PA66 Hardness: Rockwell R120, Shore D & Conditioned Values

A datasheet says R120. Your incoming inspection on the same grade reads 108. Nothing changed in the resin.

That gap is the whole story of PA66 hardness, and it catches more engineers than any other nylon number. PA66 is the rare high-volume engineering plastic whose hardness has a shelf life. ABS, PC, and POM hold their figure once the part leaves the mold, because they absorb almost no water. Nylon’s figure tracks how much moisture it has taken on.

This guide covers PA66 hardness dry and conditioned on every measurement system, explains why water moves the number, shows how fillers shift it, and shows how to keep a validated hardness band meaningful when you buy. The complete guide to plastics hardness covers the rest of the field.

What Is PA66 Hardness?

PA66 hardness is its resistance to indentation under a defined load. Unfilled grades read Rockwell R119–R120 dry as molded, dropping to roughly R104–R108 once conditioned to 50% relative humidity. The same material reads roughly M79–M85 on the M scale, about Shore D 80–85 dry and 75–80 conditioned, and 160 MPa by ball indentation dry.

Search results for nylon 66 hardness, polyamide 66 hardness, and PA66 hardness all point at the same material, and every one of those figures carries a condition, a scale letter, and a test method. Strip away any of the three and a specification turns back into a number. Hardness is a surface test only: an unfilled grade runs about 5.0 kJ/m² Charpy notched impact and can still shatter in a cold drop.

Why PA66 is quoted on the Rockwell R scale

Unfilled nylon is softer than PC, POM, or glass-filled grades, so labs test it on the Rockwell R scale: a 1/2-inch (12.7 mm) steel ball under a 60 kgf major load. Harder polymers use the M scale, a 1/4-inch ball under 100 kgf. PA66 Rockwell hardness for unfilled grades reads roughly R120, and POM also carries an R120 figure, but POM’s specification value is M94: two identical-looking numbers, two different tests, and no valid conversion between them. Our Rockwell hardness of plastics article walks through every scale letter.

The test methods, and why the condition matters most

Rockwell R and M come from ASTM D785 or ISO 2039-2, and MatWeb’s Rockwell hardness reference documents the ball diameters and loads behind each scale. Ball indentation comes from ISO 2039-1 in MPa, Shore D from ASTM D2240 or ISO 868, and water absorption from ISO 62 or ASTM D570. That last method decides which of the other three applies: a POM value stands on its own, but a nylon value doesn’t, because water plasticizes polyamide.

Need a PA66 hardness value you can defend on a drawing? Request a quote for branded prime nylon 66 with datasheets on the correct scale, condition, and test standard.

PA66 Hardness Dry vs Conditioned: The Two Numbers

No datasheet assembles the full nylon 66 hardness chart, because each one reports only the state its customers ask for.

Dry as molded: Rockwell R119–R120

Dry as molded means tested shortly after molding, holding roughly 0.1 to 0.3% moisture. GEHR Plastics publishes 160 MPa (23,200 psi) ball indentation under ISO 2039 for PA 6.6 dry . DAM appears on most datasheets because it’s the easiest state to reproduce, and it’s also the state your part spends the least time in.

Conditioned at 50% RH: the value parts actually run at

Conditioning brings the specimen into equilibrium with a controlled atmosphere, 23 °C at 50% relative humidity per ISO 291. Conditioned, unfilled PA66 reads roughly R104 to R108. This is the honest number for a part in a climate-controlled plant, and the one most datasheets omit.

The gap on one real grade

A representative unfilled PA66 sold as Nypol® PA A3 publishes Rockwell R 120 dry as molded and R 104 conditioned, both to ASTM D785, in one table with the condition labelled. That is a 16-point drop on the same material, produced by water alone, which is why the conditioned band is grade- and method-dependent.

Yusuf, a quality manager at an underhood connector molder in Bursa, learned the difference the hard way. His team molded a glass-filled PA66 bracket, tested the audit plaques dry, and shipped them inside the validated R-scale band. The customer rejected the lot anyway, because their reference coupon had sat in a 50% RH lab and read eight points lower. The parts were fine; the specification was not.

Why Moisture Moves PA66 Hardness

Water isn’t an inert filler in nylon; it bonds to the polymer and changes how the chains move.

