Nylon 6 vs Nylon 66 — the difference that matters

Nylon 6 vs Nylon 66 — the difference that matters.

PA6 and PA66 look identical and are often used interchangeably. For many applications they are interchangeable. But PA66 has higher strength, higher heat resistance, and better creep performance — at 15-25% higher cost. This guide covers when the upgrade matters and when PA6 is the better economic choice.

§ 01 / THE

The chemistry difference

PA6 (Nylon 6) is polymerized from caprolactam — a 6-carbon ring. PA66 (Nylon 66) is polymerized from hexamethylenediamine + adipic acid (6+6 carbon building blocks). The "66" in the name means 6 carbons in each monomer.

Practical implications:

PA66 has higher crystallinity due to its more regular structure
PA66 melts higher (260 °C vs 220 °C for PA6)
PA66 has better creep resistance at elevated temperatures
PA6 absorbs water faster (though both are hygroscopic)
PA6 processes easier (lower melt viscosity, better mold filling)

§ 02 / PROPERTY

Property comparison

Property	PA6	PA66
Tensile strength	70 MPa	85 MPa
Flexural modulus	2,500 MPa	2,800 MPa
Heat deflection temp (0.46 MPa)	180 °C	200 °C
Heat deflection temp (1.8 MPa)	70 °C	90 °C
Melt temperature	220 °C	260 °C
Continuous service temp	80 °C	100 °C
Water absorption (24h)	1.5%	1.2%
Water absorption (saturated)	10%	8.5%
Density	1.14 g/cm³	1.14 g/cm³
Cost (per kg)	~$3.50	~$4.20

PA66 wins on strength, stiffness, and heat resistance. PA6 wins on processability and cost. Both absorb significant water, which affects dimensions and properties — a common design issue for both grades.

§ 03 / THE

The water absorption issue

Both nylons absorb water from ambient humidity. This causes:

Dimensional growth (up to 2% at saturation)
Reduced stiffness (modulus drops 40-50% when saturated)
Improved impact strength (water plasticizes the polymer)
Faster creep under load

Design implications:

Dry-machined tolerances shift when parts equilibrate to ambient humidity — allow 0.5-1% dimensional change
For tight-tolerance applications, condition parts to service humidity before final measurement
Structural calculations should use saturated properties, not dry data sheet values
For outdoor or wet service, consider PA12 (0.3% water absorption) or acetal instead

§ 04 / GLASS-FILLED

Glass-filled variants

PA6-GF30 (30% glass fiber) and PA66-GF30 dramatically change properties:

Property	Unfilled	+30% Glass Fiber
Tensile strength (PA66)	85 MPa	180 MPa
Flexural modulus (PA66)	2,800 MPa	9,500 MPa
Heat deflection (1.8 MPa)	90 °C	250 °C
Machinability	Good	Tool-aggressive (carbide only)
Water absorption	8.5%	4%
Relative cost	1.0× (baseline)	1.6×

Glass fill triples the stiffness. Used for structural parts under load (automotive engine covers, industrial housings). Downside: much harder to machine, shorter tool life, rougher surface finish possible.

§ 05 / WHEN

When to spec PA6 vs PA66

Standard industrial mechanical parts → PA6

Cost-effective, widely stocked, good properties. For gears, bushings, bumper blocks under moderate load. 80% of nylon applications.

Automotive under-hood → PA66 or PA66-GF30

Service temperature above 80 °C combined with mechanical load. PA66 handles 100 °C continuous; PA66-GF30 goes to 150 °C+.

High-load structural → PA66-GF30 or higher fill

Stiffness comparable to aluminum at half the weight. Used where glass fill is needed for stiffness, not just strength.

Fast-cycle injection molding → PA6

Lower melt temperature means faster cycles. For high-volume parts where cycle time matters, PA6 saves cost over PA66.

Dimensional stability priority → PA12 or acetal

Neither PA6 nor PA66 is good for tight dimensional stability in variable humidity. PA12 absorbs 1/10 the water. For precision optics or instrument parts, consider acetal or PA12 instead.

READY WHEN YOU ARE

Nylon parts — PA6 or PA66?

Email [email protected] with your drawing. Tell us the service temperature and humidity range — we'll recommend the appropriate grade and call out any water-absorption design considerations.

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