In short
- PA66 melts 40 °C higher (260 °C) than PA6 and resists creep better; it is the first candidate for parts running continuously above 100 °C.
- PA6 is tougher, fills more easily and has a glossier surface; it is usually preferred for visible parts and thin-walled moulds.
- Both families absorb moisture (PA6 about 9–10%, PA66 8–8.5% saturated); conditioning must be planned where dimensional stability matters.
- Glass fibre raises stiffness and HDT markedly in both; GF30 grades give about 240–250 °C HDT for PA66 and 200–210 °C for PA6.
When choosing between PA6 and PA66, the first question is not price but the real temperature and environment the part works in. Although the two families are branches of the same monomer chemistry, they differ in melting point, crystallisation rate, moisture behaviour and processing window. This article explains the comparison asked most often in quote discussions, using the criteria our technical team applies.
What is the basic difference?
PA6 is made from a single monomer, caprolactam; PA66 from two, hexamethylenediamine and adipic acid. This structural difference gives PA66 a more ordered crystal structure: a melting point of about 260 °C (220 °C for PA6), higher stiffness and creep resistance, and less dimensional change under heat. PA6 crystallises more slowly, which makes it tougher, glossier and easier to fill in the mould.
| Criterion | PA6 | PA66 |
|---|---|---|
| Melting point | 220 °C | 260 °C |
| Density | 1.13 g/cm³ | 1.14 g/cm³ |
| Tensile strength (dry) | 75–85 MPa | 80–90 MPa |
| Moisture uptake (saturated) | 9–10% | 8–8.5% |
| Processing temperature | 230–260 °C | 275–295 °C |
| Typical advantage | Toughness, surface, easy processing | Temperature, stiffness, creep |
Values are typical ranges for the material families; the binding values are in the supplier data sheet sent with the quotation.
When does temperature make PA66 mandatory?
If the part runs continuously above 100 °C or sees short peaks above 150 °C, PA66 is the right starting point. Radiator tanks, intake manifolds and cooling-circuit parts in the engine bay are made from glass-fibre PA66 for this reason. PA6 is comfortable in the 80–100 °C band and sufficient for gears, bushings, cable ties and power-tool housings. If the temperature goes above 150 °C and stays there, both families are at their limit; then PA4.6 or PPA should be considered.
Why does moisture matter?
Both polyamides absorb moisture from the air, and a part that absorbs moisture both softens and grows. Dry PA6 is brittle while conditioned PA6 is very tough, which is why impact tests are usually run on conditioned specimens. If dimensional stability is critical there are two routes: condition the part after moulding in a controlled way, or choose a family with low moisture uptake. PA66 absorbs slightly less than PA6, but the real difference appears in the long-chain polyamides (PA6.10, PA6.12, PA12).
Does glass fibre close the gap?
Glass fibre doubles or triples stiffness in both families and lifts the heat deflection temperature close to the melting point. In GF30 grades PA66 gives about 240–250 °C HDT and PA6 200–210 °C; the reinforcement does not close the gap, it moves both families up together. On the other hand, glass-fibre PA6 fills more easily and has a better surface, which is why PA6 GF30 is often chosen for visible structural parts.
Cost and supply
PA66 feedstock is usually more expensive and its price is more sensitive to supply swings than PA6. Our technical team therefore suggests starting with PA6 if the part runs below 100 °C, and moving to PA66 only when temperature and creep really demand it. Natural and black, GF15 and GF30 grades of both families are in our stock programme; equivalent comparisons are always made with two suppliers' data sheets side by side.
A short checklist
- Continuous operating temperature above 100 °C? If yes, PA66.
- Is the part visible and surface gloss important? If yes, PA6.
- Tight tolerances and a humid environment? Consider a long-chain polyamide or PPA.
- Creep under load? PA66 or a glass-fibre grade.
- Thin walls and long flow paths? PA6 fills more easily.
Do not decide without a sample: we send both candidates for a trial and share starting values for drying and mould temperature in writing.
Frequently asked
If I replace PA6 with PA66, do I need to change the mould?
Usually not, but mould temperature and drying change: PA66 needs a higher melt temperature (275–295 °C) and a hotter mould; shrinkage is similar but filling thin walls can become harder. We do not recommend switching without a sample trial.
Is PA66 always more expensive than PA6?
Mostly yes; PA66 feedstock (adipic acid and HMDA) is more expensive and its price swings with supply. If your part runs below 100 °C, PA6 is usually sufficient and more economical.
Which absorbs less moisture?
PA66 absorbs slightly less (about 8–8.5% saturated versus 9–10% for PA6). If moisture is the real problem, long-chain polyamides (PA6.10, PA6.12, PA12) or PPA perform far better.
About this article: Forbest Plastik technical team — Application-based material selection experience in engineering plastics, elastomers and compounds. Values are typical family ranges; the binding value is the supplier data sheet.