The mechanism in one paragraph

PA66 chains are held together partly by hydrogen bonds between amide groups. Water molecules are small enough to reach those sites and occupy them, which breaks chain-to-chain bonding, increases chain mobility, and plasticizes the polymer, so the surface resists indentation less. The effect reverses when the part is dried.

The same mechanism in numbers you can check

Hardness is the visible symptom, and the modulus collapse behind it is far larger. BASF Ultramid® A3K, the reference unfilled PA66, loses roughly 70% of its tensile modulus on conditioning, from about 3,000 to 3,600 MPa dry down to around 1,100 MPa, and tensile stress at yield falls about 40%, from roughly 85 MPa to 50 MPa (ISO 527). Conditioned nylon is also softer and tougher, so the common claim that nylon loses 20 to 40% of its properties to moisture is right for stiffness and wrong for ductility.

“Conditioned at 50% RH” doesn’t mean equilibrium

Here is the caveat almost no datasheet prints. ISO 291 defines the conditioning atmosphere, not the conditioning time. Datasheet practice commonly means about 40 hours at 23 °C and 50% RH.

True equilibrium is different: a 4 mm PA66 specimen can take more than a year to get there, because water diffuses through nylon slowly. A “conditioned” value is therefore a snapshot, so ask which protocol produced it. The same absorbed water drives the thermal behavior in our PA66 melting point and heat resistance guide.

Nylon 66 Hardness by Grade: Unfilled, Glass-Filled and Modified

Unfilled nylon is the baseline. Every additive family moves the number, and the direction matters more than the size.

Glass-filled PA66 hardness: the filler raises the reading

Glass fibre is the one common additive that makes PA66 harder, and the evidence is direct. Celanese Zytel® 70G33L, a 33% glass-fibre PA66, is published at Rockwell M 101 under ISO 2039-2 . Set that against roughly M79 to M85 unfilled and the direction is unambiguous. The same glass lifts tensile modulus to about 10,000 MPa and heat deflection temperature to 250 °C.

For anyone searching “PA66 GF30 hardness,” this is the point most datasheets bury: a reinforced grade is normally reported on the M scale like any other nylon. At very high loadings a lab can report on a heavier Rockwell scale where the M-scale ball no longer fits, and that number is then not comparable to an unfilled nylon value.

Everything else in that table trades hardness away. Impact-modified and flame-retardant (UL94 V-0) grades soften the surface, so if you need V-0 and a hardness floor, specify both. Heat-stabilized grades publish most completely: Leona™ 14G50 lists Rockwell M for both the dry and the 50% RH state.

Is PA66 Harder Than POM?

Sometimes. The determining variable is water.

Dry, PA66 wins on ball indentation. Conditioned, POM wins.

Dry, unfilled PA66 reads about 160 MPa by ball indentation against roughly 140 MPa for unfilled POM, so PA66 is the harder material dry. Conditioned, the ranking reverses: PA66 absorbs around 2.5% water at 23 °C over 24 hours and keeps softening toward equilibrium, while POM absorbs under 0.25% and barely moves. In humid or wet service, POM is the harder material and stays that way for the life of the part. Our POM hardness guide owns the acetal side.

Anneke, a design engineer at a pump manufacturer in Eindhoven, found where that ranking breaks. Her team specified PA66 for a coolant pump gear because the dry datasheet showed it harder than the acetal alternative. In continuous wet service the nylon gear grew, the mesh tightened, and the drive drew more current than the motor was rated for. Switching to POM solved it, not because POM is harder, but because POM holds its dimensions through humidity.

PA66 versus nylon 6, ABS, and polycarbonate

Bare numbers are only comparable when the scale letters and the condition match.

Comparing PA66 hardness vs PA6, PA66 is harder by roughly 12% dry and far more once conditioned. Hardness also doesn’t predict wear: unfilled PA66 runs near 0.11 coefficient of friction by ASTM D3702 with a wear rate of 1×10⁻⁵ to 5×10⁻⁵ in/h at PV2 , and the engineering plastic hardness comparison puts the full field side by side.

