Wet filament: warning signs, which materials absorb the most, and drying

How moisture shows up in a print

Bambu Lab explains the mechanism: water absorbed from the air turns to vapor inside the hot nozzle, the molten filament expands, flows erratically and forms bubbles. The visible results are stringing, oozing, holes, rough surfaces and reduced strength, and they get worse the higher the humidity and the longer the spool sits exposed ^[1]. Its filament guide adds under-extrusion to the list ^[2]. Prusa says the first sign is usually a poor surface; a lot of water causes weak layer adhesion, blobs, and bubbling and smoke during extrusion ^[5].

Symptom, likely cause, what to check

Confirming it is moisture

1. Listen. Popping or crackling during the first layers points to water ^[3].
2. Swap the spool. Prusa suggests trying a different spool of the same filament; moist filament strings heavily ^[8]. Reprinting the same file from a freshly dried spool isolates moisture from settings.
3. Know the baseline. Some filaments are naturally stringier than others, so compare against earlier prints with the same material ^[5].
4. Read the retraction tower. If a tower still strings at its top, OrcaSlicer’s guide says to dry the filament and check that the nozzle is not leaking ^[9]. The stringing guide covers the settings side.

Which filaments take up the most water

Bambu Lab’s data sheets list a saturated water absorption rate measured at 25 °C and 55% RH. The drying column is from the Bambu Lab Wiki filament guide, except the support row, which is from its drying guide ^{[2, 1]}.

The absorption figure alone does not predict the drying requirement. PC has the lowest value in the table yet is marked Required, while PLA at 0.43% is only Recommended. Treat the manufacturer’s drying instruction as the deciding factor, not the percentage.

Prusa describes most FFF materials as hygroscopic. PLA changes little as it absorbs humidity, polyamide changes significantly, and polyamide, polypropylene, PVA and BVOH should be stored dry or dried before every print ^[5]. Badly stored nylon can absorb water up to 10% of the filament’s weight ^[6]. Polymaker recommends printing its CoPA and PA-CF nylons straight from a heated dryer for the whole print ^[4], and Bambu Lab warns that nylon and PC absorb moisture within hours in humid air ^[3]. See the nylon guide for that family.

Drying: follow the spool’s data sheet

Drying settings differ by brand even within one material. Prusa’s table lists 45 °C for 6 hours for Prusament PLA and 55 °C for 6 hours for its PETG ^[5], while Bambu Lab’s data sheets list 50 °C for 8 hours for PLA Basic and 65 °C for 8 hours for PETG Basic in a forced-air oven ^{[13, 14]}. The drying chart collects published values by product, and the drying planner schedules a batch. Methods are compared in how to dry filament.

• Do not overshoot. Prusa warns that exceeding the recommended temperature softens the filament until the windings stick together, and that sticking to the recommended times prevents overdrying ^[5].
• Choose the oven carefully. The two makers disagree here. Prusa allows a home oven with an external thermometer, while warning that home ovens are imprecise and often cannot hold low temperatures ^[5]. Bambu Lab’s data sheet calls microwave and kitchen ovens incompatible and specifies a forced-air oven with the filament kept away from the heater ^[13].
• Check the spool. Older Prusament black spools loosen above 45 °C, while the newer spools tolerate 90 °C ^[5]. Bambu Lab notes some third-party spools may deform during drying ^[1].
• Printer beds. Bambu Lab’s heated-bed drying method applies to its enclosed printers; open-frame models such as the A1, A1 mini and P1P cannot be used ^[1].

Safety: when drying on a heated bed, Bambu Lab says to wear gloves when flipping the spool midway to avoid burns, and to wait until the spool is no longer hot to the touch before removing it ^[1].

Keeping it dry afterward

Bambu Lab’s data sheets specify printing and storage below 20% RH in a sealed container with desiccant ^[13]. After drying, store moisture-sensitive filament immediately in a sealed container or AMS with fresh desiccant and use it soon ^[1]. Prusa suggests a dry box or vacuum bags with desiccant, giving the most hygroscopic materials priority if space is limited ^[5].