Pick up a toothbrush and you are holding the argument for PA612. Bristles have to stay springy while soaking wet, several times a day, for months. Polyamide 6 would absorb water, plasticise and go limp; polyamide 12 would be soft to begin with. The 6/12 copolymer holds a useful stiffness wet or dry, and it has owned that application for decades.
That is the whole proposition, and it is a narrower one than it sounds — but where it applies, nothing else quite substitutes.
What "6/12" actually specifies
The name records the two building blocks: a six-carbon diamine condensed with a twelve-carbon diacid. The result places amide groups further apart than PA6 does and closer together than PA12 does, so every property that depends on amide density lands between the two.
- Water uptake is well below PA6's and above PA12's.
- Stiffness is above PA12's and a little below PA6's, at 45 to 70 MPa tensile.
- Shrinkage is 0.6% to 1.4%, between PA6's 0.8%–2.0% and PA12's 0.5%–1.2%.
- Density is 1.04 to 1.08 g/cm³, again squarely between them.
There is nothing surprising in any of that, which is the appeal. PA612 is the polyamide whose behaviour you can interpolate.
On the spool that density works out at 392.22 m of 1.75 mm filament per kilogram, at 2.55 g per metre — more length than PA6 gives you and less than PA12. Your own reel may sit a little either side of that, and the length and weight converter will tell you where.
Wet stiffness is the property worth paying for
Every aliphatic polyamide softens as it takes up water, because water molecules insert themselves between chains and break the hydrogen bonds holding them together. The size of that effect scales with how many amide groups there are to hydrate.
For most printed parts this hardly matters, which is why PA6 remains popular. It matters enormously for anything that is wet in service, or that lives in a workshop with no humidity control and must hold a spring rate: a clip that has to keep its grip, a bushing that has to keep its clearance, a fitting that has to keep its seal. A PA6 version of those parts is a different part in August than it was in January.
Why unfilled PA612 is printable at all
Bed adhesion follows PA6 exactly — Garolite or a polyamide-specific sheet, because plain polyetherimide does not hold a nylon reliably — and the thermal settings sit where the family sits, with the nozzle at 250 to 275 °C, the bed at 60 to 90 °C, an enclosure closed and cooling off.
Drying at 80 °C for 8 to 12 hours. Less thirsty than PA6 is not the same as dry, and the melt still hydrolyses when damp — the resulting part looks rough and snaps along layers, and no reprint from the same wet spool recovers it.
Its lower shrinkage than PA6 makes unfilled PA612 genuinely printable on a good enclosed machine, which is more than can be said for unfilled PA6 at any size. That is a quiet but real advantage: you can have a nylon part without the fibre reinforcement, and therefore without the brittleness and the hardened nozzle.
A note on the naming, because it causes real confusion
Polyamide names use two conventions and they collide here. A single number — PA6, PA11, PA12 — means the polymer was made from one monomer that contains both the acid and the amine ends. Two numbers means two monomers, the first counting the carbons in the diamine and the second in the diacid.
So PA612 is a two-monomer copolymer, and it is a different material from PA6 and PA12 rather than a blend of them. It is also a different material from PA66, which pairs the same six-carbon diamine with a six-carbon diacid and is the stiffest and most common industrial nylon. Sellers write these names inconsistently — PA6/12, PA-612, nylon 612 — and a listing that cannot keep the notation straight is a listing worth asking questions about.
The bristle question, and the figure that answers it
PA66 is the material the toothbrush industry moved away from, and it is worth being precise about why, because the obvious explanation is wrong. PA66 is not an unusually thirsty nylon. It takes up marginally less water than PA6 does — roughly 8.5% at saturation against PA6's 9.5%, and about 2.5% against 2.8% for a part conditioned in ordinary indoor air. That ordering follows from crystallinity: PA66's chains pack more regularly than PA6's, leaving less accessible amorphous phase for water to occupy, and the same mechanism is why PA66 melts higher.
The comparison that matters here is not PA66 against PA6 at all. It is PA66 against PA612, and by the same test method it is not close: Ensinger publishes 8.5% at saturation and 0.45% at 24-hour immersion for PA66, against 3.0% and 0.25% for PA612 (ASTM D570, 23 °C — both figures from the same publisher's datasheets, which is what makes them comparable). A bristle spends its whole working life wet. A PA66 filament in that service loses stiffness as it takes water up, splays, and stops cleaning; a PA612 filament, holding a third as much water at worst, keeps most of its bending resistance. That property — wet stiffness retention, not dryness in the abstract — is what DuPont's Tynex 612 bristle line was built around and what kept it the standard.
What PA612 is genuinely bad at
- Availability. This is the material's biggest practical problem. Few brands stock it, colours are limited, and a project that depends on being able to reorder in a year is a project at risk.
- Price, at $45 to $85 per kilogram, close to PA12 and well above PA6, for properties that sit between them.
- Being obviously better than the alternatives. For any given part, PA6 is usually cheaper and PA12 usually more stable. PA612 wins on the specific combination, and if your part does not need that combination the money is wasted.
- Open-frame printers, like every polyamide here.
- Adhesives and paint, defeated by the same chemical resistance that makes it useful.
- Heat. Deflection between 55 and 95 °C depending on load and crystallinity; this is a moisture-stability material, not a high-temperature one.
Where to choose it
Wear parts and bushings in damp environments. Clips, catches and spring elements that must hold their force. Fluid fittings and cable glands. Brush and comb-like geometries, which is where the industrial volume still goes. Replacement parts for existing PA612 components, where matching the water uptake matters as much as matching the shape.
The decision is a two-step one. If the part will be dry and stiffness is the priority, PA6 is cheaper and stiffer. If dimensional stability outranks everything, PA12 absorbs even less. PA612 is what you choose when you need most of the first and most of the second at once — and the PA6 against PA12 comparison is the clearest way to see the gap this copolymer is filling.