Nylon & PA12 Guide
PA6, PA66 and PA12: hygroscopicity, wear, and SLS/MJF printed properties
Nylon (polyamide) is the workhorse engineering plastic for wear, toughness and fatigue. PA6 and PA66 are molded/extruded grades with high strength but strong moisture absorption that changes dimensions and stiffness; PA12 absorbs far less water and is the standard SLS/MJF 3D printing material. For printed parts, PA12 delivers UTS ≈48 MPa with layer-dependent ductility and excellent chemical resistance.
1. Grades and moisture
PA66 (UTS ≈83 MPa dry) and PA6 (UTS ≈75–85 MPa) are the strongest commodity nylons, used for gears, bushes, fasteners and structural brackets. Their weakness is hygroscopicity: they absorb up to 8% moisture by weight, which plasticizes the material — strength drops and dimensions grow — so parts must be conditioned or designed with moisture in mind.
PA12 absorbs only ~1.5% water, giving stable dimensions and electrical properties; it is tougher and more chemical-resistant than PA6/66 at slightly lower strength. It is the default powder for SLS and MJF printing and common in fuel, pneumatic and cable applications.
2. Printed PA12 (SLS / MJF)
SLS/MJF nylon prints at layer heights 0.1–0.12 mm with typical tolerance ±0.3% (±0.3 mm). Printed PA12 has density ≈1.0 g/cm³, UTS ≈48 MPa, elongation roughly 15–40% depending on orientation and infill, and excellent fatigue and impact behavior — strong enough for functional brackets, ducts and snap fits.
Printed parts are porous on the surface (0.1–0.3 mm Ra); dyeing, media blasting and sealers improve cosmetics and reduce fluid permeability. Minimum wall is ~0.7–1.0 mm; unsupported overhangs are not a constraint because un-sintered powder supports the geometry.
3. Wear and friction
Nylon is self-lubricating and wears well against steel: PA66 with internal lubrication (MOS2, PTFE) is a standard gear material. Its low friction coefficient (0.2–0.4 against steel) and fatigue resistance make it preferable to POM where impact or noise matters.
For printed gears, PA12 fatigue life is good but inferior to molded PA66; consider glass-filled printed grades (PA12-GF) for stiffness. Avoid nylon in hot, wet acid environments — hydrolysis degrades the polymer above ~60 °C in moisture.
4. Selection guidance
Molded/CNC: PA66 for strength and temperature (to ~90 °C continuous), PA6 for toughness and cost, PA12 for dimensional stability and low moisture. Printed: PA12 for functional prototypes and end-use brackets; PA12-GF for stiffness; MJF gives finer detail than SLS on small features.
Design clearances for moisture growth in molded nylon, specify conditioning (water absorption to equilibrium) for precision fits, and add a shrinkage allowance if the part will be post-annealed.
Quick reference
| Grade | Density g/cm³ | UTS MPa | Water abs. 24h | HDT °C | Notes |
|---|---|---|---|---|---|
| PA6 | 1.13 | 75–85 | 1.3–1.8% | 80 (1.8MPa) | Tough, cost-effective |
| PA66 | 1.14 | 83 | 1.0–1.5% | 95 | Strongest molded nylon |
| PA12 | 1.01 | 48 (printed) | ~0.3% | 95 | SLS/MJF standard |
| PA12-GF | 1.25 | 70 (printed) | ~0.3% | 130 | Stiff, low creep |
Sources & references
- ISO 1874 — Polyamide moulding materials
- ASTM D638 — Tensile
- ASTM D570 — Water absorption
- Xometry Learning Center — Nylon PA12 Guide (public engineering reference)