You can hear this one before you can see it. Sharp ticks or a soft hiss from the nozzle while it prints, sometimes a visible wisp of vapour, and an extrusion that comes out rough, bubbled and slightly furry.
Do not confuse it with the rhythmic clunk of an extruder skipping. That noise comes from the motor and means force, not steam — see extruder clicking. Hold a hand near the hotend and listen: popping originates at the tip.
What the water is doing
Several of the polymers used in printing absorb water into the body of the filament, not merely onto its surface. A spool that feels perfectly dry can hold enough water by weight to ruin every print on it.
That water travels down with the filament into the melt zone, passes 100 °C long before the plastic melts, and flashes to steam. Steam occupies over a thousand times the volume of the water that made it, so each pocket blows a small crater in the extrusion on its way out. The result is a bead full of voids: weaker, rougher, dimensionally unreliable, and often accompanied by stringing that no retraction setting will control.
The part that drying cannot undo
For polyamides, the copolyesters and polycarbonate there is a second effect, and it is permanent. At melt temperature, water attacks the polymer backbone and cuts long chains into short ones. Short chains give weak parts and a runny, uncontrollable melt.
Drying the spool prevents this happening to the filament that has not yet been printed. It does not restore the parts already made from wet material, and it does not help a spool that has been printed wet repeatedly — that damage happened in the hotend, one metre at a time.
Drying schedules by material
These are the figures this site stores for each material, and they are worth following rather than averaging.
| Material | Temperature | Time |
|---|---|---|
| PLA | 45 °C | 4–6 hours (optional) |
| PETG, PCTG | 65 °C | 4–6 hours |
| ABS, ASA, HIPS | 65 °C | 4–6 hours |
| TPU 95A | 60 °C | 4–6 hours |
| PET | 70 °C | 6–8 hours |
| Nylon (PA6, PA12, PA612) | 80 °C | 8–12 hours |
| Polycarbonate | 90 °C | 6–12 hours |
| PVA | 45 °C | 6–12 hours |
The temperature column is a ceiling as much as a target. PLA and PVA are dried low because both soften near their listed figure; a spool dried at a nylon temperature will sag, weld its own coils together, and become unusable in a way no amount of care afterwards fixes. The nylons need the heat and the hours, and the reason they get both is that they hold more water than anything else here.
About kitchen ovens
A domestic oven's thermostat swings widely around its setpoint and its lowest setting is often above what PLA or PVA tolerates. There is also the plainer risk of leaving plastic unattended in an appliance designed to reach several hundred degrees.
A filament dryer or a food dehydrator with a genuine temperature readout is the sane tool. If an oven is the only option, use an independent thermometer inside it, stay in the room, and do not attempt it for the low-temperature materials at all.
Printing straight from a dry box
For nylon, PVA and any fibre-filled grade, drying and then leaving the spool on an open holder undoes the work within hours in a humid room. Those materials want to be printed from a sealed container with fresh desiccant, feeding through a bulkhead fitting. Everything else is a compromise that works in a dry climate and fails in a wet one.
Three shortcuts that do not dry filament
Printing hotter to drive the water off. The steam is generated inside the nozzle either way; more heat produces more violent boiling, worse voids and faster hydrolysis.
Reusing exhausted desiccant. Indicating silica gel changes colour precisely so you can tell, and beads that have gone through several spools without being regenerated are ballast.
A quick two hours for a badly wet nylon spool. Water diffuses out of a solid slowly, and the surface dries long before the core does — which is why a spool can print acceptably for the first hundred metres and badly afterwards.
Which materials do this
Polyamides worst by a wide margin, then PVA and BVOH, which dissolve in water and therefore chase it. PETG picks up moisture fast enough that it is the most common material people meet this on, and the most common complaint about PETG in general has a drying fix. TPU bubbles and prints furry. PLA is the least affected and still degrades on a shelf over months — a spool that snaps between the holder and the extruder has usually been both wet and old.