Everyone knows wet filament prints badly. Far fewer people know that "wet filament" describes two entirely separate problems happening at once, and that one of them goes away in a dryer while the other is permanent. That distinction decides whether a rescued spool is as good as new or quietly compromised.
Mechanism one: water flashing to steam
This is the visible one. Filament that has absorbed moisture carries it into the melt zone, where the temperature is far above water's boiling point at atmospheric pressure. The water turns to vapour instantly and expands enormously, and it has to go somewhere.
What comes out is a bead full of voids, laid down by a nozzle that is intermittently spitting. The consequences follow directly:
- Popping and hissing at the nozzle, which is the vapour escaping audibly.
- Fine hairs and stringing that retraction tuning cannot remove, because the pressure spikes are not coming from the extruder.
- A hazy, rough or pitted surface where a glossy one is expected. Voids at the surface scatter light.
- Under-extrusion and gaps, because part of the volume the slicer asked for is steam rather than plastic.
- Weak layers, since a bead full of bubbles has less material at the interface to weld with.
All of this is mechanical, all of it stops the moment the water is gone, and all of it is what a dryer fixes.
Mechanism two: the water attacks the polymer
The second mechanism is chemical and it does not undo. At melt temperatures, water molecules cleave certain bonds in a polymer backbone — hydrolysis — and each cleavage turns one long chain into two shorter ones.
Chain length is where a thermoplastic's strength comes from. Long chains entangle with their neighbours and have to be pulled past each other to break the material; short ones slide apart more easily. Cutting them shortens the average, and the part that comes out is measurably weaker even when it looks perfect.
The polymers that suffer are the ones with hydrolysable links in the backbone: the polyamides, polycarbonate, and the polyesters including PET and PLA. Their stored notes say so in plain terms — wet PET hydrolyses in the melt and the part comes out weak; wet polycarbonate prints cloudy and weak. Our sister site PeriodicDeck covers the chemistry of why a backbone's bond types decide so much of a material's behaviour.
The practical consequence is uncomfortable: drying a spool restores its printability, not its molecular weight. If a nylon spool has been run through a hot nozzle while saturated, the plastic that came out is permanently shorter-chained. Dry it and the surface defects vanish, so the next print looks fine — and it is not quite the part it would have been.
Why nylon is the worst case and polypropylene the best
Water is a polar molecule and it goes where it is welcome. Nylon's repeating amide group is strongly polar and hydrogen-bonds water directly into the bulk of the polymer, not merely onto its surface, which is why PA6's stored note describes a spool left out overnight in a humid room as already too wet to print well. It also has hydrolysable amide links, so it collects both mechanisms at once.
Polypropylene sits at the opposite end. Its backbone is plain carbon and hydrogen, entirely non-polar, with nothing for water to attach to and nothing to hydrolyse. It is one of the very few filaments on this site where drying is genuinely optional.
Everything else falls between. PETG absorbs quickly but is more tolerant of the consequences; PLA absorbs slowly but is a polyester and does degrade if it is printed wet; TPU takes up water readily and prints furry when it has.
How water gets in, and how fast
Filament does not soak up water indefinitely — it equilibrates with the humidity of the air around it. Each polymer has an equilibrium moisture content at a given relative humidity, and the spool moves toward that figure at a rate set by how deep the water has to travel and how warm it is.
Three things follow, and they explain most of what people observe:
- A spool in a humid room reaches a bad state faster than one in a dry room reaches the same state. The driving force is the difference between the air and the plastic.
- The outer wraps go wet first. This is why a spool can print badly for an hour and then improve, and why the first metres off a rescued spool are the worst.
- Warm and humid is much worse than cool and humid, because diffusion speeds up with temperature. A spool stored above a radiator in a damp house is the worst place in the building.
What this means for how you work
Three practical positions follow from the two mechanisms.
Judge a spool by its symptoms, not its age. An unopened spool three years old in a sealed bag with desiccant is fine. An opened spool three weeks old in a humid workshop may not be.
Dry before a print that matters, not after it fails. The steam damage is fixable retrospectively; the chain scission is not, and it happened during the failed print.
Treat structural nylon and polycarbonate differently from everything else. For those, drying is not a quality step, it is a strength requirement, and printing from a heated dry box rather than a dried spool is the difference between the part you designed and a weaker one that looks the same.
The procedure itself — temperatures, times, equipment and how to verify the result — is on drying filament properly, and keeping a dry spool dry between prints is a storage problem rather than a drying one.