts with molding itself: drying is mandatory before processing. Wet pellets hydrolyze at melt temperature, and the result is splay, bubbles, reduced molecular weight and poor mechanical properties. PA66 typically needs drying to below about 0.1–0.2% moisture, often around 80 °C for 4–6 hours in a desiccant dryer.

Design rules for moisture-tolerant PA66 parts

• Dimension to the service state. Define whether the drawing is DAM or conditioned and state the conditioning explicitly.
• Allow clearance for growth. When a PA66 part mates with a metal or a stable plastic part, leave room for moisture-driven expansion.
• Use glass-filled grades where stiffness and lower swell matter, but remember anisotropy: fiber orientation adds a directional warp component.
• Avoid tight fits across the moisture range. A press fit that is correct dry will become an interference fit, or jam, once the part absorbs water.
• Design for conditioning time. Thick sections take far longer to reach equilibrium; a thick part may take weeks to stabilize.
• Consider moisture as a design variable, not a defect — for seals and bearings, some swell can be beneficial.

Conditioning: how to get stable parts

Conditioning accelerates the moisture uptake that would otherwise take weeks:

• Hot-water or steam conditioning — immerse or steam parts to reach equilibrium quickly, then measure.
• Humidity conditioning — hold parts in a controlled humidity chamber at a defined temperature and RH to a defined equilibrium.
• Time in service — the slowest option; parts may take weeks to months to stabilize depending on wall thickness.

Whatever the method, define the target moisture content in the specification and verify it by weight gain, then measure dimensions at that state. Measuring a dry-molded part and calling it "the dimension" is the single most common source of PA66 fit problems.

Processing notes

• Dry the resin to the supplier's spec before molding; use a desiccant dryer, not just a hot-air oven.
• Verify moisture with a moisture analyzer, not by feel.
• Keep the dryer sealed and avoid long residence times in the hopper.
• Control mold temperature — a hotter mold gives better crystallinity and more stable (if slightly more warped) parts.
• Package parts in moisture-barrier bags if they are to be measured or assembled dry.

When PA66 is the wrong choice

If a part must hold tight tolerances across wide humidity swings with no allowance for conditioning, PA66 is a poor fit. Consider:

• A low-absorbing engineering plastic such as PBT, POM or PPS for tight-tolerance, moisture-stable applications.
• A metal where dimensional stability is paramount.
• PA66 only where its toughness, wear resistance and temperature range justify the tolerance compromise.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Drawing states DAM or conditioned, not ambiguous
• Service-condition moisture content defined
• Clearance allowed for moisture growth
• Fits checked across the full moisture range
• Glass-filled grade used only where anisotropy is acceptable
• Resin dried to spec and verified by moisture analyzer
• Conditioning method and target moisture defined
• Thick sections allowed extra conditioning time
• Parts stored in moisture-barrier packaging if measured dry
• Material re-evaluated if tolerances cannot accommodate swell

PA66 rewards the designer who treats moisture as a known quantity. Dry it before molding, condition it before measuring, and dimension it for the environment it will live in — then its excellent mechanical properties can be used without fighting a moving target.