Bottom line first: If your part is in long-term contact with water or demands excellent dimensional stability and low friction, choose POM (polyoxymethylene / acetal). If the part faces high temperatures, impact loads, or needs higher mechanical strength, choose PA66 (nylon 66). Their prices are close — the cost of choosing the wrong material far exceeds the price difference between them.
1. Material Property Comparison Table
| Property | POM (homo/copolymer) | PA66 (Nylon 66) |
|---|---|---|
| Density (g/cm³) | 1.41–1.42 | 1.13–1.15 |
| Tensile strength (MPa, ISO 527) | 60–70 | 75–85 (dry) / 50–60 (conditioned) |
| Flexural modulus (GPa) | 2.6–3.1 | 2.8–3.3 (dry) / 1.2–1.5 (conditioned) |
| Notched impact strength (kJ/m², ISO 179) | 6–9 | 5–7 (dry) / 15–20 (conditioned) |
| HDT @1.8MPa (°C) | 95–110 | 70–90 (dry) / 240+ glass-filled |
| Continuous use temperature (°C) | -40 to 100 | -40 to 120 (dry) |
| Water absorption 24h (%, ISO 62) | 0.2–0.25 | 1.0–1.5 (up to 8% at saturation) |
| Coefficient of friction (vs steel) | 0.20–0.32 | 0.30–0.40 |
| Molding shrinkage (%) | 1.8–2.2 | 0.8–1.5 |
Data source: typical grade values under ISO 527 / ISO 179 / ISO 75 test conditions (e.g., DuPont Delrin 500P, Ascend Vydyne, BASF Ultramid A3K). Always verify against the supplier’s TDS before purchasing.
2. The Essence of the Performance Difference
Moisture absorption is the biggest dividing line. The amide groups in PA66 are hydrophilic: at moisture saturation, tensile strength drops about 30% and dimensions swell 0.5%–2%, while impact toughness increases 2–3x. POM absorbs only 1/5 as much water, keeping dimensions virtually unchanged in humid environments — the core reason precision gears and snap-fits default to POM.
PA66 has the higher thermal ceiling. Glass-filled PA66 (e.g., PA66-GF30) reaches an HDT above 240°C and is widely used around engines. POM loses strength rapidly above 110°C and has a narrow processing window (it decomposes and releases formaldehyde above ~230°C), making it unsuitable for hot environments.
Chemical resistance differs by direction. POM resists organic solvents and fuels well but not strong acids (avoid below pH 4). PA66 resists alkalis and oils, but suffers stress cracking in calcium chloride or zinc chloride solutions — many chassis part failures in de-icing salt environments trace back to this.
3. Application Scenario Analysis
- Choose POM: precision gears, bearing cages, snap-fits and slide rails, plumbing valve parts, food conveyor chain plates, fuel system floats. Keywords: low friction, wear resistance, dimensional stability, water contact.
- Choose PA66: automotive intake manifolds, cooling fans, engine covers, power tool housings, cable ties, rail insulation parts. Keywords: heat resistance, impact resistance, glass-fiber reinforceable.
- Either works: general structural parts and appliance internals — decide by processing and cost.
4. Cost-Benefit Assessment
| Item | POM | PA66 |
|---|---|---|
| 2026 general-grade market price (USD/ton) | 1,550–1,950 | 2,250–2,800 |
| Glass-filled grade (USD/ton) | 1,950–2,400 (GF25) | 2,500–3,200 (GF30) |
| Density impact on cost | 25% denser — more material per part | Lower density — less material per part volume |
| Processing cost | High shrinkage requires precise mold compensation | Requires drying (moisture <0.2%), slightly higher energy |
Note: PA66 pricing is volatile due to adiponitrile supply (it spiked sharply in 2021). For long-term contracts, negotiate raw-material indexation clauses; POM pricing is comparatively stable. On a per-part basis, PA66’s 20% lower density often narrows the real cost gap well below the per-ton difference.
5. Selection Recommendations
- Water/humid environment + precision fit → POM. No hesitation.
- Continuous service above 100°C → PA66 (prefer glass-filled grades).
- Sliding friction parts → POM; for even lower friction, choose PTFE-modified grades.
- Impact or vibration loads → PA66 — its toughness advantage after conditioning is significant.
- Exposure to de-icing salts / inorganic salts → avoid PA66; use POM or PA12 instead.
- Before volume purchase, request TDS and batch COA, retest key properties per ISO 294 specimens, and run a 500–1,000 piece pilot validation for import-substitute grades.
Action advice: List your operating conditions across four dimensions — temperature, humidity, load, and media — then score them against the comparison table above. 80% of selection debates resolve in 10 minutes. When in doubt, request free samples of both materials for side-by-side testing: samples are free, field failures are expensive.
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