Polyamide 6 is made by opening the caprolactam ring and stitching the units into one long chain, so every sixth carbon along the backbone carries an amide group. Those amide groups are the entire story. They hydrogen-bond to the amide groups on neighbouring chains, and that network of bonds is what makes nylon tough, abrasion-resistant, slippery against steel and stable at temperatures that would slump a copolyester.
The awkward part is that a water molecule is also very good at hydrogen bonding, and it competes.
Water does not just spoil the print, it changes the part
Absorbed water wedges itself between chains and pushes them apart, breaking some of the chain-to-chain bonds and replacing them with chain-to-water bonds. The polymer becomes more flexible, tougher against impact, less stiff — and physically larger. A nylon part conditioned in humid air is measurably bigger than the same part fresh from a dry oven, and it will keep drifting for weeks until it reaches equilibrium with its surroundings.
No other filament on this site does that. It is why nylon is the wrong answer for a precision spacer in a damp workshop and the right answer for a gear that has to survive being hit. If you want the chemistry behind why an amide backbone behaves this way while a polyester one does not, PeriodicDeck is a useful companion.
Inside the hotend the same affinity is destructive rather than interesting. Water flashes to steam, blows the extrudate into foam, and — worse — hydrolyses the chain, cutting long molecules into short ones. A part printed from wet nylon is not merely rough; it is chemically shorter and permanently weaker.
- Dry at 80 °C for 8 to 12 hours before a long job.
- Print from a heated dry box, not just from a dried spool. PA6 re-absorbs enough in a few hours of open air to undo the drying.
- If extrusion starts hissing, popping or leaving a blistered surface mid-print, stop. It will not improve.
Two percent is a lot
Shrinkage runs from 0.8% up to 2.0%, the largest range of any material here, and the reason is crystallinity: as PA6 cools, the chains fold into ordered regions that occupy less space than the melt did. A 60 mm gear at the top of that range lands at 58.8 mm — you would model it at 61.22 mm to get 60, a correction of 1.22 mm. Work your own figure in the shrinkage compensation calculator, because the exact value depends on your chamber temperature and how fast the part cooled.
That contraction is also why unfilled PA6 lifts off the plate so aggressively. The stress is real and large, and it wins against most adhesion strategies. Garolite — a woven-glass epoxy laminate, sold as G10 — is the standard surface, because nylon bonds to it well while hot and releases as it cools. Plain PEI does not hold it.
Nozzle 250 to 280 °C, bed 60 to 90 °C, enclosure required, part cooling off.
Post-processing nylon is genuinely different
Two things you can do to a nylon part that you cannot do to most prints:
Dye it. Nylon takes fabric dye readily in hot water — the same amide groups that grab water grab acid dyes. A grey or natural print comes out an even, through-thickness colour that no amount of spray paint matches, and the dye penetrates the layer lines rather than sitting on top of them.
Anneal it. Held above its glass transition, PA6 crystallises further, becoming stiffer and raising its usable temperature. It also shrinks again while doing so, so anneal the test piece and measure it before you anneal the part that has to fit something.
What PA6 is genuinely bad at
- Holding a dimension. Between printing shrinkage and moisture uptake, a nylon part is the least dimensionally trustworthy thing you can print. Design clearances, not press fits.
- Being glued. Its solvent resistance and low surface energy defeat most adhesives. Mechanical fasteners, heat staking or a printed-in captive nut are the honest answers.
- Unattended printing on an open machine. Not merely worse — a large unfilled PA6 part on an open frame is a warping failure waiting to be discovered in the morning.
- Cost. $35 to $70 per kilogram, several times PLA, before you account for the dryer you also had to buy.
- Being the beginner's "strong material". Filled nylon exists largely because unfilled PA6 is so hard to print that the fibre-reinforced grades are what most people should have bought. That is not a criticism of PA6; it is what the PA6 versus PA6-CF comparison is for.
Where it is worth all of that
Gears and worm drives, cable guides and chain links, bushings running against steel, living hinges that must survive thousands of cycles, tool handles, and anything that has to take a hit without cracking. Nylon's combination of low friction and high abrasion resistance is difficult to get any other way on a desktop machine.
At 1.14 g/cm³ a 1 kg spool of 1.75 mm PA6 holds 364.69 m — more than PLA, because nylon is lighter than most people assume. Weighing a spool to estimate what is left is less reliable here than anywhere else, though, since absorbed water is part of what you are weighing. Feed the figures into the length and weight converter and treat the answer as approximate on a spool that has been open a while.