PA6 vs PA66 Nylon for Molded Parts — Which Absorption, Strength, Temperature and Cost Profile Fits Your Part?
Last updated: 2026-09-04 · Reading time: about 6 min
PA6 and PA66 differ in moisture absorption, heat resistance and processing window. PA6 absorbs 2.5–3.0 % at saturation; PA66 absorbs 1.5–2.0 %. PA66 melts at 260 °C vs 220 °C for PA6, giving PA66 a higher heat-deflection temperature under load. PA6 processes more easily and costs less; PA66 holds stiffness at higher temperature and absorbs less water. The right choice depends on which property your application cannot compromise.
See also related article.
Key facts
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Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.
How do PA6 and PA66 compare on the numbers that matter?
Six properties. Each one points toward a different application profile.
| Property | PA6 (Unfilled, Dry) | PA66 (Unfilled, Dry) | What It Means for Your Part |
|---|---|---|---|
| Moisture absorption (saturation) | 2.5–3.0 % | 1.5–2.0 % | PA66 holds dimensions better in wet environments; PA6 gains more toughness when conditioned |
| Melting point | 220 °C | 260 °C | PA66 survives higher continuous service temperature; PA6 is easier to melt and process |
| Heat deflection temperature (1.8 MPa) | 60–80 °C (dry), up to 200 °C (GF30) | 70–90 °C (dry), up to 220 °C (GF30) | PA66 holds stiffness at higher temperature; glass fill raises both grades substantially |
| Tensile modulus (dry) | 2,800–3,200 MPa | 3,000–3,500 MPa | PA66 is stiffer; PA6 is tougher and more ductile at break |
| Impact strength (notched, dry) | 5–8 kJ/m² | 3–5 kJ/m² | PA6 absorbs more energy before fracture; PA66 is more brittle in dry state |
| Mold shrinkage | 0.7–1.2 % | 0.7–1.2 % | Similar range; glass fill reduces both to 0.2–0.5 % and lowers anisotropy |
PA6 vs PA66. Two polyamide resins used for injection molded parts. PA6 (nylon 6) is polymerized from caprolactam; PA66 (nylon 6/6) is polymerized from hexamethylenediamine and adipic acid. PA6 processes more easily and absorbs more moisture; PA66 offers higher heat resistance and lower water uptake. Both are available in filled and modified grades that extend the property range.
The choice between PA6 and PA66 is a trade-off between processing cost and end-use performance. PA6 is easier to mold — lower melt temperature, wider processing window — and delivers higher impact strength when dry. PA66 gives higher heat resistance, lower moisture absorption and better stiffness retention at temperature. If your part lives under a hood or in a hot fluid, PA66 is the default. If it needs toughness at room temperature in a dry environment, PA6 often wins on cost and performance together.
How do you decide which one to specify?
Four steps. The method moves from the application requirement to the resin that meets it at lowest total cost.
- Start with the service temperature and environment. If the part operates continuously above 80 °C or in contact with hot fluids, PA66 (especially glass-filled) is the default — its higher melting point and heat-deflection temperature give it a clear edge. If the part runs at or near room temperature in a dry or indoor environment, PA6 is usually sufficient and cheaper. For parts that cycle between hot and cold, or that see under-hood temperatures with occasional peaks, PA66-GF30 is the standard automotive choice.
- Check moisture sensitivity of the critical dimension or assembly. If your part has tight tolerances that must hold in a humid environment — a gear mesh, a snap-fit that must not loosen, a bearing surface — PA66’s lower absorption means less dimensional change at equilibrium. If the part will be used dry or can be pre-conditioned, PA6’s higher absorption is not a liability and its extra toughness becomes an asset.
- Evaluate the impact and fatigue requirement. PA6 is tougher and more ductile than PA66 in the dry state, which matters for parts that see sudden loading, snapping or repeated flexure. If the part is a living hinge, a clip or a latch, PA6 often outperforms PA66 on cycle-to-fatigue-failure. If stiffness under static load matters more than impact, PA66 is the stronger choice.
- Compare landed cost including drying, cycle time and scrap. PA6 melts at a lower temperature and generally runs a faster cycle, which lowers piece cost on high-volume tools. PA66 requires higher barrel temperatures and more rigorous drying (both resins are hygroscopic, but PA66’s higher melting point means moisture causes more visible surface defects). Factor in the resin price, the cycle time difference and the expected scrap rate — on a 100,000-shot program the processing cost gap can exceed the resin price gap.
“PA6 and PA66 are not interchangeable — they are two points on a property map. Pick the one your application’s temperature, moisture and impact requirements point to, then let the filler package fine-tune the rest.”
— Tooling Engineer, aCatMould
Frequently asked questions
Can I substitute PA6 for PA66 if the price gap widens?
Sometimes, but not without re-checking the critical properties.
PA6 has lower heat resistance and higher moisture absorption — if your part was specified for PA66 because of under-hood temperature or tight dimensional stability in humidity, substituting PA6 will lose those advantages. On the other hand, if the original spec was driven by stiffness and the part runs cool and dry, a well-filled PA6 may perform acceptably at lower cost.
Never swap the base resin without re-trialing the part under the actual service conditions. Run a dimensional check after moisture conditioning and confirm the heat-deflection temperature at the application’s peak load.
Does glass fill change the PA6 vs PA66 decision?
Yes, it narrows the gap on some properties and widens it on others. Glass fill raises stiffness and heat resistance on both grades substantially — a GF30 PA6 can match the HDT of an unfilled PA66. But glass fill also increases abrasiveness (affecting mold steel selection), raises mold shrinkage anisotropy, and reduces impact strength relative to the unfilled grade.
At equal filler content, PA66-GF still outperforms PA6-GF on heat resistance and moisture absorption, but the difference is smaller than between the unfilled grades. The decision often comes down to whether the remaining gap matters for your specific application.
How does moisture conditioning affect the comparison?
Moisture acts as a plasticizer for both grades — it lowers stiffness and strength but raises impact elongation and toughness. PA6 absorbs more water, so the shift is larger: a dry PA6 part at 3 GPa modulus may drop to 1.5 GPa when saturated, while PA66 drops less because it absorbs less. If your part will be used in a humid environment, design for the conditioned properties, not the dry-as-molded values — the difference can be the margin between passing and failing.
Conditioning to equilibrium before testing gives you the real in-service numbers. Skipping this step is the most common source of a surprise in the field.
Key takeaways
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Start the decision from your service temperature and environment.
PA6 and PA66 are not interchangeable — they are two points on a property map shaped by moisture, temperature and load.
The right choice comes from matching your application’s non-negotiable requirements to the resin that meets them, then letting the filler package handle the rest. Get the base resin right and the rest of the spec follows.
Sources
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Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet