Wet filament: how to spot it and dry it

Symptom

The filament has picked up moisture from the air: bubbles or a little smoke appear when extruding, the surface comes out rough or lumpy, there is a lot of stringing and layers bond less well. It is common with spools that have been left open for a while.

Likely causes, from most to least likely

1. Most filaments are hygroscopic: they absorb water from the air if left out.
2. Especially sensitive materials, such as nylon (PA), PVA or TPU, which need drying more often than PLA.
3. Spool stored without a bag or box with desiccant.

Checks to tell which one it is

• Purge some filament and look at the strand: if it bubbles or smokes, it is wet.
• Print the same part with a dry or freshly opened spool: if strings and surface improve, it was moisture.
• Think about how long the spool has been open and where it has been kept.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Dry the spool

   Dry it in a filament dryer or an oven with a reliable temperature, at the temperature and for the time given by the filament manufacturer. As a guideline, Prusa gives for its own filaments: PLA 45 °C 6 h; PETG 55 °C 6 h; TPU 60 °C 4-6 h; ASA 80 °C 4 h.

   Check: The next print has no bubbles, less stringing and a better surface.

   If that does not fix it: If it doesn't improve, look for other causes of stringing or under-extrusion (temperature, retraction, nozzle).

2. If you use a home oven, be careful

   Home ovens don't measure temperature accurately: check it with an external thermometer, don't exceed the stated temperature (the filament softens and the windings stick together) and make sure the spool can take that heat. Stick to the time as well: over-drying isn't good either.

   Check: The filament comes out of the oven dry with no windings stuck together.

   If that does not fix it: Use a filament dryer, which controls temperature better.

3. Keep the filament dry

   After drying, store it in a closed box or a vacuum bag with desiccant (silica gel). If you can, print from a dry box.

   Check: Later prints with that spool keep coming out well.

   If that does not fix it: Dry it again before use if bubbles or strings come back.

Prevention

• Store each spool in a closed bag or box with desiccant when not in use.
• Dry the most sensitive materials (PA, PVA, TPU) before printing.
• Follow the filament manufacturer's drying temperature and time.

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Drying filament · Prusa Research
• Stringing and oozing · Prusa Research
• Under-extrusion · Prusa Research
• Clogged nozzle (CORE One, CORE One L, XL, MK4/S, MK3.9/S) · Prusa Research