Most printable plastics are hygroscopic, meaning they pull moisture out of the air and hold onto it, slowly, whether the spool looks dry or not. That water doesn't sit on the surface where it would evaporate off. It gets absorbed into the filament itself. When that filament heats up in the hot end, the water flashes to steam faster than it can escape cleanly, and it pushes its way out through the melted plastic on its way out of the nozzle.
That's the mechanism behind most of the print defects that get blamed on other things: popping or crackling sounds at the nozzle, fine strings and blobs on an otherwise clean print, a rough or matte surface where the filament used to come out glossy, and parts that snap under a load a fresh spool of the same material would have handled fine.
Which materials are most at risk
PLA is the least sensitive of the common materials, though it still absorbs moisture over months, just more slowly than the rest. PETG is moderate risk: a noticeably worse surface finish and more stringing show up within weeks of an opened spool sitting in humid air. Nylon is high risk and can visibly degrade within days in a humid shop, often needing to be dried before every print rather than occasionally. PVA, used for supports, is the worst of the group: it can absorb enough moisture to become unusable within a single humid day left uncovered.
Drying filament that's already wet
Most affected filament can be dried out and used normally afterward. A food dehydrator or a filament-specific dry box set to the material's drying temperature, well below its print temperature, run for several hours, drives the absorbed water back out. PETG and nylon both benefit from longer drying times than PLA; nylon in particular can need eight hours or more if it's been sitting exposed for a while. The filament doesn't need to be perfectly bone dry, just dry enough that steam isn't forming faster than it can escape during the print.
Keeping it dry in the first place
- Reseal the bag it came in, properly, every time. The factory bag and a fresh desiccant packet are doing real work. A spool left uncovered on a shelf overnight in a humid shop has already started absorbing moisture.
- Use a sealed container with rechargeable desiccant for active spools, not just the ones in storage. A spool currently loaded in the printer is often the one left exposed longest.
- Print from a sealed dry box when the shop's humidity is genuinely a factor. A printer running continuously in a shop without climate control benefits from feeding directly out of a sealed, desiccant-controlled box rather than an open spool holder.
The expensive version of this problem
It isn't a print that obviously failed. It's a part that looks fine, passes a quick visual check, and then fails under load later because the layer adhesion was quietly compromised by moisture the whole time it was printing. A part that's going to see real mechanical load is worth printing from filament you know was dry, not filament that was probably fine.
If a print's surface finish or strength has quietly gotten worse without a slicer change, check the filament before the printer. Dry the spool, reseal it properly afterward, and keep anything hygroscopic like nylon or PETG in sealed, desiccant-controlled storage between prints, not just on the shelf.
