Acetal vs. Nylon: Which Engineering Plastic Should You Choose?
The most frequent material decision for plastic CNC parts. Moisture absorption, dimensional stability, machinability, and a complete decision matrix.
The Single Biggest Difference: Moisture Absorption
If your part needs to hold tight tolerances in humidity, this is the single most important factor in choosing between acetal and nylon. Acetal and nylon are similar in many properties — but the moisture absorption gap is enormous and consequential. Nylon absorbs 3–9× more moisture than acetal. In humid environments, nylon bushings, gears, and precision fits swell and bind. Acetal parts stay put. If your part lives in a humid or wet environment and needs to maintain dimensional tolerances, this single data point should drive your selection.
Moisture Absorption and Dimensional Stability
If your part must maintain dimensional tolerances in a humid or wet environment, moisture absorption is the number you need to check first.
This is the decisive factor in most acetal-vs-nylon decisions. Moisture absorption is not cosmetic — it directly changes part dimensions, loosening bearing fits and changing gear mesh clearance over time.
| Material | 24-hr Immersion | Equilibrium (50% RH) | Equilibrium (saturation) | Dimensional Impact |
|---|---|---|---|---|
| Acetal POM-H (Delrin) | 0.25% | 0.5% | 0.9% | Negligible — precision fits remain stable |
| Acetal POM-C (copolymer) | 0.22% | 0.45% | 0.80% | Negligible — precision fits remain stable |
| Nylon PA12 | 0.25% | 0.70% | 1.60% | Low — lowest-absorbing nylon grade — most dimensionally stable |
| Nylon PA6/6 (dry) | 0.30% | 1.50% | 8.0% | Moderate — bearing fits may loosen in dry; tighten in wet |
| Nylon PA6 (dry) | 1.30% | 2.80% | 8.50% | High — significant swell in humid conditions; avoid for precision fits |
| Nylon PA6 (filled, MoS₂) | 1.00% | 2.20% | 6.50% | Reduced vs unfilled PA6 but still much higher than acetal |
What Moisture Absorption Means for Your Part
A 50 mm nylon PA6 bushing absorbs 8.5% moisture by weight at saturation (ASTM D570), but linear dimensional swell is not 1:1 with weight gain — PA6 swells roughly 1.5–2.5% linearly, which on a 50 mm bore means approximately 0.75–1.25 mm of growth distributed across bore and OD. A precision H7/f6 running fit with a 0.025–0.075 mm clearance band will close completely and potentially seize. The same bushing in acetal absorbs only 0.2% at 50% RH equilibrium, translating to roughly 0.1–0.2% linear swell — about 0.05–0.10 mm on that 50 mm bore — well within fit tolerance. For any precision bore, shaft, or gear that must hold dimensional tolerances in service, acetal is the correct default choice over PA6 or PA6/6.
Full Mechanical Properties Comparison
Before you finalize your material call-out, compare the actual numbers — nylon's mechanical advantage disappears once it absorbs moisture in service.
Values for dry-as-molded (DAM) condition at 23°C (73°F). Nylon properties change substantially with moisture content — the "conditioned" (wet) properties of nylon are notably lower.
| Property | Acetal POM-H | Nylon PA6/6 (DAM) | Nylon PA6 (DAM) | Winner |
|---|---|---|---|---|
| Tensile Strength (UTS) | 69 MPa | 83 MPa | 75 MPa | Nylon (DAM) — POM wins in wet conditions |
| Tensile Modulus (stiffness) | 3,100 MPa | 3,300 MPa | 2,900 MPa | Essentially equal — both lose stiffness wet |
| Elongation at Break | 40–75% | 30–60% | 50–200% | Depends on grade |
| Flexural Modulus | 2,900 MPa | 3,200 MPa | 2,700 MPa | Essentially equal |
| Notched Izod Impact | 75–130 J/m | 55–130 J/m | 50–100 J/m | Nylon PA12 and toughened grades win |
| Hardness (Rockwell M) | M80 | M79–M85 | M70–M80 | Essentially equal |
| Compressive Strength | 127 MPa | 100 MPa | 87 MPa | Acetal |
| Continuous Service Temp | 90–100°C (194–212°F) | 100–120°C (212–248°F) | 90–110°C (194–230°F) | Nylon at elevated temperature |
| Coefficient of Friction (dry, vs steel) | 0.20–0.35 | 0.25–0.45 | 0.30–0.45 | Acetal (dry) — nylon wins with lubricant |
| Density | 1.42 g/cm³ | 1.14 g/cm³ | 1.13 g/cm³ | Nylon (lighter) |
| Moisture Absorption (equil.) | 0.9% | 2.5% | 8.5% | Acetal — significant advantage |
Machinability Comparison
If you're sourcing CNC plastic parts, the machinability gap between acetal and nylon directly impacts your quote price and achievable tolerances.
For CNC-machined parts, machinability directly affects quote cost and achievable tolerances. Acetal has a meaningful machinability advantage over most nylon grades.
