Acetal vs. Nylon: Which Engineering Plastic? (2026)

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.

Section 1 of 5

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.

Section 2 of 5

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. 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	100–120°C	90–110°C	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

Section 3 of 5

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

Excellent

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

Good (with care)

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. Our free DFM review includes material recommendations — upload your CAD file and we'll flag whether your material choice is optimal for the tolerance requirements and service environment before quoting.

Get a CNC Plastic Quote with Free DFM Review

Section 4 of 5

Wear 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

Section 5 of 5

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	Nylon PA6/6 or PEEK	POM maximum continuous service 100°C; nylon PA6/6 up to 120°C
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)

Frequently Asked Questions

Is acetal better than nylon?

Neither material is universally better — it depends on the application. Acetal wins for dimensional stability in humid environments, lower moisture absorption, and stiffer precision sliding parts. Nylon wins for higher impact toughness, better performance at elevated temperatures, and when self-lubrication under load is needed. Identify your primary design driver first.

Does nylon absorb more moisture than acetal?

Yes, significantly. Nylon PA6 absorbs 1.5–3.5% moisture at equilibrium; PA6/6 absorbs 2.5–8% in saturated conditions. Acetal (POM) absorbs less than 0.9% at equilibrium and less than 0.25% in 24 hours. The practical result: nylon parts in humid environments swell dimensionally, while acetal parts remain essentially stable at the same conditions.

Which is better for plastic gears — acetal or nylon?

Acetal is the default choice for most plastic gears. It is stiffer, more dimensionally stable, and has better tooth geometry control in humid environments. Nylon is preferred when impact loads are severe (nylon absorbs shock better), when the gear runs in a lubricated environment (nylon with lubricant can outlast dry acetal), or when operating temperatures exceed 100°C where nylon outperforms POM.

Which machines better — acetal or nylon?

Acetal machines significantly better than most nylon grades. POM produces short, clean chips with no stringiness, holds tighter tolerances (±0.001 in vs ±0.002–0.003 in for nylon), and is less prone to workpiece deflection under cutting forces. Nylon tends to produce long, stringy chips that can jam tooling and has more thermal sensitivity, requiring careful attention to cutting heat.

Can I substitute acetal for nylon in an existing design?

Often yes, but check three things first: (1) Does the nylon part rely on its toughness to absorb impact? Acetal is more brittle in notched impact. (2) Does the mating shaft or contact surface need the self-lubricating properties of nylon with lubricant? (3) Does the operating temperature exceed 100°C? If none of these conditions apply, acetal is typically a direct drop-in with better dimensional performance.

What is the cost difference between acetal and nylon?

Natural acetal rod and plate stock is typically 10–25% more expensive than equivalent nylon PA6/6 stock per pound. However, machining costs are often lower with acetal due to better chip control, tighter tolerances in fewer passes, and reduced scrap rates. The total cost of a finished acetal part is frequently comparable to or lower than the equivalent nylon part.