Acetal vs Nylon — the mechanical plastic choice.

Key observations:

• Similar strength and stiffness — for most mechanical work they're interchangeable
• Acetal absorbs 10× less water (major factor)
• Nylon is 1/5 the cost (major factor)
• Acetal has lower friction and better wear (matters for bearings, gears)

This is the biggest practical difference between the two:

Acetal: absorbs 0.8% water at saturation. Dimensional change ~0.5%. Mechanical properties barely affected.

Nylon: absorbs 8-10% water at saturation. Dimensional change 2-3%. Strength drops 40-50%. Modulus drops 50%+.

For precision mechanical parts (tight tolerances, consistent properties in variable humidity), acetal is strongly preferred. For parts where dimensional drift is irrelevant or service is always dry, nylon wins on cost.

Parts that should not be nylon:

• Precision instrument components (optics, measurement devices)
• Fuel-contact parts (some fuel chemistry hydrolyzes nylon)
• Marine or high-humidity service without protective coating
• Anywhere dimensional tolerance below ±0.1 mm is required

For sliding applications, acetal beats nylon:

Coefficient of friction vs steel: acetal 0.20, nylon 0.28. Small but consistent difference.

Wear rate: acetal wears 2-3× slower than nylon against steel in dry sliding.

Self-lubricating behavior: both are self-lubricating but acetal is better — lower torque, quieter operation, longer life.

For gears, bearings, bushings, cam followers: acetal is the default unless there's a cost-driven reason for nylon. Nylon with molybdenum disulfide or PTFE fill (Nylatron grades) closes the gap but costs more than plain acetal.

At 5-7× the cost per kg, acetal doesn't make sense for many applications where nylon works:

• Non-critical mechanical parts (handles, spacers, brackets under moderate load)
• High-volume injection molded parts where material cost dominates
• Parts that will be replaced periodically anyway (wear parts in maintenance cycles)
• Coated or painted surfaces where water absorption can be mitigated

For machined parts (where material cost is a smaller fraction of total part cost), the nylon-to-acetal premium matters less. For injection-molded parts in high volumes, the 5-7× material cost delta is significant.