Branded PA66 Grades and Their Hardness Baselines

Prime manufacturers publish hardness with the scale and test method attached, which is what makes their data the reference point.

Prime unfilled PA66 lands around R120 dry and M80 on the M scale, so a wildly different figure for a standard grade is worth questioning. Ultramid®, Durethan®, Zytel®, Vydyne®, Technyl®, and Amilan® are the families your drawings are most likely to name.

Sourcing Consistency: Keeping a PA66 Hardness Spec Meaningful

On nylon, hardness is both a design property and a fast QC check, and drift tells you what is in the bag.

Rafael, an incoming-inspection lead at a molder in Monterrey, watched Rockwell R readings on a new PA66 lot come in at 99 against a validated band of 108 to 115. The parts still molded, so nobody flagged it at first. A moisture-content check on the certificate of analysis came back at 0.45%, well above specification. The lot had taken on water in storage, and drying it brought the reading back inside the band.

What a shifting reading means, and what to check on a COA

Lot-to-lot Rockwell drift on nylon points to one of five things:

• Off-spec or substituted resin
• Regrind blended into virgin
• A filler-content change
• A moisture difference at the time of test
• A crystallinity shift from cooling

Confirm the grade designation, the melt flow index (ASTM D1238 or ISO 1133), the moisture content, the filler content, and the lot number, then cross-check them against the manufacturer’s datasheet. Moisture mass fraction explains more off-band readings on nylon than any other single number.

How Yifuhui supplies hardness-consistent PA66

We hold branded prime PA66 from the BASF Ultramid®, Lanxess/Envalior Durethan®, Celanese Zytel®, and Ascend Vydyne® families, with traceable lots and full COA and MSDS documentation that includes moisture content. Minimum order is 25 kg, FOB Shanghai, so the hardness band you validated is the band you reorder. See our PA66 resin supplier page for datasheets on your grade.

Frequently Asked Questions

What is the hardness of PA66?

Rockwell R119–R120 dry as molded (ASTM D785), falling to roughly R104–R108 conditioned at 50% RH. The same material reads approximately Shore D 80–85 dry, 75–80 conditioned, M79–M85 on the M scale, and 160 MPa by ball indentation dry.

Why does PA66 hardness change with moisture?

PA66 hardness changes because water molecules hydrogen-bond to the amide groups in the chain, breaking chain-to-chain bonding and acting as a plasticizer, so the material loses indentation resistance and stiffness together. Conditioning drops unfilled PA66 from about R120 to R104 or below, and drying reverses it.

Is PA66 harder than POM?

Dry, yes by ball indentation: 160 MPa for PA66 against about 140 MPa for POM. Conditioned, no, and POM stays harder because it absorbs under 0.25% water against nylon’s roughly 2.5%. The two also sit on different Rockwell scales, so their R values can’t be compared.

Is nylon 66 harder than nylon 6?

Yes. Both read about R120 dry, but after moisture PA66 holds around R108 while PA6 falls to roughly R85, so PA66 holds its band through humid service where PA6 doesn’t.

What is the Shore D hardness of PA66?

PA66 Shore D hardness is approximately 80–85 dry and 75–80 conditioned. Treat Shore D as a screening scale for nylon rather than a specification scale, because Rockwell R or M is what prime datasheets publish (Shore hardness scale explained).

Conclusion

PA66 hardness comes down to two numbers and one rule. The numbers: Rockwell R119–R120 dry as molded and roughly R104–R108 conditioned. The rule: a nylon hardness value without its condition, its scale, and its method isn’t a specification.

The mechanism is water, not grade. Absorbed moisture plasticizes the amide hydrogen bonding, which is why unfilled PA66 loses about 40% of its yield strength and 70% of its modulus on conditioning. Water is also why POM wins in wet precision service, and why hardness drift is one of the fastest signals of badly stored resin.

Specifying nylon 66 for a part with a validated hardness, a gear mesh, or a connector tolerance? Request a quote for branded prime PA66 (Ultramid®, Durethan®, Zytel®, Vydyne®), 25 kg minimum order, full COA documentation including moisture content, FOB Shanghai, response within 24 hours. To gain a deeper understanding of ABS Plastic Hardness and Polycarbonate Hardness, please refer to our accompanying guide.