Acetal vs Nylon: Precision vs. Toughness Compared

Acetal (POM) and nylon (PA) are the two most widely machined engineering thermoplastics in North American industrial shops. They share similar stiffness and strength on paper, but behave very differently in real parts: acetal stays dimensionally stable in wet or humid environments while nylon absorbs 2–9% moisture by weight, swelling enough to shift a close-tolerance bore. Nylon, in turn, outperforms acetal in impact resistance and operates at higher continuous temperatures. Picking the wrong material means out-of-tolerance parts or premature fatigue failure.

TL;DR

Moisture: Acetal absorbs ~0.2% moisture (24 hr); nylon PA6 absorbs 1.6–1.8% in 24 hours and 2–3% at equilibrium, with nylon 6/10 and 6/12 lower (~1%), and PA6 in water-saturated conditions reaching up to 9%.
Dimensional stability: Acetal holds tighter tolerances in humid or wet service — the clear choice for precision gears and close-clearance bearings.
Impact resistance: Nylon absorbs more impact energy; acetal is stiffer but more notch-sensitive.
Temperature: Nylon runs to 210–250°F (99–121°C) continuous depending on grade; acetal is rated to ~180–185°F (82–85°C).
Friction: Acetal has a lower coefficient of friction (~0.20–0.35) than standard nylon (~0.30–0.45) unlubricated; both improve with lubrication.
Chemical resistance: Acetal resists weak acids and most organics; nylon is vulnerable to strong acids and phenols.
Cost: Comparable at standard grades; specialty filled grades vary widely.

Comparison Table

When to Choose Acetal

Precision Machined Parts in Humid Environments

Acetal's 0.2% moisture absorption is roughly one-eighth of nylon PA6's equilibrium uptake in water. For close-tolerance bores, gears, and valve spools that see water, steam, or high humidity, acetal holds dimensions that nylon cannot. A ±0.001″ tolerance bore that works fine in acetal may loosen by several thousandths in nylon once equilibrated — enough to cause gear noise or bearing play.

High-Cycle Fatigue Applications

Acetal (especially homopolymer Delrin) has outstanding fatigue resistance — among the best of any unfilled thermoplastic. Snap-fit clips, cam followers, and flex-cycling components that operate millions of cycles favor acetal over standard nylon grades.

Low-Friction Dry Running

Acetal's lower dry coefficient of friction and its tendency to generate a thin self-lubricating film make it the preferred choice for unlubricated gears, cams, and bushings. In applications where re-lubrication is impractical — food machinery washed frequently, for example — acetal outperforms unlubricated nylon.

Electrical Insulation with Dimensional Stability

Acetal's moisture stability preserves its dielectric properties in variable humidity. For precision electrical insulators, connector housings, and coil forms that need consistent dimensions and insulation values, acetal is more predictable than nylon.

When to Choose Nylon

Impact-Resistant Structural Parts

Nylon absorbs impact energy through a combination of ductility and moisture-plasticized toughness. In applications such as cable clamps, brackets, and pump housings that must survive drop impacts or shock loads, nylon's higher impact resistance is a meaningful advantage. At elevated moisture content, nylon becomes even tougher — a beneficial effect when the service environment is wet.

Higher Temperature Service

Nylon 6/6 continuous-use ratings reach 210–220°F (99–104°C); specialty grades (PA6/10, PA12) extend further. Acetal is limited to 180°F (82°C) continuous. Automotive under-hood components, hot-water plumbing fittings, and industrial oven conveyor parts running above 185°F require nylon rather than acetal.

Structural Load-Bearing with Steel Contact

Nylon's compressive strength and ability to work against harder counterfaces (steel shafts, racks) without galling is well established. In heavy-load bushings, wear pads, and rollers where the mating surface is steel, nylon's surface behavior often equals or exceeds acetal's. The moisture absorption that hurts dimensional precision actually improves toughness and impact resistance in these applications.

Wide Range of Specialty Grades

Nylon offers broader grade variety: PA6, PA6/6, PA6/10, PA6/12, PA11, PA12, cast nylon, and numerous filled variants (MoS₂, glass-filled, carbon-filled). Cast nylon grades can produce very large billets at lower cost than acetal for large-section wear parts. If you need thick sections or large-diameter rod, cast nylon is often more economical than acetal.

Specs Head-to-Head

Moisture and Dimensional Stability

This is the defining difference. Acetal POM has equilibrium moisture absorption around 0.8% in ambient air. Nylon PA6/6 equilibrates at 3% in ambient conditions and up to 8–9% when immersed in water. Each 1% of moisture absorbed causes dimensional expansion of roughly 0.2–0.3% in nylon — significant in close-tolerance assemblies. For a 1.000″ bore in nylon submerged in water for an extended period, you may see the bore grow to 1.016–1.027″. Acetal's bore would remain within 0.003″ of nominal. If your part tolerances are tighter than ±0.005″ and the environment is humid or wet, acetal is the correct choice.

Mechanical Properties

Dry mechanical properties are similar — nylon PA6/6 is slightly stronger in tensile (11,000–12,400 psi vs 9,500–10,000 psi for acetal) but the acetal advantage in fatigue endurance limits often matters more in cyclic applications. Acetal is stiffer under dry conditions; moisture-saturated nylon loses stiffness measurably (flexural modulus can drop 30–40% at saturation). Acetal's notched impact strength is moderate (1.2–2.3 ft-lb/in); nylon can reach 3.5 ft-lb/in and higher in toughened grades.

Thermal Performance

Nylon 6/6's continuous service rating of 210°F (99°C) is about 30°F above acetal's practical ceiling. Heat deflection temperatures follow the same pattern (275°F for PA6/6 vs 230°F for acetal at 264 psi). In applications where sterilization cycles, hot wash-down, or proximity to heated components is involved, nylon's thermal advantage is real and often decisive.

Chemical Resistance

Both materials handle water, oils, aliphatic hydrocarbons, and weak alkalis. Acetal has better resistance to weak acids; nylon is attacked by strong acids (including dilute hydrochloric and sulfuric at elevated temperatures) and phenols. Acetal is also attacked by oxidizing acids and chlorinated hydrocarbons. Neither is suitable for aggressive chemical service — consider PTFE or UHMW for harsh chemistry.

Machinability

Both materials machine well with standard carbide tooling. Acetal produces tighter tolerance parts without the risk of moisture-driven dimensional change between machining and installation. Nylon requires consideration of moisture conditioning if close tolerances are needed in wet service — parts are sometimes pre-soaked to achieve equilibrium dimensions before final machining.

Cost & Availability

Acetal and standard nylon (PA6, PA6/6) are priced similarly at commodity grades — roughly $2–5 per pound for extruded rod and sheet. Specialty nylons (PA12, oil-filled, glass-filled) carry premiums. Large-section cast nylon can be significantly cheaper per pound than acetal for billets above 6″ diameter. Both materials are stocked in a wide range of rod, sheet, and tube sizes, and are available in FDA-compliant grades.

Common Alternatives

UHMW polyethylene — Better abrasion resistance and lower friction than either; limited to 180°F; low stiffness. See acetal vs UHMW.
PET-P (Ertalyte) — Better fatigue resistance and creep resistance than acetal or nylon at elevated temperature; higher cost. See acetal vs PET-P.
PEEK — When temperature exceeds 250°F and chemical exposure is severe; 5–10× cost premium.

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