Every nylon is a chain of carbon segments joined by amide groups, and the number in the name is the count of carbons between them. Polyamide 6 places an amide every sixth carbon. Polyamide 12 places one every twelfth — so along a given length of chain there are half as many of the polar groups that grab water, hydrogen-bond to neighbouring chains and drive crystallisation.
Almost every practical difference between the two polyamides is a consequence of that ratio, which makes PA12 one of the few materials whose behaviour you can predict from its name.
Half the amides, most of the moisture problem gone
PA6 will happily take up a few percent of its own mass in water from ordinary indoor air, swelling and softening as it does. PA12 takes up a fraction of that, reaches equilibrium sooner, and moves far less in the process. A PA12 part measured in a dry workshop in winter and again in a humid August is close to the same part; a PA6 one is not.
That does not excuse you from drying it. Absorbed water still hydrolyses the melt and still steams in the nozzle, so 8 to 12 hours at 80 °C before a serious print remains the standard. The difference is on the other side of the process: PA12 stays trustworthy in service, where PA6 keeps negotiating with the weather.
Shrinkage follows the same logic. Fewer amide groups mean weaker chain-to-chain attraction and less aggressive crystallisation, so PA12 contracts 0.5% to 1.2% against PA6's 0.8% to 2.0%. It still warps — it is still a semi-crystalline polyamide and it still wants an enclosure — but it warps at a level a careful setup can manage, where unfilled PA6 often cannot.
The lightest common filament that is not a foaming trick
PA12's density is around 1.01 g/cm³, near enough to water that a print sits neutrally buoyant. That is remarkable for an engineering plastic and it has a direct effect on what a spool is worth.
A 1 kg reel of 1.75 mm PA12 holds 411.64 m of filament. The same kilogram of PA6, at 1.14 g/cm³, holds 364.69 m — 46.94 m less, because PA6 is 11.4% denser. So although PA12 is priced higher per kilogram, $45 to $90 against PA6's $35 to $70, part of that gap closes as soon as you compare metres rather than mass. Run your own two spools through the length and weight converter before concluding it is the expensive option.
Low density also means light parts, which is precisely why PA12 dominates in powder-bed printing: it is the standard material of industrial SLS, and a surprising number of "nylon" service-bureau parts are this polymer rather than PA6.
Fuel, oil and hydraulic fluid
PA12 is the polymer of automotive fuel lines, brake and pneumatic tubing and cable jacketing, and it holds that position on chemical resistance. It shrugs off petrol, diesel, most oils, greases and hydraulic fluids at temperatures where a copolyester would swell and a styrenic would craze.
For a printed part this is a genuinely rare capability. A funnel, a fitting, a pump housing or a workshop tool that will live in contact with solvents is a job PA12 can take and most desktop filaments cannot.
A profile that has to allow for softness
The plate is where PA12 is kinder than its stiffer relative: textured PEI often holds it, where PA6 usually demands a Garolite sheet, and the reduced warping means a modest chamber temperature goes further than you would expect. Set the nozzle between 250 and 275 °C with the bed at 60 to 90 °C, keep the enclosure closed, and leave part cooling off or very low.
The tuning note specific to PA12: it is soft. Tensile lands at 40 to 60 MPa and the stiffness is visibly below PA6's, so a thin PA12 wall flexes where you expected it to hold. Add wall count rather than infill, and expect to redesign a part that was dimensioned for a stiffer material.
What PA12 is genuinely bad at
- Rigidity. If the part must not deflect, this is the wrong nylon and possibly the wrong family.
- Price, both per kilogram and in availability — fewer brands stock it, and colour choice is thin.
- Open-frame printers. Better than PA6, still not viable for anything large.
- Precision without a dry chamber. Better than PA6, still a polyamide.
- Being glued or painted. Its chemical resistance is exactly what stops adhesives and coatings keying to it.
- High temperature. Heat deflection is quoted between 50 and 90 °C depending on load and crystallinity, which is a wide band and a reminder that a PA12 part near its limit is a part whose behaviour depends on how it cooled.
Where it is worth choosing over PA6
Snap fits and living hinges that must survive thousands of cycles without absorbing their way out of tolerance. Fluid-contact parts. Lightweight housings where every gram counts. Anything that has to hold a dimension in a workshop with no humidity control.
Where PA6 still wins is stiffness, abrasion resistance under load and cost, which is why gears and bushings usually stay with it. The PA6 against PA12 comparison sets the two out property by property, and the honest summary is that this is a choice between dimensional peace of mind and mechanical performance.