Acetal (POM) Machinability
- Short, clean chips — no stringiness at standard feeds and speeds
- Tight tolerances: ±0.001 in routinely achievable on good lathe/mill setups
- Low thermal sensitivity — moderate heat generation, sharp tools required
- Excellent surface finish without specialized tooling
- No moisture conditioning required before machining
- Low spring-back in thin-wall sections vs. nylon
Nylon (PA6, PA6/6) Machinability
- Stringy, long chips that require careful chip management at bores and slots
- Wider tolerances typical: ±0.002–0.003 in due to workpiece compliance
- Thermally sensitive — high cutting speeds generate heat and smear surface
- Moisture-conditioned nylon machines differently than dry nylon (softer)
- Requires moisture sealing (oil immersion) before tight-tolerance machining
- Tapping nylon threads requires larger pilot holes — nylon swells into threads
Not Sure Which Plastic Fits Your Application?
MakerStage machines both acetal and nylon grades for CNC precision parts. Upload your CAD file and the quote review can flag whether the material choice fits the tolerance requirements and service environment before production.
Get a Plastic Parts QuoteWear Resistance and Friction
If your design includes a sliding or bearing surface, whether the contact runs dry or lubricated determines which material gives you better wear life.
Both materials are used for sliding and bearing applications, but they behave differently depending on whether the contact is dry or lubricated.
Dry Sliding — Acetal Wins
In dry sliding applications (no external lubricant), acetal has a lower coefficient of friction against steel (0.20–0.35) than unfilled nylon (0.30–0.45). POM also develops a thin transfer film on the mating surface that reduces friction over time.
- POM dry CoF vs steel: 0.20–0.35
- PA6/6 dry CoF vs steel: 0.30–0.45
- PA6 dry CoF vs steel: 0.35–0.50
- POM Taber wear index: 6–12 mg/1000 cycles (H18, 1 kg)
- PA6/6 Taber wear index: 8–15 mg/1000 cycles
Lubricated — Nylon Can Outperform
In lubricated sliding (grease or oil present), MoS₂-filled or oil-impregnated nylon grades can match or exceed acetal wear performance. The lubricant bridges the CoF gap. In gearboxes with splash lubrication, nylon gears can run cooler and quieter than acetal.
- MoS₂-filled nylon CoF vs steel: 0.10–0.20 (lubricated)
- Nylon self-lubricating grades: PA6 + 18% MoS₂
- Lubricated nylon wear rate: lower than dry acetal in some tests
- Acetal outperforms in: dry, intermittent, high-load cycles
- Nylon outperforms in: continuous lubricated, high-speed, lower load
Application Decision Matrix
Your material choice should be driven by your specific service conditions, not habit — use this matrix to validate the call before locking your drawing.
Use this matrix to pick the right material. Default to acetal for precision sliding parts; switch to nylon when impact toughness or elevated temperature is the driver.
| Application / Requirement | Recommended | Rationale |
|---|---|---|
| Precision bushing in humid environment | Acetal POM | Dimensional stability — acetal absorbs <0.9% vs 2.5–8% for PA6/6 |
| Plastic gear (dry running) | Acetal POM | Better dimensional stability, lower dry CoF, cleaner tooth profile in humidity |
| Plastic gear (splash lubricated) | Nylon PA6/6 or PA12 | Lubrication bridges CoF gap; nylon higher impact toughness protects teeth |
| High-impact structural bracket | Nylon PA6/6 or PA12 | Nylon notched Izod can reach 1,000 J/m in toughened grades; acetal ~130 J/m max |
| Precision jig or fixture | Acetal POM | Dimensional stability critical; acetal holds tolerances across humidity cycles |
| Food contact part (natural grade) | Acetal POM | FDA-compliant natural acetal; simpler compliance than nylon FDA certification |
| Cam follower or roller (dry) | Acetal POM | Better wear resistance and lower CoF in dry rolling/sliding contact |
| Continuous service > 120°C (248°F) | Nylon PA6/6 or PEEK | POM maximum continuous service 100°C (212°F); nylon PA6/6 up to 120°C (248°F) |
| Snap-fit or living hinge | Nylon PA12 | Higher elongation and fatigue resistance for repeated flex cycles |
| General structural plastic part (low humidity) | Either — cost decides | Both suitable; acetal machines better; nylon lighter (1.13 vs 1.42 g/cm³) |
Further Reading
- What Is Acetal (POM/Delrin)? Complete Engineer's Guide — properties, grades, chemical resistance, and machinability.
- Acetal vs. PEEK: When Is PEEK Worth the Cost? — upgrade path for high-temperature and aggressive chemical environments.
- CNC Machining Acetal (POM/Delrin): Speeds, Feeds, and Design Rules — full machining reference with DFM checklist.
- Material Selection Guide — broader framework covering metals, plastics, and composites.
Frequently Asked Questions
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