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Should I use nylon or acetal?
Use acetal for precision gears, bushings and snap fits in changing humidity: it absorbs about 0.2% moisture at 50% RH, so dimensions hold. Use nylon 66 when parts see higher heat, impact or abrasion: it melts at 500 °F (262 °C) versus about 330 °F (166 °C) for acetal and is tougher, but it absorbs about 2.5% water and can grow 0.5–0.7%.
Nylon 66 (PA66)
Acetal (POM)
By the numbers
Priced on the same representative part: housing, injection molding, 10,000 pcs.
Tensile strengthMPa
PA66
83
POM
62
Densityg/cm³
PA66
1.14
POM
1.41
Max service temp°C
PA66
100
POM
100
Raw material cost$/kg
PA66
3.8
POM
3
Which should you pick?
Pick PA66 when
- Heat approaches solder or under-hood temperatures
- Parts see impact, shock or abrasion
- The part is a cable tie, clip or tough fastener
- Glass-filled grades may be needed later for stiffness
Pick POM when
- Tight tolerances must hold in humid or wet service
- Low friction against steel or itself matters
- Parts are machined on screw machines or CNC to tight tolerances
- Springs, clips and snap fits need high fatigue resistance
Key differences
| PA66 | POM | |
|---|---|---|
| Moisture absorption | About 2.5% at 50% RH; up to about 8% saturated | About 0.2% at 50% RH |
| Effect of moisture | Parts grow 0.5–0.7%; stiffness can drop by more than half | Negligible |
| Design data | Design with conditioned properties, not dry-as-molded | Published dry data is representative |
| Chemical weak spots | Strong acids, phenols and zinc chloride | Strong acids, oxidizers and sustained chlorinated water |
| Flame rating | V-2 and V-0 grades available | UL 94 HB in standard grades |
| Bonding | Bondable with surface prep | Very difficult to bond or paint |
Questions
Is Delrin better than nylon?
For precision, yes; for toughness and heat, no. Delrin is a brand of acetal homopolymer: stiffer, lower in friction and nearly unaffected by humidity. Nylon 66 is tougher, handles higher temperatures and resists abrasion better.
Which is better for gears, nylon or acetal?
Acetal for precision gear trains in varying humidity, since it holds tooth dimensions and runs with low friction. Nylon for gears that see shock loads or heat, where toughness and quiet running matter more than exact backlash.
Which is stronger, nylon 66 or acetal?
Dry as molded, nylon 66 runs about 12,000 psi (83 MPa) tensile versus about 9,000 psi (62 MPa) for acetal copolymer. Conditioned at 50% RH, nylon loses much of that strength and stiffness, so the practical gap is smaller.
Try both on your part.
The builder prices the same geometry in every suitable process and material, with DFM for each.