Nylon Plastic FAQ: Moisture, Grades, Machining & More

Nylon generates more technical questions than almost any other engineering plastic, primarily because of its moisture absorption behavior and the wide range of grades and production methods available. These are the questions engineers, machinists, and buyers ask most often.

TL;DR

Moisture is the dominant variable — design for it, don't ignore it
Cast Nylon 6 for large or heavy parts; extruded Nylon 6/6 for bar-feed CNC and tight tolerances
Filled grades (MoS₂, glass) are NOT food-safe — only unfilled natural qualifies
Nylon can be used outdoors short-term; long-term UV exposure degrades unfilled grades
Bonding requires surface prep — nylon is not easily adhesive-bonded without treatment
Glass-filled nylon doubles stiffness but is abrasive to mating surfaces and cuts

Q: How do I deal with nylon's moisture absorption in a precision part?

Moisture absorption is the most common design problem with nylon. A bushing or bearing bore machined to a precise clearance in a dry shop can tighten in a humid field environment as the nylon absorbs moisture and swells.

Three approaches, in order of reliability:

Design for the wet state: Calculate the expected moisture content at operating conditions (typically 2–4% at 50% RH, up to 9% submerged) and add the corresponding dimensional growth to your clearance. For a 1" bore in an environment at 50% RH, add ~0.010"–0.015" bore clearance beyond normal bearing tolerance.
Pre-condition the part: Immerse the finished nylon part in water at room temperature for 24–72 hours before final fit. The part absorbs moisture and expands; fit and adjust to the conditioned dimension. This is especially useful for parts that will operate fully submerged (marine, pump housings).
Switch materials: For close-clearance bore/shaft fits where moisture variation cannot be tolerated, Delrin absorbs less than 0.25% moisture and is dimensionally nearly immune to humidity changes. It's the correct substitution for moisture-sensitive applications.

For parts in dry environments (sealed assemblies, indoor office equipment), moisture is rarely a practical problem — unfilled nylon holds dimension very well at low humidity.

Q: What is the real difference between cast and extruded nylon — does it matter for my part?

Yes, it matters primarily for two reasons: available size range and residual stress.

Cast Nylon 6 is polymerized at atmospheric pressure directly in a mold. No pressure means no orientation stress in the finished billet. You can machine deep cuts and thin walls without stress-relief distortion. Available in rod to 12" diameter and plate to 4" thick × 24" × 96" — the only practical source for large nylon shapes.

Extruded Nylon 6/6 is pushed through a die under pressure, creating orientation stress in the material and a slightly harder skin. For bar-feed CNC operations on small-diameter rod (under 3"), extruded nylon is faster to source in standard lengths and has better diameter tolerances over the bar length. Above 3" diameter or 1" plate thickness, cast is the practical standard.

If you are making a 1" diameter rod with turned bushings on a bar-feed lathe, extruded Nylon 6/6 is the right call. If you are making an 8" diameter sheave blank, cast Nylon 6 is the only reasonable option. See the nylon grades guide for full selection criteria.

Q: Can nylon be dyed or painted?

Dyeing: Yes. Nylon's polyamide structure accepts acid and reactive dyes well — chemically identical to nylon textile fiber. Acid dyes in hot-water baths at 180°F–200°F produce penetrating colors without meaningfully affecting mechanical properties. Shop-level dyeing requires a temperature-controlled bath; practical mainly for small custom parts.

Painting: Nylon is difficult to paint without surface preparation. Abrade with 120-grit, wipe with isopropyl alcohol, then apply a nylon-adhesion primer. Without primer, paints delaminate under flex or thermal cycling.

Stock colors: Natural (off-white), black, and blue are standard. Green is available in some cast Nylon 6 grades. Non-standard colors typically require a custom pour with minimum quantities.

Q: Is glass-filled nylon better than unfilled for my bushing or gear?

For bearing and gear applications: generally no. Glass-filled nylon (GF30 or GF33 at 33% by weight glass fiber) dramatically increases stiffness and reduces thermal expansion, but it has a higher coefficient of friction than unfilled nylon and is abrasive to mating steel surfaces.

Use glass-filled nylon for:

Structural brackets, housings, and manifolds where deflection is the failure mode
Parts with wide temperature swings where dimensional stability from low CTE is essential
Applications where creep under sustained load is a concern and the mating surface is sacrificial or hardened to 60+ Rc

Do not use glass-filled nylon for:

Any food-contact application (glass fiber is not FDA-approved for food contact)
Bearings or sliding contact surfaces running against soft steel or aluminum — the glass fiber will abrade the mating part rapidly
Parts with thin walls or sharp corners — GF30 is notch-sensitive and will crack where unfilled nylon would flex

For comparison of all nylon grades, see the nylon grade selection guide.

Q: Can nylon be used outdoors?

