Why two closely related aliphatic polyamides flow differently in the mould, take up moisture, wear, look on the surface and hold dimensions differently — an in-depth engineering analysis for designers, process engineers and buyers.
Working principle. The difference between PA6 and PA66 originates at the molecular level — in chain symmetry and crystallisation rate. From this follow all the practical consequences: shrinkage, heat resistance, creep, moisture absorption, friction, surface and chemical resistance. That is why a correct comparison always begins with the mechanism, not with a single line of a TDS.
1. Where PA6 and PA66 sit in the polyamide "family"
PA6 and PA66 account for over 80 % of global polyamide consumption in engineering applications, but they are only two members of a broader class. Polyamides are classified by the number of carbon atoms in the monomer units: the length of the carbon chain directly governs the affinity for moisture, flexibility, thermal threshold and chemical resistance.
- Short chain (high stiffness and heat resistance): PA46 — highly crystalline, Tm ≈ 295 °C, retains stiffness at extreme temperatures; often outperforms PA66 in transmission and electrical components.
- Standard chain (the workhorses): PA6 and PA66 — a balance of strength, processability and cost at a high affinity for atmospheric moisture.
- Long chain (high dimensional stability): PA11, PA12, PA610, PA612 — a longer chain "dilutes" the polar amide groups, so moisture absorption is minimal, while dimensional stability and low-temperature resistance are the best in class.
- High-performance and amorphous aromatic: polyphthalamides (PPA) with aromatic rings in the chain (Tm > 300 °C) and amorphous PA (PA6I/6T) — transparent, without a pronounced melting point.
This context matters: when standard PA6/PA66 fall short on heat, moisture or dimensions, the answer lies not between them but in moving to long-chain, carbon-filled or specialised systems. As a separate note, additive manufacturing (3D printing) is dominated precisely by PA12 and PA11, not by PA6/PA66.
2. The chemical basis of the difference
The fundamental distinction arises at the synthesis level and determines all subsequent behaviour in the melt and in the solid state.
Thanks to its linear symmetrical structure, PA66 packs more densely and forms crystalline zones quickly on cooling. This gives a higher melting point (≈ 255–265 °C versus ≈ 220 °C for PA6), higher crystalline-phase density, better creep resistance and lower moisture absorption — but also a narrower processing window. PA6 crystallises more slowly and more isotropically, which makes it technologically "forgiving" and better on the surface.
3. Melt rheology and flow kinetics
This is exactly where most comparisons stop at "both flow", even though the difference is critical for part geometry and tooling.
Viscosity and fillability. In the melt, PA6 has lower viscosity and higher flowability — it easily fills long flow-path/thickness ratios, thin-walled electronics housings and complex internal ribbing without extreme injection pressures and high shear stresses. PA66 has a noticeably higher melt viscosity and a narrower processing window, which calls for a more disciplined temperature regime, precise gate placement and reliable mould venting to guard against short shots and burning.
Shrinkage and anisotropy. The rapid crystallisation of PA66 produces higher shrinkage (typically 1.5–2.5 % for unfilled grades), which is strongly anisotropic (different along and across the flow) — this raises the risk of warpage in asymmetric parts. PA6 shrinks less and more isotropically (1.0–2.0 %), so it holds tolerances "off the mould" more easily. With glass reinforcement the shrinkage of both drops sharply (to 0.3–0.8 %), but the difference between the longitudinal and transverse directions becomes even more pronounced — hence the critical role of gate location.
| Parameter | PA6 | PA66 |
|---|---|---|
| Melt temperature | ≈ 230–260 °C | ≈ 270–300 °C |
| Mould temperature | ≈ 40–90 °C | ≈ 60–100 °C |
| Melt viscosity | lower, high flowability | higher, narrower window |
| Shrinkage (unfilled) | 1.0–2.0 %, more isotropic | 1.5–2.5 %, anisotropic |
| Shrinkage (GF30) | ≈ 0.3–0.7 % | ≈ 0.4–0.8 % |
| Drying before moulding | mandatory for both; residual moisture in the melt causes chain hydrolysis and a drop in viscosity/strength | |
4. Thermal behaviour: HDT, service temperature, creep
Heat resistance in high-temperature environments (under-hood space, industrial equipment) is the main differentiator of PA6 and PA66.
Unfilled PA6 and PA66 have a relatively low heat deflection temperature (HDT) under load. Glass fibre "locks" the polymer matrix and prevents chain slippage on heating: PA66-GF30 reaches HDT ≈ 250 °C, whereas PA6-GF30 — about 205 °C. This margin is critical during short-term thermal peaks (paint ovens, local radiation from the exhaust).
