Understanding moisture absorption in nylon — how it affects mechanical properties, dimensional stability, processing, and design strategies to manage it.
Nylon’s Moisture Sensitivity: A Critical Design Variable
Of all common engineering thermoplastics, nylon absorbs the most moisture. PA6 reaches 9.5% by weight at saturation, while PA66 reaches 8.5%. This is not a minor property — it fundamentally alters the material’s mechanical behavior, dimensions, electrical properties, and appearance.
Designers and engineers who ignore moisture absorption face a common failure mode: parts that fit perfectly when molded (dry) but swell, distort, or change flexibility after exposure to normal humidity. A nylon gear that meshes perfectly in the factory may bind and wear prematurely in a humid warehouse. Understanding and controlling moisture effects is essential for reliable nylon part design.
Mechanism: How Nylon Absorbs Water
Nylon’s molecular structure contains amide groups (-CONH-) that form hydrogen bonds with water molecules. This is the same hydrogen-bonding mechanism that makes nylon strong in the first place — but water molecules occupy hydrogen-bonding sites that were previously linking nylon chains together, reducing inter-chain forces.
**Absorption Rate**:
The rate of moisture absorption depends on thickness and humidity:
| Sample Thickness | Time to Reach 50% Saturation | Time to 90% Saturation |
|—|—|—|
| 1mm film | 2 hours | 8 hours |
| 2mm sheet | 8 hours | 36 hours |
| 3mm rod | 18 hours | 72 hours |
| 6mm plate | 3 days | 12 days |
This is why thin-walled parts reach equilibrium faster — and why thick sections can retain dry conditions in the core while the surface is saturated.
**Drying Reverses the Process**:
Heating nylon above 80°C drives off absorbed moisture. At 100°C for 4-6 hours, PA6 and PA66 reach dry-as-molded condition. However, the dimensional changes from drying are not fully reversed — the part does not shrink back to its dry-molded dimensions because the polymer chains have reorganized.
Effects on Mechanical Properties
**Property Changes from Dry (0% RH) to Conditioned (50% RH)**:
| Property | PA6 Dry | PA6 Conditioned | Change |
|—|—|—|—|
| Tensile Strength | 95 MPa | 75 MPa | -21% |
| Tensile Modulus | 3.2 GPa | 2.0 GPa | -37% |
| Flexural Modulus | 3.0 GPa | 1.8 GPa | -40% |
| Elongation at Break | 80% | 180% | +125% |
| Notched Izod | 45 J/m | 80 J/m | +78% |
| Hardness (Shore D) | 82 | 74 | -10% |
**Key insight**: Moisture acts as a plasticizer for nylon. The material becomes softer, weaker in tension and flexure, but significantly tougher. Impact resistance nearly doubles in conditioned nylon vs. dry nylon.
**Design Implication**: If you design to dry property values, your conditioned parts will be 20-40% weaker than calculated. Always design to the highest moisture condition the part will experience in service.
**Glass fiber reinforcement mitigates moisture effects** — GF30 grades show only 10-15% strength reduction from dry to conditioned (vs. 20-25% for unfilled). The glass fiber network is unaffected by moisture; only the nylon matrix is plasticized.
Dimensional Effects and Warpage
Moisture absorption causes linear expansion in nylon:
**Dimensional Change from Dry to Saturated**:
| Grade | Linear Expansion per % Moisture |
|—|—|
| PA6 | 0.25-0.30% |
| PA66 | 0.22-0.28% |
| PA12 | 0.12% |
| PA6-GF30 | 0.08% |
| PA66-GF30 | 0.07% |
A PA6 bushing with 50mm OD, molded dry, will expand to approximately 50.35mm at 50% RH saturation (0.35% change × 50mm = 0.175mm). If the assembly requires 50.0-50.1mm fit, this is a critical tolerance issue.
**Unequal Moisture Distribution Causes Warpage**:
In thick sections, the outer surface absorbs moisture while the core remains dry. This creates differential swelling — the surface wants to expand while the core resists. The result is warpage (bowing, distortion) even in parts with symmetrical geometry.
**Design Strategies**:
1. **Anneal before final dimensioning** — Heat treat parts at 120-130°C for 1-2 hours to crystallize and stabilize dimensions before machining or assembly
2. **Condition to equilibrium** — Allow parts to reach uniform moisture content before final assembly
3. **Use GF or CF reinforcement** — Fiber reinforcement reduces moisture-induced expansion by 70-80%
4. **Specify PA12** — At 1.5% saturation vs. 8-9% for PA6/66, PA12’s dimensional change is negligible
Processing: Drying Requirements
Excess moisture in nylon during injection molding causes catastrophic defects:
**Moisture Defects**:
– **Bubbles and voids**: Steam formed during injection creates internal voids
– **Silver streaks**: Water vapor flashing off during injection creates surface streaks
– **Reduced molecular weight**: Hydrolysis during processing weakens the material
– **Reduced mechanical properties**: Even if surface looks good, the material is degraded
**Required Drying Parameters**:
| Material | Drying Temperature | Drying Time | Max Moisture Content |
|—|—|—|—|
| PA6 | 80°C | 4-6 hours | 0.20% |
| PA66 | 80-85°C | 4-6 hours | 0.15% |
| PA12 | 80°C | 3-4 hours | 0.10% |
| PA6-GF30 | 80°C | 4-6 hours | 0.15% |
| PA66-GF30 | 85°C | 4-6 hours | 0.12% |
**Drying Equipment**: Desiccant dryers are mandatory for nylon. Hot air dryers are insufficient because they cannot remove moisture below the surface. Desiccant dryers with dew point below -40°C are required.
**Moisture Analyzers**: Use Karl Fischer titration or loss-on-drying to verify material moisture before processing critical parts. Most production facilities check every batch.
PA12 vs. PA6/PA66: When to Choose Low-Moisture Grades
For applications where moisture is unavoidable, PA12 is the logical choice:
**Applications where PA12’s low absorption is essential**:
– **Underwater or marine components**: PA12 maintains properties in submerged conditions where PA6/66 would absorb 5-8%
– **Outdoor exposed parts**: PA12’s 1.5% saturation vs. 8-9% means far less dimensional change through seasonal humidity cycles
– **Food processing (steam cleaning)**: PA12 resists steam exposure better than PA6/66
– **Fluid metering components**: Dimensional stability in humid air is critical for precision metering
– **Cable conduits**: PA12 handles underground moisture without swelling
**Cost vs. Benefit**:
PA12 costs approximately 2-3× more than PA66. The premium is justified when:
1. Field failures from moisture-induced swelling are costly
2. Dimensional tolerances are tight (±0.05mm or tighter)
3. The part is exposed to water, humidity, or steam
4. Assembly requires parts at equilibrium condition before fit testing
**Hybrid Approach**:
For many applications, PA66-GF30 achieves a practical balance: the glass fiber reinforcement reduces moisture absorption by ~70% (effective absorption drops from 8.5% to ~2.5%), and the GF network limits dimensional change. This is why PA66-GF30 is the automotive default — it handles under-hood humidity without the premium cost of PA12.
Whether you need technical guidance on selecting the right nylon grade for your specific application, or want to discuss pricing and supply options for PA6-CF, PA66-GF, or standard nylon materials, our engineering team is ready to help. Nylonplastic.com supplies industrial-grade nylon materials to manufacturers in North America, Europe, and Southeast Asia.
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Contact our technical team for nylon grade recommendations, pricing for bulk orders, or samples for testing. We supply PA6-CF carbon fiber reinforced nylon in 1.75mm and 2.85mm diameters, plus full range of PA6, PA66, PA12, and GF-reinforced grades.
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**FAQs**
**Q: What is the difference between PA6 and PA66?**
A: PA66 (nylon 66) has a higher melting point (265°C vs. 225°C) and better chemical resistance than PA6. PA6 offers better impact resistance and is more cost-effective. PA66 is preferred for high-temperature and under-hood automotive applications; PA6 is common for general engineering.
**Q: How much does glass fiber reinforcement improve nylon?**
A: Adding 30% glass fiber increases tensile strength by 100-120% (from ~80 MPa to ~170 MPa) and flexural modulus by 250-300% (from ~2.8 GPa to ~9 GPa). However, it also reduces impact resistance and increases warpage.
**Q: What is carbon fiber reinforced nylon used for?**
A: Carbon fiber reinforced nylon is used for structural components requiring high stiffness-to-weight ratio, ESD-sensitive applications (electronics packaging, fuel systems), and precision parts requiring dimensional stability. nylonplastic.com supplies PA6-CF for FDM 3D printing and injection molding applications.
**Q: How do I prevent moisture absorption problems in nylon parts?**
A: Dry nylon resin to below 0.2% moisture content before molding (4-6 hours at 80-85°C in desiccant dryer). For dimensional-critical parts, anneal after molding (1-2 hours at 120-130°C) to stabilize crystallinity. Use glass or carbon fiber reinforcement to reduce moisture-induced dimensional change by 70-80%.
**Q: Can nylon be used for food contact applications?**
A: Yes. Both PA6 and PA66 have FDA 21 CFR §177.1500 compliance for food contact. EU Regulation 10/2011 compliance requires specific compound selection with documented SML testing. Always verify specific grade compliance with your supplier.
