Guides / Materials
Acetal or Nylon: Choosing Between the Two Default Bearing Plastics
September 21, 2026
Nylon and acetal cover most of the bearing and gear work in machined plastics. They sit at similar prices, machine on the same equipment and appear on the same shortlists. The decision between them comes down to four questions, and moisture answers two of them.
Moisture is the first question
Acetal takes up roughly 0.2 percent water in 24 hours of immersion and under one percent at saturation. Nylon takes up several times that, somewhere between 1.5 and 8.5 percent depending on the grade and on how humid the air is, and it grows as it absorbs.
That growth is real and it moves parts. A nylon bushing machined dry in a conditioned shop and then fitted in a humid plant can lose most of its running clearance. Shops that machine nylon regularly either condition the stock before cutting or machine the bore deliberately oversize and let it close up.
Acetal does not need that management. If your part has a tight bore, a gear mesh or a running fit that has to stay put, acetal is the safer starting point.
Impact and shock are the second
Nylon wins here, and the moisture that causes the dimensional trouble is what makes it win. Absorbed water plasticises nylon and raises its toughness. A nylon roller that gets hit repeatedly survives where an acetal one can crack.
Acetal is notch sensitive. Sharp internal corners, thin ribs and abrupt section changes concentrate stress and start cracks. If your part sees shock loads, either move to nylon or take the radii on the acetal drawing seriously.
Creep and stiffness are the third
Acetal has the higher tensile modulus of the two, and it holds load better over time. A preloaded acetal part relaxes less than a nylon one. For a gear tooth that must keep its profile under a constant torque, or a clamped part that must not settle, acetal is the more predictable choice.
Temperature is the fourth
Nylon carries a higher continuous service temperature than acetal. Acetal is generally held to somewhere around 90 degrees C in continuous service, a little more for short periods. Above that, nylon buys you headroom, and above roughly 120 degrees C neither is the answer and you should be looking at PEEK or PPS.
Side by side
| Question | Acetal (POM) | Nylon (PA6, PA66) |
|---|---|---|
| Water absorption | About 0.2 percent in 24 hours | 1.5 to 8.5 percent depending on grade |
| Dimensional stability | High | Moves with humidity |
| Stiffness | Higher | Lower, drops further when wet |
| Creep under sustained load | Better | Worse |
| Impact and shock | Notch sensitive | Tougher, especially conditioned |
| Continuous service temperature | Around 90 degrees C | Higher |
| Friction against steel, dry | Low and consistent | Low, rises when dry and hot |
| Machined tolerance you can hold | Tighter | Needs a moisture allowance |
Practical rules
Pick acetal when:
- The part runs wet or in a humid plant and the fit matters
- The part is a precision gear, cam or bushing with a tight bore
- Load is held for a long time and the part must not relax
- The tolerance on the drawing is plus or minus 0.05 mm or tighter
Pick nylon when:
- The part takes impact or shock loading
- Compressive load on a bushing outside diameter is high
- The bore tolerance can carry a moisture allowance
- Service temperature sits above what acetal will take
What this means for your RFQ
If you are unsure, say so on the RFQ. A shop that machines both will tell you which one it would run, and it will usually tell you the clearance change that comes with the swap. What you should not do is leave the drawing saying “plastic” and let each shop guess, because the quotes then are not comparable.
If the part has a tight running fit, put the fit and the service environment on the drawing rather than only the nominal dimension. That one line saves more arguments than any tolerance note.
Ready to quote a part? Send your RFQ and we match you with shops that machine both materials. Reply within 24 business hours.
Frequently asked questions
Which is stronger, acetal or nylon?
Dry, acetal is stiffer and holds a dimension better. Nylon that has taken up moisture is tougher and absorbs impact better. Neither number decides a part on its own, because nylon in service is rarely dry and acetal in service rarely changes.
Does nylon really grow that much?
Yes. Depending on grade and humidity, nylon takes up somewhere between 1.5 and 8.5 percent water and grows with it. Acetal takes up roughly 0.2 percent in 24 hours. On a 50 mm bore that difference is the gap between a running fit and a seized one.
Can I swap nylon for acetal on an existing drawing?
Only with a fit review. Acetal is stiffer and less forgiving of shock, and the clearances a nylon part needed for moisture growth will be loose in acetal. Treat it as a redesign of the fits, not a material substitution.
Need a quote for this part?
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