Wet filament: recognise it, dry it and store it

How to recognise it

Filament absorbs moisture from the air. Bambu Lab explains that during printing the water vaporises suddenly in the hot nozzle: the molten plastic expands, extrudes erratically and forms bubbles, causing stringing, oozing, small holes, a rough surface and weaker parts. Prusa gives a poorer surface as the first sign and, in more severe cases, weak layer adhesion, blobs, bubbling and smoke during extrusion.

The easiest sign to catch is audible: popping or crackling as the nozzle extrudes, sometimes with small puffs of steam. A strand extruded by hand also shows it: bubbles and an uneven surface. Prusa also lists moisture among the causes of clogs and under-extrusion, because steam pockets disrupt the flow.

How to dry: checks and steps

Temperatures and times depend on the material and on the filament brand: the first reference is the filament maker's label or data sheet, then the printer or dryer maker's table. Official tables don't match: for example Prusa gives 45 °C for 6 hours for PLA and 55 °C for 6 hours for PETG, while Bambu's table for a forced-air oven lists 50 °C for 8 hours for PLA Basic and 60–65 °C for 8 hours for PETG. These values are for their own filaments and spools: don't treat them as universal.

1. First confirm that moisture is really the problem: listen to the extrusion and, if possible, print the same file with a freshly opened or already dried spool.
2. Check the spool's maximum temperature. The limit is given by the filament maker; Bambu notes some third-party spools may deform. If the filament is spool-less, mount it on a reusable spool before drying.
3. Use a filament dryer or a forced-air oven with stable temperature, with the spool away from heating elements. Bambu advises against microwave and kitchen ovens; Prusa warns that household ovens fluctuate and recommends an external thermometer to check the real temperature.
4. Only if specified in your model’s manual: On enclosed Bambu printers (X, H series, P2S, P1S) you can dry on the heated bed with the spool covered, flipping it halfway with gloves; Bambu excludes open-frame printers (A1, A1 mini, A2L, P1P). Unload the filament from the extruder and clear the bed first.
5. With AMS 2 Pro or AMS HT, use the mode intended for the material: Bambu states the AMS 2 Pro reaches 65 °C and may not fully dry some materials, such as TPU.
6. At the end, let the spool cool until it is no longer hot and put it straight into a sealed container with desiccant.

Which materials are most sensitive

According to Prusa, the materials that most often need drying are polyamide (nylon), polypropylene, PVA/BVOH, TPU and PEI, while PLA is relatively resistant. In Bambu Lab's table, drying before use is «required» for TPU, PC, PVA, PA and filled PA, PET-CF and PLA Wood, and «recommended» for PLA, PETG, ABS and ASA.

Bambu adds two useful warnings. Polyamide-based filaments left in open air for about three months absorb so much moisture that they become very hard to dry in a normal convection oven within the spool's temperature limit: if you won't use them for a long time, seal them with desiccant. PC, on the other hand, can become brittle after repeated heating and cooling cycles: keep it sealed after the first drying to reduce further drying. Bambu's material guide also notes that damp PVA and BVOH become too soft to pass through the AMS.

Storage after drying

Bambu Lab states that indoor air is usually at 45–65% relative humidity and that most filaments stay dry only below 20%: in those conditions they remain dry for about 2–7 days depending on the material, while at around 55% freshly dried filament can absorb enough moisture to affect print quality within 2–12 hours. So keep it in a sealed container or the AMS with desiccant, ideally with a hygrometer to check the actual humidity. Prusa recommends a closed box with desiccant, a drybox or vacuum bags with desiccant.

Desiccant runs out: in Bambu's silica gel the indicator beads change colour when it is saturated. It can be regenerated in an oven without exceeding 100 °C, in a heat-resistant container not used for food, or replaced.

When buying or replacing something makes sense

Wet filament does not require replacing printer parts: before changing the nozzle or extruder because of stringing, bubbles or under-extrusion, always test with dry filament. A dryer or an airtight box with a hygrometer becomes worthwhile if you often print sensitive materials such as nylon, TPU, PC or PVA, or live in a humid climate; for PLA used up quickly, good storage is often enough. Replace desiccant when it is saturated and no longer regenerates. According to Bambu, a polyamide spool left in open air for months can be very hard to recover with an ordinary oven.

How to verify the result

Record the model and variant, material, slicer profile and change made. Repeat the same test and compare the result with a photo taken before the adjustment. Keep the change only if it improves the defect without introducing others. Use your model’s instructions for temperatures, calibration and disassembly; this guide is not based on our own hands-on tests.