Designing Around Moisture

Nylon absorbs moisture from the air, swelling roughly 0.2% in dimension for every 1% of moisture taken up. A PA6 part that fits perfectly in winter may bind in a humid summer. Design allowances:

· Budget running clearance generously — 1.5–2× the metal-equivalent value

· For precision parts, condition the stock in a humid environment before final machining

· Use PA12 or POM where dimensions must stay stable across seasons

· Avoid long unsupported spans that amplify the effect of expansion

· Remember that moisture also plasticises nylon, raising toughness and reducing stiffness

Machining and Annealing

Cast nylon machines freely with sharp, high-rake carbide tooling at high spindle speeds; heat build-up is the main enemy, so avoid dwelling in the cut. Stress-relieve thick sections after rough machining, then finish to tolerance. For a comparison with acetal in geared applications, see our POM vs PA66 guide.

Typical duties for cast nylon include large spur and helical gears, sheaves, rope pulleys, slide blocks, and wear pads on heavy machinery. In each case the material earns its place through toughness and cost rather than precision: nylon gears run quietly and absorb shock where a steel gear would transmit it into the gearbox. Where tolerances must hold to a hundredth of a millimetre in humid air, POM or PEEK will serve you better — and a supplier who says so before you order is worth keeping.

Related reading: POM vs PA66 comparison, POM Gears Design Guide, and Engineering Plastic Tolerance Standards.