For engineering and sourcing teams
Controlling Nylon Dimensions in Humid Service?
Moisture changes stiffness, toughness and part dimensions. Buyers should define both the inspection condition and the real use environment before approving a nylon tolerance.
- Specify whether dimensions apply dry-as-molded or conditioned
- Match drying and storage controls to the selected nylon grade
- Review metal inserts, fits and wall sections for differential movement
Request a moisture and tolerance reviewView nylon molding services
Why Moisture Absorption is the Defining Characteristic of Nylon
No other engineering thermoplastic absorbs moisture as dramatically as nylon. PA6 can absorb up to 9.5% by weight at saturation — more than any other common engineering plastic. Even PA66, with its tighter molecular packing, reaches 8.5%. PA12 reaches only 1.5%, making it the nylon of choice for wet environments.
This moisture absorption is not a defect — it is a fundamental property arising from the hydrogen-bonding amide groups in the polyamide chain. Water molecules penetrate the amorphous regions of the polymer and plasticize it, fundamentally altering mechanical, thermal, and dimensional properties.
For buyers and engineers specifying nylon, understanding moisture effects is not optional. Every nylon datasheet value — tensile strength, modulus, impact resistance — is measured on conditioned specimens. The same part, tested dry-as-molded, can show tensile strength 40% higher but impact resistance 70% lower than conditioned values.
How Nylon Absorbs Moisture: The Science
Moisture absorption in nylon follows a classic Fickian diffusion process with two phases:
- Phase 1 (rapid): Surface saturation within hours — parts appear “wet” but interior is still dry. This is the most dangerous phase for dimensional instability and surface blistering.
- Phase 2 (slow): True equilibrium through bulk diffusion — can take weeks or months for thick sections (>10mm wall thickness)
At 50% relative humidity (23°C), PA6 reaches approximately 2.5% moisture content within 24 hours. Full saturation at 95% RH can take 2-4 weeks. The actual equilibrium moisture content depends on:
| Nylon Grade | Max Moisture Absorption (%) | Equilibrium at 50% RH | Equilibrium at 95% RH |
|---|---|---|---|
| PA6 | 9.0-9.5% | 2.5-3.0% | 8.5-9.0% |
| PA66 | 8.0-8.5% | 2.0-2.5% | 7.5-8.0% |
| PA12 | 1.4-1.6% | 0.7-0.9% | 1.3-1.5% |
| PA6-GF30 | 6.0-7.0% | 1.5-2.0% | 5.5-6.5% |
| PA66-GF30 | 5.0-6.0% | 1.3-1.8% | 4.5-5.5% |
| PA46 | 7.0-7.5% | 2.0-2.3% | 6.5-7.0% |
Effects on Mechanical Properties
Moisture absorption simultaneously improves some properties while degrading others — a trade-off that must be understood for accurate part design.
| Property | Dry As Molded | Conditioned (50% RH) | Change |
|---|---|---|---|
| Tensile Strength (MPa) | 80-90 | 55-65 | -28% |
| Tensile Modulus (MPa) | 3000-3300 | 1800-2100 | -35% |
| Elongation at Break | 40-60% | 150-250% | +250% |
| Izod Impact (kJ/m²) | 3-5 | 6-10 | +100% |
| Flexural Modulus (MPa) | 2800-3100 | 1500-1800 | -40% |
| HDT (1.82 MPa, °C) | 65-75 | 55-65 | -15°C |
Effects on Dimensional Stability
Moisture absorption causes dimensional changes that can be significant for precision parts. The dimensional change is not linear — it follows a differential swelling pattern between skin (which equilibrates first) and core (which remains dry).
- Linear expansion: PA6 expands approximately 0.15-0.20% per 1% moisture absorbed in the flow direction, and 0.40-0.50% in the transverse direction
- Anisotropic swelling: Similar to shrinkage anisotropy, moisture swelling is greater perpendicular to the mold flow direction
- Warpage from differential drying: Parts dried unevenly after conditioning can warp as surface and core dry at different rates
- Threaded inserts: Metal inserts in nylon do not change dimension with humidity — dimensional changes occur only in the nylon surrounding the insert, creating stress on the interface
A moisture specification should connect the resin, process and final inspection state.
Send the nylon grade, drawing, use humidity and critical tolerances. We can review drying, conditioning and dimensional control before production.
Measurement Methods: How to Measure Nylon Moisture Content
Accurate moisture measurement is essential for quality control in nylon processing and for verifying material condition before use.
| Method | Principle | Accuracy | Time | Best Use |
|---|---|---|---|---|
| Loss on Drying (LOD) | Weigh before/after oven drying at 103°C | ±0.1% | 2-4 hours | Quick production QC |
| Karl Fischer Titration | Chemical reaction with water molecules | ±0.01% | 10-20 min | Precision verification |
| Coulometric KF | Electrolytic Karl Fischer | ±0.001% | 15-30 min | R&D, spec testing |
| Moisture Analyzer | Automated LOD with integrated balance | ±0.05% | 20-40 min | Inline production QC |
| Dew Point Hygrometer | Equilibrium with desiccant | ±0.3% | 30-60 min | Dryer monitoring |
Control Strategies: Drying and Storage Best Practices
Effective moisture control requires a systematic approach from material storage through processing:
- Desiccant dryer: The standard for nylon drying. Use desiccant with dew point below -40°C (ideally -70°C). Maintain air dew point of -40°C throughout the drying cycle. Target 0.1-0.2% residual moisture for PA66, 0.2% for PA6.
- Drying temperatures: 80°C for PA6 and PA66 (higher temperatures risk oxidative degradation), 70-75°C for PA12
- Drying time: 4-6 hours minimum for properly sealed material; 8-12 hours for material exposed to humid air or with wall thickness >15mm
- Hopper design: Use sealed hoppers with nitrogen purge to prevent re-absorption during production runs
- Material shelf life after drying: Once dried, nylon re-absorbs moisture at approximately 0.1% per hour at 23°C/50% RH — dried material must be used within 2-4 hours or re-dried
Nylon Moisture Absorption FAQ
Should nylon dimensions be inspected dry or conditioned?
The drawing or quality agreement should state the inspection condition. Dry-as-molded and conditioned nylon can produce different dimensions and mechanical behavior.
Which nylon absorbs the least moisture?
PA12 generally absorbs much less moisture than PA6 or PA66, which often makes it easier to control in humid or wet service.
Can drying reverse dimensional change caused by moisture?
Drying removes absorbed water, but the cycle must be controlled to avoid uneven shrinkage, oxidation or distortion in thick and constrained parts.
What should be included in a moisture-control RFQ?
Include the nylon grade, drawing, operating humidity and temperature, critical fits, inspection condition, packaging requirement and annual volume.
Related Reading
At a Glance
| Decision Point | Practical Takeaway | Buyer Note |
|---|---|---|
| Drying | Needed before processing | Drying lowers surface and dimensional risk |
| Conditioning | Changes size and toughness | Spec the part in its use state |
| Storage | Controls moisture pickup | Use sealed handling for sensitive grades |
| Best use | Controlled engineering parts | Set tolerance after environment is defined |
Why Choose Nylon Plastic
Nylon Plastic helps buyers control moisture-related risk when fit, drying and storage conditions all affect final part performance.
For a nylon moisture and dimensional-risk review
Share the nylon grade, drawing, operating humidity, critical fits and annual volume. We will identify the drying, conditioning and inspection points that need agreement.
