Popping, Steam and Bubbles From the Nozzle (Wet Filament)

Likely causes, most common first

1. Spool left open in humid airMost common
   Bambu Lab says the problems get worse as humidity rises and the longer the filament is exposed. A spool left on the printer for weeks, or one that has not been used in a long time, is the usual suspect; Polymaker says it is safest to dry a long-unused spool before printing.
2. A moisture-hungry materialVery common
   Polymaker lists nylon, PVA, TPU and PETG as highly hygroscopic, ABS as moderate, and PLA as low but still affected over time. Prusa names polyamide (nylon), polypropylene, PVA and BVOH as highly hygroscopic. Bambu Lab adds that PA, PC and fiber-filled materials are especially sensitive.
3. Storage that is not really dryCommon
   Polymaker says a simple dry box is good for storage but less effective for actively drying. A box without working desiccant, or a bag that does not seal, lets moisture back in.
4. Not moisture at all: a too-hot nozzle or a naturally stringy filamentOccasional
   Prusa notes some filaments string by nature, so compare with a known-dry spool before blaming moisture. If a dry spool strings just as much, see Stringing and Oozing.

Fix it, one change at a time

1. Stop and unload the spool if it is popping hard. Bambu Lab warns that damp filament is more prone to clogging, especially engineering materials.
2. Dry it at its maker's temperature and time. Use the drying settings on the filament label or data sheet. Prusa's own table for its Prusament filaments gives, for example, 45 °C for 6 hours for PLA, 55 °C for 6 hours for PETG and 60 °C for 4 to 6 hours for TPU Maker spec; other brands differ. Do not go hotter: Prusa warns too much heat softens the filament so the turns stick together, and to stick to the recommended times to avoid overdrying.
3. Check the spool can take the heat. Prusa gives different temperature limits for its own old and new spools, and Bambu Lab notes some third-party spools are not heat-resistant enough and can deform.
4. Use a method that holds temperature. A filament dryer set for the material is simplest. Prusa notes home ovens lack precise control and many cannot go low enough, so use an external thermometer if you use one. If your printer maker supports drying on the heated bed, follow their steps exactly: Bambu Lab allows it only on its enclosed printers, has you unload the filament from the toolhead first, and says to wear gloves when handling the hot spool.
5. Store it sealed straight away in an airtight bag or box with fresh desiccant, as Prusa, Bambu Lab and Polymaker all advise.
6. Dry while printing for the worst offenders. Polymaker says nylon, PVA, TPU and PETG may need to stay in a dryer during printing in humid climates.
7. Clean the nozzle if printing wet filament left a partial clog. See Nothing Coming Out of the Nozzle.

Confirm it is fixed

Reprint the same small test with the dried spool. The popping should be gone, the strand should come out smooth, the surface should be even, and stringing should be back to what that filament normally does. If the symptoms return a few weeks later on the same spool, the storage is letting moisture back in.

Why it happens

Many filaments absorb water from the air, some a lot. Inside the hotend the plastic is heated far above the boiling point of water, so the absorbed water turns to steam in the melt. Bambu Lab explains that this makes the molten filament expand and extrude erratically and form bubbles, which gives stringing, oozing, holes, rough surfaces and lower strength. The popping you hear is small pockets of that steam bursting out at the tip.

How much it matters depends on the material. Prusa notes PLA does not change much with humidity while polyamide (nylon) is affected significantly, and Polymaker ranks nylon, PVA, TPU and PETG as the most sensitive. That is why a PLA spool can tolerate being left out far better than a nylon one, and why nylon is the material most worth keeping sealed.

Drying reverses it for most filaments, but only if the heat stays below the point where the filament softens, and the filament only stays dry if it is stored sealed afterward. For drying equipment, storage and a fuller look at each material, see Drying and Storing Filament.