How to tell if your filament is wet, and what to do

Quick checks

This guide is for anyone whose prints have suddenly turned stringy, rough or weak, or whose printer pops and crackles while extruding. It covers the signs makers link to wet filament, which materials are most at risk, simple checks, the drying tables Prusa and Bambu Lab publish, how to store a spool afterwards, and when drying isn't the answer.

Why does moisture cause print problems?

Prusa says the materials used for FFF printing "are mostly hygroscopic," meaning they "readily attract water molecules from the environment." Polymaker says absorbed moisture "turns into steam when the filament is heated in the hotend," and that this steam disrupts extrusion. Bambu Lab describes the same process: the moisture "rapidly vaporizes inside the hot nozzle," so the molten filament expands, extrudes erratically, and forms bubbles. It says these problems get worse as humidity rises or the filament stays exposed for longer.

Signs your filament is wet

Makers list overlapping symptoms:

Some material guides add their own notes. Prusa says moist polyamide "makes bubbles, which leads to uneven layers and an ugly surface," and that flexible filaments "can be hard to print when wet."

Which materials absorb moisture fastest?

Makers rank materials a little differently, but they agree on the main groups.

• Polymaker's groups: "Highly hygroscopic" for nylon, PVA, TPU and PETG. It says drying is crucial for these, and in humid climates they often need to stay in a dryer even while printing. "Moderate sensitivity" for ABS and some specialty copolymers. "Low sensitivity" for PLA, which it says "can still take on moisture over time or in damp conditions."
• Prusa: It names polyamide, polypropylene, PVA and BVOH as highly hygroscopic, and says they should be stored dry or dried before every print. It says polyamide, PVA and TPU need drying more often than PLA, and that "PLA, for example, doesn't change properties too much by uptaking humidity, whereas Polyamide is affected quite significantly." Its nylon guide says improperly stored polyamide can absorb water weighing up to 10% of the filament's weight.
• Bambu Lab: Its drying table marks drying before use as "Required" for PLA Wood, TPU, PC, PVA, Support for PA/PET, PA-based and PET-CF filaments, PPA-CF and PPS-CF, and "Recommended" for its other PLA grades, PETG, ABS and ASA.

For problems specific to the two most common materials, see our PLA printing problems and PETG printing problems guides.

How fast it happens

Bambu Lab gives the clearest timing. It says typical indoor air is around 45–65% relative humidity, and that at around 55% RH, freshly dried filament "will usually absorb enough moisture to affect print quality within 2~12 hours, depending on how hygroscopic the filament type is." Even at 20% RH, it says filaments generally stay dry for only 2–7 days.

A special case: Prusa XL

Prusa says the Original Prusa XL is more sensitive to filament moisture than single-tool printers. Unused toolheads stay heated (70 °C idle by default), so moisture is released inside the tool and causes stringing. Prusa says that on the XL "even PLA can absorb enough moisture to create printing issues," and recommends drying filament before printing on it.

Simple checks before you change settings

1. Listen. Popping or hissing during extrusion is the sign Polymaker and its stringing guide point to.
2. Swap the spool. Prusa's stringing guide suggests trying a different spool. If a freshly opened or dried spool of the same material prints cleanly, the first spool is the problem.
3. Think about its history. Polymaker says that if a spool "hasn't been used in a long time, it's safest to dry it before printing." Bambu Lab also recommends drying first when storage conditions are uncertain.
4. Don't blame every string on moisture. Prusa says some filaments are "naturally more stringy than others," so compare results before deciding. Our stringing guide covers the settings side.
5. Measure storage humidity. Bambu suggests a hygrometer in the sealed container to check the real humidity level.

A factory-sealed spool is a useful reference. Polymaker says it dries every spool before vacuum-sealing it with desiccant, so a new spool should be ready to print "provided the seal is intact."

How to dry it

Use the filament maker's settings for that exact product first. If it publishes none, use your printer maker's table. Polymaker says to follow the recommended drying times and temperatures for your filament "to avoid damaging or deforming it."

Prusa's drying table

Prusa publishes these temperatures and times for Prusament materials. It says not to exceed them, "or the filament will soften and stick together," and to stick to the times "to prevent overdrying."

