Nylon vs Delrin: Machinability, Moisture Absorption & Dimensional Stability
Nylon (polyamide) and Delrin (acetal/POM) are the two engineering plastics that come up most often when an OEM part is moving off metal. They look similar on a datasheet and behave very differently on a machine and in service. The short version: Delrin wins on dimensional stability and machinability, nylon wins on impact toughness and abrasion. Olympus Machining is an ITAR-registered precision CNC machining shop in Hanover, Pennsylvania that machines both.
Property Comparison
| Property | Delrin (Acetal / POM-H) | Nylon 6/6 (PA66) |
|---|---|---|
| Tensile strength | ~10,000 psi (70 MPa) | ~11,000 psi (76 MPa) dry, ~8,000 psi conditioned |
| Water absorption (24 hr) | ~0.2% | ~1.2–1.5% (up to 6–8% saturated) |
| Dimensional change, humid service | ~0.2% max | 0.5–2% growth |
| Continuous service temp | ~180°F (82°C) | ~210°F (99°C) |
| Coefficient of friction (dry, vs steel) | ~0.20 | ~0.25–0.30 |
| Impact / notch toughness | Moderate, notch-sensitive | High, ductile |
| Machinability | Excellent — brass-like | Fair — elastic, burr-prone |
| Practical machined tolerance | ±0.001–0.002″ | ±0.005″ typical, tighter with stabilized stock |
| Chemical resistance | Good to fuels/solvents; poor to strong acids | Good to oils/greases; attacked by acids and phenols |
| Relative material cost | Baseline | Similar; cast nylon slightly higher |
Values are typical published ranges for unfilled grades. Filled and cast grades (MoS2 nylon, glass-filled PA66, Acetron POM-C) shift these numbers — send the specified grade with your drawing.
Moisture Absorption Is the Deciding Factor
Nylon is hygroscopic. A part machined to print at 20% relative humidity can gain 0.5–2% in linear dimension after weeks in a humid plant or submerged service, and stiffness drops as it conditions. On a 4-inch bore that is up to 0.080 inch of growth — enough to close a running clearance or bind a bearing fit. This is also why inspecting a nylon part is a conditioned-state question, not just a CMM question.
Delrin absorbs about 0.2% and effectively stops there. That is why acetal is the default when a plastic part has to hold a press fit, a gear tooth profile, a seal groove, or a bearing clearance across changing environments. If your drawing has geometric tolerances under 0.005 inch, specify Delrin unless a nylon-specific property forces the choice.
Machinability on the Shop Floor
- Delrin: sharp polished carbide, high positive rake, 500–1,000 SFM, air blast or light flood. Clean chips, minimal burrs, Ra 0.8 µm as-machined.
- Nylon: same tool geometry but lighter depth of cut and higher feed to prevent rubbing. The material's elasticity pushes away from the cutter, so finishing passes are spring passes and edges need deburring.
- Heat: both melt rather than gall. Neither tolerates a dull tool — smeared edges and dimensional drift show up immediately.
- Stress relief: thick extruded stock in either material holds residual stress. We rough oversize, allow it to relax, then finish — standard practice on sections over about 2 inches.
- Workholding: soft jaws and vacuum or full-support fixturing; plastics deflect under clamp pressure that a metal part would ignore.
Cast nylon (Nylon 6, MC901) machines noticeably better than extruded Nylon 6/6 — lower internal stress, more predictable finish — and is the grade we recommend when a part must be nylon but the tolerances are still meaningful.
Application-Specific Trade-Offs
Specify Delrin when…
Gears and gear racks, precision bushings and sleeve bearings, seal glands, valve and manifold bodies, electrical insulators, snap fits, fluid-handling fittings, and any part with tolerances tighter than ±0.005 inch or a required press fit.
Specify nylon when…
Wear pads and slide plates, sheaves and rollers, sprockets, impact bumpers and guards, cable and chain guides, and high-load low-speed bearings where shock loading would crack notch-sensitive acetal.
Neither — go to PEEK
Continuous service above 210°F, autoclave cycling, aggressive chemical exposure, or implant-adjacent medical work. See our PEEK CNC machining page.
Neither — go to UHMW
Abrasive sliding contact, food-line wear strips, and impact at low temperature where the lowest coefficient of friction matters more than tolerance.
Design Notes for Either Material
- • Call out the specific grade (Delrin 150, Acetron POM-C, Nylon 6/6, MC901 cast nylon) — substitutions change fit.
- • State whether tolerances apply as-machined or in the conditioned/in-service state for nylon parts.
- • Keep wall sections above 0.060 inch and avoid sharp internal corners; both materials are notch-sensitive under load.
- • For threads under load, prefer coarse threads or threaded metal inserts over fine threads cut directly in plastic.
- • Plan for thermal expansion roughly 8–10× that of steel when a plastic part is captured in a metal assembly.
Not Sure Which One Your Part Needs?
Send a drawing or STEP file with the service environment — temperature, humidity, load, and mating material. A CAM engineer reviews every quote and will flag where a material change tightens your achievable tolerance or lowers cost.
Submit a DrawingFrequently Asked Questions
Is Delrin or nylon better for tight-tolerance CNC parts?
Delrin (acetal/POM). It absorbs roughly 0.2% water in 24 hours versus 1.5–3% for Nylon 6/6, so machined dimensions stay stable. Delrin holds ±0.002 inch on milled features and ±0.001 inch on turned features; unstabilized nylon can grow 0.5–2% in service as it takes on moisture.
Which machines better, nylon or Delrin?
Delrin machines better. It cuts like brass with low cutting forces, clean chip formation, and a fine as-machined finish. Nylon is tougher and more elastic, so it deflects under tool pressure, tends to leave burrs and stringy chips, and usually needs sharp positive-rake tooling, light finishing passes, and supportive workholding.
When should I specify nylon instead of Delrin?
Choose nylon when impact toughness, abrasion resistance, or higher continuous service temperature matters more than dimensional precision — for example wear pads, sheaves, rollers, sprockets, and structural bumpers. Cast Nylon 6 (MC901) and MoS2-filled grades also machine more stably than extruded Nylon 6/6.
Is acetal the same as polyamide?
No. Acetal is polyoxymethylene (POM) — sold as Delrin (homopolymer) or Acetron/Celcon (copolymer). Polyamide is the chemical family sold as nylon (PA6, PA6/6, PA12). They are different polymers with different moisture behavior, stiffness, and machining characteristics.
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Last reviewed: May 12, 2026