Delrin vs Nylon for Machined Parts: The Moisture Decides
Strength, machinability, wear, temperature, and cost — plus where PEEK fits. The engineering-plastics decision, made properly.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 8 min read
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The 30-Second Answer
Delrin is DuPont's brand of acetal homopolymer (POM-H); nylon is the polyamide family, usually PA6 or PA66 for machined parts. Mechanically they are neighbors — but one behaves like a metal and the other behaves like a sponge. Choose Delrin when tolerances, low friction, and dimensional stability matter: it absorbs almost no water (~0.2% in 24 hours) and machines to size predictably. Choose nylon for toughness and abrasion resistance in dry environments — and accept that the part will absorb moisture and grow. Everything else follows from that single difference.
The Numbers, Side by Side
| Property | Delrin (POM-H) | Nylon 6/6 | PEEK |
|---|---|---|---|
| Tensile strength | ~10,000–11,000 psi | ~12,000 psi (dry) | 100–115 MPa (14,500–16,700 psi) |
| Flexural modulus | ~420,000–450,000 psi | ~410,000 psi (dry) | ~600,000 psi |
| Moisture absorption, 24 h | ~0.2–0.25% | ~1.2–1.5% | ~0.05–0.2% |
| Moisture at saturation | ~0.8–0.9% | up to 8–9% | very low |
| Friction coefficient | ~0.20–0.21 | low, humidity-dependent | very low |
| Machinability | Excellent, holds size | Good, size drifts with moisture | Good, rigid and abrasive on tools |
| Material cost (approx.) | $5–15/kg | similar to Delrin | $90–400/kg |
Read the moisture rows twice — they drive everything else in this comparison. PEEK is included only as a reference point for what the next tier up costs and buys.
Moisture: The Difference That Actually Decides
Nylon is hygroscopic: PA6 and PA66 absorb up to 8–9% of their weight in water at saturation, and even a normal 50%-humidity room pulls them to 2–3% moisture content. Water acts as a plasticizer — the material gets tougher but softer and, critically for machined parts, it grows: roughly 0.5–0.6% dimensionally at 50% RH. On a 50 mm bore that is a quarter-millimeter of drift — enough to turn a precision fit into a bind or a rattle.
Delrin absorbs about 0.2% in 24 hours and under 1% at saturation — dimensionally, it does not move. This is why Delrin owns the precision-plastic-parts niche: gears that keep their mesh, bushings that keep their clearance, insulators that keep their dimensions from the day they are machined to the day they are scrapped. If your nylon part must hold a tolerance in service, either design the moisture growth into the fit or change the material.
Machinability: Both Good, One Predictable
Both are genuinely CNC-friendly — free-cutting, kind to tools, no coolant drama. Delrin machines with crisp chips and, more importantly, holds the size you cut: what measures 25.00 mm off the machine measures 25.00 mm at the customer. Nylon machines well too, but its dimensions ride the humidity between your inspection report and the assembly line. For tight-tolerance turning and milling, Delrin is simply the lower-risk material. We run both daily — see our Delrin machining and nylon machining pages for the process detail.
Friction, Wear, and the 4:1 Rule
Delrin runs a friction coefficient around 0.20–0.21 with good bearing behavior out of the box — the default for gears, bushings, sliders, and low-load bearings. Nylon fights back on toughness and abrasion resistance in dry service. The tiebreaker is moisture: wear data puts acetal bearings at roughly 4:1 life over nylon in high-moisture conditions, because wet nylon swells and softens while Delrin ignores the weather. Dry and abusive: nylon has a case. Wet, humid, or washdown: Delrin, no contest.
Temperature and Chemicals
Both are moderate-temperature materials — comfortable in the sub-100°C range for continuous service, with grade-dependent headroom beyond that. For chemicals, Delrin shrugs off most solvents, fuels, and oils but dislikes strong acids, strong bases, and UV exposure (stabilized grades exist for outdoor use). Nylon handles hydrocarbons well but its moisture behavior compounds in wet chemical environments. Either way: list the actual chemistry on the RFQ and we will confirm compatibility before cutting — substitution guesses are how plastic parts fail quietly.
Where PEEK Fits
PEEK is the answer when neither Delrin nor nylon survives: continuous heat well past their range, steam, aggressive chemicals, or real structural loads (100–115 MPa tensile, melting point 343°C, moisture uptake near zero). The price is the filter — roughly $90–400/kg against $5–15/kg for Delrin, plus more demanding machining. Our rule on quotes: if Delrin survives the environment, PEEK is a luxury; if it does not, PEEK is cheap compared to a field failure. Details on our PEEK machining page. When you do step up to it, our PEEK machining guide covers the grades and the annealing rule.
For the full Delrin-to-PEEK decision, see our PEEK vs Delrin comparison.
Selection Rules We Apply on RFQs
Tight tolerances, gears, bushings, anything that must hold size? Delrin. The dimensionally stable default.
Dry environment, impact or abrasion, cost-sensitive wear part? Nylon — conditioned before machining, with moisture growth designed into the fits.
Washdown, marine, outdoor humidity? Delrin. The 4:1 wet-wear rule is not subtle.
Strong acids, bases, or permanent UV? Neither, without a conversation — flag it on the RFQ.
Heat, steam, aggressive chemistry, or structural loads? Step up to PEEK and accept the price, or talk to us about filled grades in between.
FAQ
Is Delrin stronger than nylon?
Not quite — dry nylon 6/6 actually posts a slightly higher tensile strength (around 12,000 psi vs 10,000–11,000 psi for Delrin), and conditioned nylon is tougher on impact. The catch is the word “dry”: nylon’s properties are moisture-dependent, while Delrin's are stable. In a humid environment the comparison shifts toward Delrin, which keeps its dimensions and properties regardless of the weather.
Which is better for gears and bushings, Delrin or nylon?
Both are classic bearing materials, and in dry service either works. Delrin's edge is consistency: a friction coefficient around 0.20–0.21 and negligible moisture uptake mean the bore you machined is the bore that ships a year later. Industry wear data puts acetal bearings at roughly 4:1 life over nylon in high-moisture conditions, because nylon swells and softens as it absorbs water. Wet or humidity-cycling environment: Delrin. Dry, impact-heavy, abrasive environment: nylon can outlast it.
Can nylon be machined to tight tolerances?
It machines well on the day — the problem is the week after. Nylon absorbs moisture from the air and grows (about 0.5–0.6% dimensionally at 50% relative humidity), which is enough to open up a press fit or bind a sliding one. If the part has close-tolerance features and lives anywhere but a climate-controlled room, either condition the nylon before machining and design for the growth, or switch to Delrin and stop thinking about it.
When is PEEK worth it over Delrin?
When the environment leaves no choice: continuous temperatures above Delrin's comfort zone, aggressive chemicals, steam, or structural loads where 100+ MPa tensile strength matters. PEEK delivers that at roughly ten times the material price ($90–400/kg vs $5–15/kg for Delrin). The honest rule: if Delrin survives the environment, PEEK is a luxury; if Delrin does not survive, PEEK is a bargain compared to field failures.
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