Creep. Because of denser packing and higher crystallinity, PA66 resists creep considerably better under sustained load. For a gear, a threaded joint or a structural hinge under constant force, PA6 slowly "relaxes" over months — with a loss of tightening torque or loss of alignment. PA66 retains structural preload substantially longer.
5. Mechanics: stiffness versus impact strength
If the part is a rigid loaded chassis, specify PA66. If it has to survive sudden impacts, repeated snap-fits or cyclic vibration without cracking — choose PA6. Unfilled polyamides deliver excellent toughness but not the modulus for rigid frames; compounding with glass (30–35 % by mass) turns them into structural materials.
| Property | PA6 | PA66 | PA6-GF30 | PA66-GF30 |
|---|---|---|---|---|
| Melting point | ≈ 220–223 °C | ≈ 255–265 °C | ≈ 220 °C | ≈ 255–265 °C |
| Density, g/cm³ | ≈ 1.13 | ≈ 1.14 | ≈ 1.35 | ≈ 1.36 |
| Tensile strength, MPa | ≈ 75–85 | ≈ 80–90 | ≈ 175–185 | ≈ 190–200 |
| Elastic modulus, GPa | ≈ 2.8–3.2 | ≈ 3.0–3.6 | ≈ 9–10 | ≈ 9.5–10.5 |
| HDT A (1.8 MPa), °C | ≈ 55–70 | ≈ 70–85 | ≈ 200–210 | ≈ 245–255 |
| Moisture absorption, equilibrium 50 % RH, % | ≈ 2.8 | ≈ 2.5 | ≈ 1.5 | ≈ 1.3 |
| Moisture absorption, saturation, % | ≈ 9.5 | ≈ 8.5 | ≈ 6.0 | ≈ 5.5 |
The values are typical for the class, not the data sheet of a specific grade; verify exact data for your part against the TDS and on a trial moulding.
6. Moisture absorption: the hidden engineering risk
Both materials are hydrophilic. Absorbed water acts as a plasticiser and changes properties over time:
- tensile strength and stiffness drop significantly;
- impact strength and elongation increase;
- part dimensions grow (swelling).
PA6 absorbs moisture faster and has a higher capacity: ≈ 9.5 % at full saturation and ≈ 2.8 % at equilibrium (23 °C, 50 % RH). PA66 — approximately 8.5 % and 2.5 % respectively. The difference of 0.3–1.0 % by mass may seem negligible, but it produces substantial dimensional deviations: an unfilled dry-as-moulded (DAM) PA6 part can swell by 2.5–3.0 % in volume at full saturation — enough to seize up assemblies with tight tolerances or guides. PA66 grows less.
7. Friction and wear
PA66's higher surface hardness gives a lower coefficient of friction against steel and other polymers. Combined with a higher thermal threshold, unfilled or internally lubricated (PTFE / MoS₂) PA66 outperforms PA6 in sliding assemblies. For unlubricated kinematics — conveyor guides, bushing bearings, cams, gear drives — PA66 resists abrasive wear and local frictional overheating better. If even lower wear is needed, consider PTFE / carbon-fibre compounds or polyacetals (POM) as an alternative class for friction pairs.
8. Surface cosmetics and fibre "read-through"
This is an aspect almost always ignored in a formal substitution, yet it is decisive for visible parts. PA6 delivers noticeably better surface quality: because it flows easily and crystallises slowly, a resin-rich layer forms next to the mould wall, "hiding" the glass fibre beneath a surface skin.
When moulding PA66-GF30, rapid crystallisation often prematurely "freezes" the flow front, and the fibres orient themselves toward the mould surface. The result is a matte, rough surface with visible silvery streaks (the fibre "read-through" effect). For external control handles, visible housings and aesthetic elements, PA6-GF30 delivers a glossy premium surface that PA66 cannot achieve without expensive cyclic heat-and-cool tooling (RHCM).
9. Chemical compatibility
Polyamides have excellent general resistance to hydrocarbons and organic solvents, but are vulnerable to acids and strong bases.
