Nylon PA66 vs Acetal POM: Complete Engineering Comparison (2026) — Temperature, Strength, Cost

Which Material Should You Choose?Choose Nylon PA66 if:Higher-load mechanical componentsgears, bearings, and bushings requiring toughness and fatigue resistanceapplications involving impact or vibrationChoose Acetal POM (Polyoxymethylene)...

Nylon PA66 and Acetal POM (Delrin, Celcon) are the two most widely used engineering thermoplastics for mechanical components — gears, bearings, bushings, fasteners, and structural parts. They are similarly priced and both process easily on standard injection molding machines, but they differ fundamentally in how they interact with moisture, their friction and wear characteristics, and their mechanical toughness. The choice between them is typically determined by three factors: moisture sensitivity, toughness requirement, and dimensional precision.

Property Comparison

Tensile Strength (dry as molded): Nylon 66: 82 MPa / Acetal: 62 MPa. Nylon is 32% stronger in the dry condition. However, Nylon absorbs moisture (up to 2.5% at 50% RH equilibrium), which reduces tensile strength to ~55 MPa while increasing toughness. Acetal's moisture absorption is minimal (0.22%), so its properties are stable across humidity ranges. Impact Toughness: Nylon is significantly tougher than Acetal — Nylon 66 notched Izod is 50 J/m (dry) increasing to 100+ J/m when moist; Acetal is 60-70 J/m and does not change with moisture. Nylon's toughness advantage grows in moist or impact-loaded conditions. Friction & Wear: Acetal has a lower coefficient of friction (0.10-0.20 vs 0.20-0.30 for dry Nylon) and superior resistance to creep and fatigue in bearing applications. Acetal is the preferred material for precision gears and unlubricated bearings where dimensional stability and low friction are critical. Nylon develops a beneficial surface film when lubricated, giving excellent bearing performance in greased or oil-lubricated applications. Moisture Absorption: Nylon 66: 1.5% (24h), 2.5% (equilibrium at 50% RH) / Acetal: 0.22% (24h), 0.25% (equilibrium). This is the most consequential difference — Nylon's moisture absorption causes dimensional swelling (~0.7% linear at 2.5% moisture), property changes (toughness increases, strength decreases, modulus drops), and requires moisture conditioning of parts before assembly in precision applications. Acetal's dimensional stability is excellent regardless of humidity. Chemical Resistance: Nylon resists hydrocarbons, oils, and fuels — it is widely used in fuel system components (fuel rails, quick-connect fittings, filter bodies). However, Nylon is attacked by strong acids and oxidizing agents. Acetal resists a wide range of solvents and chemicals but is attacked by strong acids and oxidizers; critically, Acetal is NOT resistant to hot water or steam — it depolymerizes (unzips) in hot water above 60°C, a catastrophic failure mode. HDT at 1.82 MPa: Nylon 66: 75°C (dry) / Acetal: 85°C. Acetal's higher HDT provides a modest advantage in short-term heat resistance, but Nylon 66 GF30 (HDT 245°C) far exceeds Acetal's capability for under-hood applications. Cost: Both materials are in the $4-10/kg range — Nylon 66 is slightly less expensive, but the difference is typically not a deciding factor.

Decision Matrix

Choose Nylon When: (1) Impact toughness or fatigue resistance is critical — gears subject to shock loads, snap-fit assemblies, living hinges; (2) The part contacts fuels, oils, or greases — Nylon is the standard for fuel system components; (3) Higher mechanical loads require the stronger material — structural brackets, lift points, load-bearing bushings; (4) The application temperature exceeds Acetal's capability (>85°C HDT) — with glass fiber reinforcement, Nylon 66 GF30 reaches 245°C HDT; (5) The operating environment allows for or benefits from moisture absorption (e.g., bearings that self-lubricate when moist). Choose Acetal When: (1) Dimensional precision is paramount — precision gears, valve bodies, conveyor components where ±0.05mm tolerances must be maintained regardless of humidity; (2) Low friction in unlubricated conditions is needed — Acetal's 0.10-0.20 friction coefficient makes it the preferred unlubricated bearing material; (3) The part must not change properties or dimensions with moisture exposure — outdoor equipment, plumbing fixtures, precision assemblies; (4) Creep resistance under sustained load is critical — Acetal creeps less than Nylon, maintaining spring-back in snap-fits and clamp force in fasteners; (5) The application does NOT involve hot water or steam — Acetal's catastrophic depolymerization in hot water is an absolute disqualification for plumbing, food processing, or steam applications.

Comparison at a Glance

Equivalents & Cross-References

Related Diagnostics & Materials

References & Industry Standards

• ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
• UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
• MatWeb. Material Property Data for Engineering Thermoplastics. matweb.com
• ISO 1043. Plastics — Symbols and Abbreviated Terms. iso.org