PA66 moisture and CNC dimensional stability

The short answer

**PA66 (nylon 66)** CNC parts move with **moisture**. Dimensional stability on the print is not the same as stability on the bench or in the field. Package humidity assumptions, conditioning notes, and tolerance strategy so [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands stay meaningful instead of fighting swell you never declared.

This guide is for buyers choosing PA66 for functional prototypes and low-volume CNC plastics. Illustrations use an **Example part** only. Related material context: [PA66 vs aluminum CNC prototypes](https://3dwk.com/resources/pa66-vs-aluminum-cnc-prototypes), [choosing engineering plastics](https://3dwk.com/resources/choosing-engineering-plastics-peek-pom-nylon), [POM vs PEEK](https://3dwk.com/resources/pom-vs-peek-cnc-functional-prototypes).

Why moisture dominates PA66 metrology

PA66 absorbs water from air. As-machined dry stock can grow after days in humid storage; “pass CMM in the shop” may not equal “fit after shipping.” POM and PEEK behave differently—do not paste PA66 swell rules onto those RFQs. Background: [CNC cost drivers](https://3dwk.com/resources/cnc-machining-cost-drivers-buyer-guide), [surface roughness Ra](https://3dwk.com/resources/surface-roughness-ra-for-machined-parts).

**rates.v1.1** planning (estimates): PA66 bar ~$5/kg, CNC machine-time coeff ×0.8—cheap spindle narrative does not erase moisture risk. Compare to 17-4PH (~$9/kg, ×1.4), IN718/IN625 (~$55–60/kg, ×2.5), CoCr (×2.2). Line totals above configured USD thresholds still auto-route to **needs_manual** on hard alloys; PA66 usually stays automatic unless GD&T or thin walls force review.

Tolerance strategy that shops can hold

Default non-mates to generous general tolerances. Tighten only mates that must assemble dry or conditioned—say which. Avoid ±0.02 mm true position across a large nylon plate without stating measurement humidity. Related: [GD&T basics](https://3dwk.com/resources/gdt-basics-for-buyers), [flatness / parallelism / perpendicularity](https://3dwk.com/resources/flatness-parallelism-perpendicularity-machined-parts), [DFM checklist](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote).

Illustration 1 shows an **Example part** moisture timeline: as-machined → 24 h shop air → sealed bag versus open bin.

Stock, conditioning and RFQ notes

State: virgin vs regrind preference if you care, dry-as-machined vs humidity-conditioned acceptance, and packaging (sealed bag with desiccant vs open). If the part must fit aluminum mates that do not swell, call the assembly state. Thin walls amplify percent change—see [pockets / thin walls / undercuts](https://3dwk.com/resources/cnc-pockets-thin-walls-undercuts).

Do not invent “aerospace dry nylon” certificates. A one-line conditioning note beats a vague “hold tight.”

Design habits that reduce moisture fights

• Prefer thicker local bosses for inserts rather than knife-edge walls
• Avoid large unsupported spans that bow with swell
• Use clearance that absorbs expected growth on non-critical fits
• Consider POM when moisture stability matters more than PA66 toughness—compare honestly on parallel RFQs

Inserts and tapped holes: call helical insert allowance; nylon threads behave differently than aluminum—[thread inserts vs tapped holes](https://3dwk.com/resources/thread-inserts-vs-tapped-holes-aluminum), [designing threads](https://3dwk.com/resources/designing-threads-and-tapped-holes).

Illustration 2 contrasts a thin “PA66, ±0.02 all over” note versus a moisture-aware RFQ block and clearer estimate path.

Finish and marking

PA66 rarely wants metal-style anodize notes. Laser mark / pad print cosmetics should not redefine critical dims. Avoid plating callouts copied from aluminum drawings. If wear matters, state polymer-on-metal duty rather than Type III language.

Qty, lead time and estimate packaging

Prototype qty 1–5 often ships as-machined dry with a bag note. Bridge qty may need consistent conditioning SOPs—say so. Re-parse when qty changes—[how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost). Files: STEP + PDF—[what files to send](https://3dwk.com/resources/what-files-to-send-for-cnc-quote). Ambiguous packs widen bands—[what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).

Example part walkthrough

**Example part:** PA66 sensor bracket, ~80×50×12 mm, two M4 inserts, ISO 2768-m, bore clearance for Ø6 pin stated at 23 °C / ~50% RH acceptance, sealed bag after machine. Expect a clearer planning band on [quote-new.php](https://3dwk.com/quote-new.php) than “PA66, hold ±0.02 everywhere, no humidity note.” No fake customer CMM reports.

What to do next

Declare moisture/conditioning assumptions, then upload on [quote-new.php](https://3dwk.com/quote-new.php). Service pages: [CNC machining](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php). **rates.v1.1** PA66 figures are estimates, not invoices.

Storage, shipping and receiving checks

Seal bags help, but receiving inspection in a different climate can still surprise. Agree whether the buyer re-conditions parts before CMM and whether the supplier’s as-machined report is informational or contractual. Write that once on the RFQ so the mid is not fighting a hidden quality clause. Related: [reading a preliminary quote](https://3dwk.com/resources/reading-preliminary-online-quote-min-mid-max), [CNC lead time](https://3dwk.com/resources/cnc-machining-lead-time-buyer-guide).

When aluminum is the clearer mate

If the nylon part only exists to mock an aluminum production design, consider machining the prototype in aluminum for fit studies—see [PA66 vs aluminum CNC prototypes](https://3dwk.com/resources/pa66-vs-aluminum-cnc-prototypes) and [6061 vs 7075](https://3dwk.com/resources/6061-vs-7075-aluminum-for-machined-parts). Use PA66 when the polymer is the real functional choice, not as an accidental stand-in that inherits metal tolerances.

Common questions