| Medium | PA6 / PA66 | Comment |
|---|---|---|
| Hydrocarbons, fuels, oils | good resistance | the core strength of polyamides |
| Alcohols, glycols (antifreeze) | moderate / good | hot glycol at temperature reduces service life |
| Weak bases | moderate | depends on concentration and temperature |
| Acids (even dilute) | low | hydrolysis of amide bonds, loss of strength |
| Strong bases, hot water/steam | low / limited | hydrolysis; for hot water consider PPA, PA46 or PA612 |
| Zinc chloride (road salt) | low in PA6/PA66 | stress cracking; advantage in PA610/PA612 |
10. Summary of key differences
| Criterion | Advantage | Why |
|---|---|---|
| Heat resistance, short-term peaks | PA66 | higher Tm and HDT (GF30: 250 versus 205 °C) |
| Creep resistance | PA66 | denser packing, higher crystallinity |
| Dimensional stability in a humid environment | PA66 | lower moisture absorption and swelling |
| Friction and wear (unlubricated pairs) | PA66 | higher hardness, lower coefficient of friction |
| Processability, filling of thin walls | PA6 | lower viscosity, wider window |
| Holding tolerances "off the mould" | PA6 | lower and more isotropic shrinkage |
| Impact strength, snap-fits, vibration | PA6 | slower crystallisation, greater ductility |
| Surface quality (gloss, no read-through) | PA6 | resin skin hides the fibre |
| Raw-material cost | PA6 | usually 10–20 % cheaper |
How to use this analysis in material selection
The technical mechanism is the foundation, but the final decision is made based on the behaviour of the finished product: service temperature, moisture state, load duration, geometry, reinforcement, moulding regime, certification and the required service life. We have laid out the economic and scenario-based selection logic in detail in a separate piece — "PA66 vs PA6: why it is better", together with a substitution-check checklist.
Before replacing PA6 with PA66 (or vice versa): 5 control questions
1. What is the actual service temperature under load? If there are short-term thermal peaks or sustained heat — PA66 has the advantage (higher Tm/HDT); for moderate temperatures PA6 is often sufficient and cheaper.
2. Does the part carry a constant load or experience impacts? Constant preload (thread, gear, hinge) — PA66 (creep); impacts, snap-fits, vibration — PA6 (ductility).
3. How critical is dimensional accuracy in a humid environment? Tight tolerances + moisture — PA66 (less swelling); and always calculate using conditioned (50 % RH) data, not DAM data.
4. Is the part visible? For glossy visible housings PA6-GF delivers a better surface without fibre read-through; PA66-GF without RHCM tooling is matte.
5. Does the geometry tolerate PA66's processing window? Thin walls, long flow paths and complex ribbing are more easily filled by PA6; PA66 requires a disciplined regime, gating and venting.
Material Wizard grades for both bases
Material Wizard produces engineering polyamides under the Examid® brand on PA6 and PA66 bases at various reinforcement levels to our own formulation. If you have a sample or specification from another manufacturer, we will select a comparable material made to our formulation.
Examid® PA6 GF30Process base: easy moulding, better surfaceModulus 9–10 GPa · gloss without read-through · thin wallsRequest supply terms → Examid® PA66 GF30Heat-stabilised: heat, creep, wearHDT ≈ 250 °C · engine compartment, friction assembliesRequest supply terms →All grades can be purchased with delivery across Ukraine. The price depends on the base, the reinforcement level, the batch volume and lead times — check with a specialist: we will compare PA6, PA66, PA-CF and PPA by the behaviour of the finished product, without a formal substitution "by name". An overview of the entire line-up is on the polyamides hub.
Frequently asked questions
Why is PA66 harder to mould than PA6?
PA66 has a higher melt viscosity, a higher melting point and a narrower processing window. Its rapid anisotropic crystallisation increases shrinkage and the risk of warpage, so a more precise temperature regime, gating and mould venting are required.
Why does PA6 give a better surface in glass-filled grades?
PA6 crystallises slowly and flows easily, so a resin layer manages to form next to the mould wall, hiding the glass fibre. In PA66-GF the flow front "freezes" faster, the fibres come out to the surface — hence the matteness and silvery streaks (fibre read-through).
How much does moisture change the dimensions of a part?
An unfilled dry PA6 part can swell by 2.5–3.0 % in volume at full saturation; PA66 — less. For tight tolerances this is critical, so the calculation is carried out using conditioned (50 % RH) data, not DAM data.
What to choose for a friction assembly?
All else being equal, PA66 has the advantage thanks to its higher hardness and lower coefficient of friction. For harsher conditions — lubricated compounds (PTFE/MoS₂), PA-CF or polyacetal (POM).
When do you need to go beyond PA6/PA66 altogether?
For sustained high temperature, hydrolysis, hot water, aggressive chemistry or demanding dimensional stability — move to PPA, PA46, PA610/PA612 or PA12. This is a separate class of solutions.
Material Wizard (Derazhnia, Kharkiv) — a manufacturer, supplier and technology partner in the field of polymer materials. Send us the service temperature, the type of load, the geometry, the tolerance requirements and the operating conditions — we will select a grade for your specific part and support the testing. Check with a specialist.