Nylon vs. Acetal (POM): Selecting the Right General-Purpose Engineering Polymer
Nylon vs. Acetal: The Two Workhorse Engineering Polymers
Nylon (Polyamide, PA6/PA66) and Acetal (Polyoxymethylene, POM/Delrin) are the two most widely used engineering thermoplastics for mechanical components. Both are strong, tough, and inexpensive relative to high-performance polymers. But their performance profiles diverge in ways that make the selection critical for application success.
This comparison references ASTM International standards and manufacturer data from DuPont/BASF (Nylon) and DuPont/Ticona (Acetal).
Moisture Absorption — The Critical Difference
This is the single most important differentiator:
- Nylon PA66: Absorbs 1.5-2.5% moisture at 50% RH, up to 8-10% when submerged. This causes dimensional swelling (0.5-1.5%) and changes mechanical properties.
- Acetal POM: Absorbs only 0.2-0.3% moisture. Dimensional stability in humid/wet environments is excellent.
For precision components in wet environments — pump parts, valve bodies, food processing equipment — Acetal's moisture stability makes it the clear choice. Nylon's moisture absorption can be managed with glass-fiber reinforcement, but the fundamental difference remains.
Mechanical Properties
| Property | Nylon PA66 (dry) | Nylon PA66 (50% RH) | Acetal POM-H |
|---|---|---|---|
| Tensile Strength (MPa) | 82 | 55 | 70 |
| Flexural Modulus (GPa) | 2.8 | 1.2 | 2.8 |
| Notched Izod (J/m) | 55 | 160 | 75 |
| Fatigue Endurance (MPa, 10⁷ cycles) | 20 | — | 35 |
Key observations: Dry Nylon is stronger than Acetal, but conditioned Nylon loses significant stiffness. Acetal's fatigue endurance (35 MPa) is 75% higher than Nylon's (20 MPa) — critical for snap-fits, living hinges, and cyclically loaded components.
Chemical and Environmental Resistance
- Hydrocarbons: Both excellent
- Solvents: Nylon attacked by phenols and formic acid; Acetal attacked by strong acids and oxidizers
- UV resistance: Both degrade without stabilization; Acetal slightly better
- Creep resistance: Acetal significantly better than Nylon under sustained load
Processing and Cost
Both materials are commodity-priced and easily injection molded. Nylon requires drying before processing (moisture causes splay and degradation). Acetal processes easily from dry-as-molded condition. Acetal homopolymer (Delrin) is 20-40% more expensive than Nylon PA66.
Recommendations
- Gears and bearings (dry): Acetal — lower friction, better fatigue, no moisture swelling
- Gears and bearings (wet/underwater): Acetal — dimensional stability in water
- Snap-fit fasteners: Acetal — superior fatigue endurance
- Food contact components: Both FDA-compliant; Acetal for wet environments, Nylon for dry
- Impact-resistant housings: Nylon (conditioned) — higher impact toughness when moisture-plasticized
- High-temperature automotive: Nylon PA66 GF30 — heat-stabilized grades to 180°C+
- Living hinges: Acetal — can flex millions of cycles without failure
References & Industry Standards
- ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
- ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
- National Institute of Standards and Technology (NIST). Polymer Properties Database. nist.gov
- UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
- MatWeb — Material Property Data. matweb.com