Nylon has moderate UV resistance — it is not designed for long-term outdoor exposure. Unfilled natural nylon will:

Yellow within 6–12 months of direct sun exposure
Lose surface gloss and develop micro-cracking (chalking) over 2–4 years outdoors
Show measurable reduction in impact strength after 3–5 years of intense UV exposure

For short-term outdoor use (seasonal equipment, temporary fixtures): unfilled nylon is acceptable. Plan for replacement within 2–3 years in UV-intense environments.

For long-term outdoor use: Black carbon-loaded nylon offers substantially better UV resistance — the carbon black absorbs UV rather than transmitting it to the polymer backbone. Black nylon is not FDA-compliant, but for structural outdoor parts, it is the correct grade.

Best outdoor alternative to nylon: UHMW polyethylene in black has outstanding UV resistance and is commonly used for outdoor wear pads, truck bed liners, and equipment guards with decades of service life. Marine-grade HDPE (Starboard) is UV-stabilized specifically for outdoor/marine environments.

Q: What's the temperature limit for nylon, and what happens if I exceed it?

The continuous use temperature for unfilled nylon is 200°F (93°C). This means the material maintains acceptable mechanical properties — without excessive creep, deformation, or chemical degradation — at that temperature indefinitely under design loads.

Short-term exposure to 300°F (149°C) is typically tolerable for brief intervals (hot-wash cycles, steam flushes) without permanent damage, as long as load is minimal during that exposure.

What happens above 200°F (continuous):

Creep rate increases sharply — parts under load will permanently deform
Hardness and stiffness drop — bearings and gears lose dimensional accuracy
Oxidation and yellowing accelerate — cosmetic and mechanical degradation
Above 300°F: hydrolysis (if moisture is present) becomes the dominant failure mechanism

At temperatures above 200°F, consider:

Nylon 6/6 (rather than 6) — modest improvement; HDT of 185°F vs. 150°F at 264 psi
Glass-filled nylon GF33 — continuous use to ~230°F
Delrin — same 180°F continuous limit; not better than nylon here
PEEK — genuine 480°F continuous service; 10–15× the cost

Q: Can nylon be welded or bonded?

Bonding (adhesive): Nylon is considered low-adhesion — standard epoxies and cyanoacrylates bond poorly to unmodified nylon surfaces. Effective adhesive bonding requires:

Mechanical abrasion (120-grit, then clean with isopropyl alcohol)
Chemical etching with concentrated phosphoric acid or a commercial plastic adhesion promoter — increases surface energy
Two-part structural epoxy or a cyanoacrylate formulated for polyamides (Loctite makes specific PA primers)

Even with best practice, adhesive bonds in nylon are not as strong as the base material. For load-bearing joints, design mechanical retention (pins, screws, press fit) as primary and adhesive as secondary.

Vibration welding and hot-plate welding: Both work on nylon — the bond strength can approach base-material strength with proper welding parameters. Vibration welding is more common for production assemblies.

Solvent bonding: Not practical. Nylon has very limited solvent susceptibility — common solvents (acetone, MEK) do not dissolve it adequately for bonding. Formic acid and certain phenol-based solvents will attack nylon but are not practical shop-floor options.

Q: Is natural nylon really FDA-approved, and what does that actually cover?

Natural (unfilled) Nylon 6 and Nylon 6/6 comply with FDA 21 CFR 177.1500 — Nylon resins. This permits the material for use as food-contact articles under the conditions of use defined in the regulation.

What this covers:

Direct and incidental food contact (conveyors, bushings, cam followers, chutes)
Contact with aqueous, fatty, and dry foods
High-temperature contact when the applicable condition of use (A–H per 21 CFR 176.170) is met

What it does NOT mean:

Any filled grade is approved — MoS₂, glass fiber, graphite-filled nylon are not food-contact compliant
Color automatically indicates compliance — request a conformance letter for the specific grade
The FDA has reviewed or approved the specific machined part — compliance responsibility is on the fabricator

Full regulatory detail — including USDA, NSF/ANSI 51, and documentation requirements — is in the nylon FDA food-grade guide.

Q: Why does Nylatron GS outperform plain nylon in bearings?

Nylatron GS incorporates molybdenum disulfide (MoS₂) at 2–3% by weight into the cast Nylon 6 matrix. MoS₂ is a layered solid lubricant — its crystal planes shear with very low resistance at asperity contact points, reducing friction and the heat that causes nylon bearing failure.

Key performance differences vs. unfilled nylon (dry service):

Coefficient of friction: 0.12–0.18 vs. 0.20–0.35
PV limit: 8,000–10,000 psi·ft/min vs. 3,000 psi·ft/min — a 3× improvement
Wear life: 5–10× longer in dry-running bushing tests

Nylatron GS is black and not FDA-compliant. Do not use it in food contact. See the nylon applications guide for full bearing grade selection.

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Frequently asked questions — Nylon FAQ