Overview
PA66 (nylon 6/6) is the workhorse engineering plastic for metal- replacement applications — bearings, bushings, gears, wear pads. It combines high strength, excellent wear behavior, good chemical resistance to fuels and oils, and easy machinability at moderate cost. Most “engineering plastic” gear and bushing applications you encounter in industrial machinery are PA66 (or its close cousin, cast PA6).
The defining design consideration with PA66 — and the one that catches unwary engineers — is moisture absorption. PA66 absorbs water from humid air and from immersion. The polymer’s amide groups hydrogen-bond with water molecules, which plasticizes the structure and dramatically changes mechanical properties. A PA66 part designed using only dry-state properties (the ones typically published on datasheets) will be 40–50% weaker and dimensionally larger in service than the design assumed.
The moisture story — the most important PA66 fact
PA66 reaches 2.5–2.8% moisture at equilibrium in 50% RH air at 23°C — typical office conditions. Continuous immersion drives it up to 8.5% at saturation. The polymer doesn’t degrade — water absorption is reversible — but the property shifts are dramatic:
| Property | Dry | Moisture-conditioned (50% RH) | Saturated |
|---|---|---|---|
| Tensile strength | 85 MPa | ~55 MPa (-35%) | ~45 MPa (-47%) |
| Tensile modulus | 3.3 GPa | ~1.8 GPa (-45%) | ~1.5 GPa (-55%) |
| Elongation at break | 50% | ~200% | ~300% |
| Tg | 70°C | ~30°C | ~10°C |
| Notched Izod | 55 J/m | ~150 J/m | ~200 J/m |
| Dimensional growth | baseline | +0.3% | +0.5% |
The good news in this table: PA66 becomes tougher when wet (impact resistance goes up, embrittlement risk goes down). The bad news: it becomes softer, weaker, and bigger. Two implications:
- Filter and design on conditioned properties for in-service performance. Most service environments are humid; designing to dry-state numbers gives a 30–50% safety factor whether you wanted one or not.
- Account for the dimensional growth. A 100mm PA66 part grows ~0.3–0.5mm in service. Tight-tolerance fits between PA66 and metal need to account for this — either size for dry-as-installed conditions knowing it’ll grow, or pre-condition the part to equilibrium moisture before final machining.
The detail-view shows both states in propertyStates. The frontmatter
mechanical block reflects dry-state values (the conventional published
data); the conditioned values are in propertyStates.conditioned.
Machining notes
PA66 machines well — close to POM, slightly less clean. Practical recipe:
- Sharp carbide or HSS, positive rake
- Speed: 300–800 SFM (lower than POM’s 500–1500)
- Feed: 0.005–0.015 in/rev
- Coolant: recommended for finish work and deep cuts; light cuts can run dry
The PA66-specific issues are both moisture-related:
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Stock moisture content matters. PA66 stock typically arrives at ~1–2% moisture (room-equilibrated). For tight-tolerance machining, dry the stock at 80°C / 4 hr to baseline-dry condition, machine, then either ship dry-packed or allow controlled re-conditioning before inspection. Otherwise, dimensions drift as the part equilibrates to ambient humidity during machining.
-
Post-machining dimensional growth. Even with careful machining, the finished part will absorb moisture in service and grow by ~0.3–0.5%. For metal-replacement parts with tight fits to mating metal parts (which don’t grow), size the PA66 part 0.3% small at manufacture so the as-installed-and-conditioned fit is correct.
Surface finish achievable is ~16 Ra readily. Glass-filled PA66 is much more abrasive — expect tool life ~30% of unfilled, and consider PCD or coated carbide for production runs.
Variant selection guidance
- Extruded PA66 (TECAMID/Sustamid/Zytel 101) — the default for machined parts from stock shape. Best mechanical balance, FDA grades available.
- 30% Glass-Filled (Zytel 70G33 / Ultramid A3WG6) — pick when stiffness or dimensional stability matters more than impact resistance. The standard automotive under-hood PA66.
- Cast Nylon (PA6 cast) — pick for very large or thick parts where extruded stock isn’t economical, or where the slightly higher max- service temperature (~110°C) matters. Common bearing/gear material in industrial machinery.
- MD (MoS₂-filled cast nylon) — bearing and gear grade. Self- lubricating, gray/black color, lower friction than unfilled.
- Oil-Filled cast nylon (Nyloil) — internal oil lubrication for slideways and high-cycle bearings. Migrates oil to wear surface.
- Heat-Stabilized PA66 — high-temperature continuous service in under-hood automotive or electrical equipment up to 150°C.
Failure modes worth designing around
Moisture-driven property shift is the dominant field failure mode for PA66 parts designed by engineers unfamiliar with nylon. A bearing that calculates a 3× safety factor on tensile in the dry state has a 2× safety factor in service after humidity equilibrium. Always design with conditioned properties as the design state.
Acid hydrolysis is the second common failure. PA66’s amide backbone is vulnerable to acid attack — strong acids cause chain scission that embrittles the polymer over months to years. Not for acid pump components.
Hot-water hydrolysis is the longer-term version of the above. PA66 in continuous hot water above ~80°C will degrade over years through slow hydrolysis. For continuous hot water service, specialty PA grades (PA46, PPA, or hydrolysis-stabilized PA66) are the right choice.
Heat-aging embrittlement at sustained 100°C+ in air. Standard PA66 oxidizes and embrittles. Heat-stabilized grades extend service life ~3–5× in this regime.
UV degradation if unprotected. Outdoor PA66 yellows and embrittles within a year. Use heat-stabilized + UV grades with carbon pigment.
Applications by industry
- Industrial machinery — bearings, bushings, gears, sprockets, wear pads. The #1 use space. Replaces bronze and steel in many bearing applications, eliminates external lubrication.
- Automotive — under-hood structural parts (intake manifolds, engine covers, air handling), connector housings, fuel system components, fasteners. GF30 PA66 dominates automotive engineering plastic by volume.
- Electrical and electronics — connector bodies, switches, terminal blocks, cable ties. FR-grade PA66 is the workhorse for UL94 V-2 electrical applications.
- Aerospace — secondary structure, brackets, interior components, cable management. PA66 GF30 and heat-stabilized grades.
- Power tools — housings, gears, structural components. Glass-filled PA66 dominates due to good impact, decent heat resistance, and machinability.
- Food and beverage — gears, wear pads, conveyor parts using FDA grades. POM-C is more common for direct fluid contact; PA66 dominates the bearing and gear positions.
- Hardware and fasteners — cable ties (PA66 dominates this segment), wall anchors, small structural fasteners.