Prusa's nylon guide separately says to dry polyamide "at least 4 hours at a temperature below 90 °C."

Bambu Lab's drying table

Bambu Lab publishes figures for its own filaments across several methods. Temperatures are in °C and times in hours, copied from its table. We left out its desiccant, flip-interval and cover columns. "Not compatible" means Bambu says that unit can't dry that material.

Notes from Bambu's page:

• AMS 2 Pro suits filaments needing no more than 65 °C. For TPU, Bambu suggests the heated bed or AMS HT instead. When drying a high-temperature filament, take the low-temperature spools out.
• AMS HT can dry all filaments, but Bambu says some, such as PVA, PPS-CF and PPA-CF, may still not dry fully.
• Heated-bed drying works on the enclosed H series, X series, X2D, P2S and P1S. Bambu says the open-frame P1P, A1, A1 mini and A2L can't be used to dry filament. Cover the spool and flip it midway, as its instructions describe. The P1S has no built-in drying feature, so you set the bed temperature and time it yourself.

The two tables don't match

Prusa and Bambu publish different numbers for the same material type. PETG, for example, is 55 °C for 6 hours in Prusa's table and 60–65 °C for 8 hours in a forced-air oven in Bambu's. Bambu's own PETG figures don't match each other either: its filament guide gives 70 °C for 8 hours in a blast drying oven, and its PETG Basic data sheet gives 65 °C for 8 hours. Each table is written for that maker's own filaments, so use the one that matches your spool, or the filament maker's own figures.

After drying, print the same small test again. If the popping, stringing or roughness has gone, moisture was the cause. Our dryer vs dry box comparison explains the equipment options. If moisture keeps coming back and you decide a dryer is worth it, our filament dryer shortlist lists options.

Storage after drying

Drying only lasts as long as the storage does.

• Seal it straight away. Bambu says to store moisture-sensitive filament in a sealed container or AMS with effective desiccant immediately after drying, and use it as soon as possible.
• Use desiccant. Prusa recommends a closed box with desiccant, or vacuum bags with a desiccant inside. Polymaker suggests airtight bags, dry boxes with desiccant, or vacuum containers.
• Watch the desiccant. Bambu says its AMS silica-gel desiccant has yellow indicator beads that turn green as they absorb moisture. If the whole pack looks dark green, replace or regenerate it.
• Print from a dry environment if you need to. Polymaker says highly hygroscopic materials often need to stay in a dryer while printing in humid climates. Bambu says that when you print while an AMS 2 Pro or AMS HT is drying, it lowers the drying temperature, for example to 45 °C when drying PLA.
• Prioritize. If storage space is short, Prusa suggests prioritizing the most hygroscopic filaments.

Our filament storage guide covers containers, desiccants and hygrometers in more detail.

When won't drying help?

Drying fixes moisture, not every print problem. It's unlikely to help when:

• Nylon has sat in open air for months. Bambu says PA-based filaments left in open air for around 3 months absorb so much moisture that they become "very difficult to dry" in a convection oven at 80–90 °C.
• The spool has been dried too often. Bambu says PC "may become brittle after repeated 'heating–cooling' drying cycles," which raises the risk of filament breaking during printing. Prusa also warns against overdrying.
• It was dried too hot. Prusa says filament dried above its temperature can "soften and stick together," and that high temperatures damage filament "even if left for less time than recommended."
• The stringing is a settings problem. If a fresh spool strings just as badly, look at temperature and retraction instead. See our stringing guide.
• The filament snaps rather than pops. Brittleness can come from age, tension or the material itself, not just moisture. Our brittle filament guide covers how to tell the difference.

If layers split or parts are weak even with dry filament, our layer adhesion guide covers temperature, cooling, drafts and flow.

A sensible order

1. Listen for popping or hissing, and look for new stringing, roughness or bubbles.
2. Print the same test with a freshly opened or dried spool of the same material.
3. Dry the suspect spool once, at the filament maker's settings or your printer maker's table.
4. Print the test again and compare.
5. Store the spool sealed with desiccant, and check the humidity with a hygrometer.
6. If nothing changed, look at slicer settings or other causes rather than drying again.

Dryers & dry boxes: what to check before you buy ↗ Filament storage: what to check before you buy ↗