Polyamides are chains of carbon segments joined by amide groups, and the numbers in the names count the carbons between those joins. That single ratio is the family's organising principle, because the amide groups are what do everything interesting — they hydrogen-bond to each other, giving nylon its toughness, abrasion resistance and heat capability, and they hydrogen-bond to water, which is where the trouble starts.
More amides per unit length means a stiffer, stronger, more heat-capable polymer that absorbs more water and shrinks more. Fewer means the opposite. Every trade-off in this family is that one sentence.
| Material | Amide spacing | Choose it for |
|---|---|---|
| PA6 | one every six carbons | maximum stiffness and abrasion resistance, if you can control humidity |
| PA612 | between the two | parts that must stay stiff while wet |
| PA12 | one every twelve carbons | dimensional stability, fuel and oil contact, and light weight |
When to reach for a polyamide
For a specific mechanical profile that is genuinely hard to get otherwise: low friction against steel, high abrasion resistance, and the ability to flex repeatedly without cracking. Gears, bushings, cable guides, living hinges, tool handles and snap fits are where this family lives, and a nylon gear will outlast a copolyester one by a margin that is not close.
The price of admission is high, and it is worth being honest about it before buying a spool. Nylons run $35 to $90 per kilogram, need an enclosure, need a plate they will actually stick to, and need drying equipment that reaches 80 °C. If your part does not specifically need what nylon offers, almost anything else on this site is a better use of an afternoon.
Water is the family problem, and it is two problems
During the print, damp polyamide degrades chemically as well as cosmetically. The individual pages set out the mechanism; the family-level point is that this damage is permanent, so a rough-looking nylon print is not something to push through and tidy up afterwards.
Afterwards, the finished part keeps negotiating with the weather. A polyamide component grows and softens as the air gets humid and reverses as it dries, which rules the family out of precision work in an unconditioned space and makes it excellent wherever a bit of give is an advantage.
The family drying figure is 8 to 12 hours at 80 °C for all three members, and an ordinary sealed box with desiccant is not enough while a job is actually running.
Shrinkage, and why most people should buy the filled version
Unfilled polyamides contract 0.5% to 2.0% depending on member, the largest range on this site, and PA6 sits at the top of it. That is why unfilled nylon lifts off the plate on any part with a real footprint, and why so many people conclude that nylon is impossible.
Short-fibre reinforcement cuts that contraction to a fraction and turns the family into something a good workshop machine can print reliably. Those grades live with the composites rather than here, and for most functional parts they are the honest recommendation — unfilled polyamide is the connoisseur's choice, not the practical one.
What the machine has to provide
The temperatures across the three members cluster between 250 and 280 °C at the nozzle with the bed somewhere from 60 to 90 °C, and every one of them wants an enclosure with the part-cooling fan off. What genuinely distinguishes this family at the machine is the plate: polyamides do not bond reliably to plain polyetherimide, and Garolite — a woven-glass epoxy laminate — has become the standard surface because it grips a hot nylon and lets go of a cold one.
Two post-processing capabilities are unique to this family and worth knowing. Nylon takes fabric dye in hot water, giving an even through-thickness colour no spray paint matches. And it anneals, gaining stiffness and heat resistance while shrinking again — so anneal the test coupon before the part that has to fit.
Which of the three
Dry environment, stiffness first? PA6. Wet or humid service, and the part must keep its spring? PA612. Fuel, oils, light weight or a workshop with no humidity control? PA12.
And if the part is large, flat, or simply has to work: buy it fibre-filled.