Damp filament: symptoms, drying times per material, and storing it dry

Last month this spool printed beautifully. Today, same file, same profile, there are strings everywhere, the surface is rough, and the nozzle makes a faint popping sound every few seconds. Nothing on the printer changed. What changed is the filament: it has pulled moisture out of the air. In a humid climate like ours this happens far faster than guides written for dry countries suggest.

Why moisture ruins prints

Many filament plastics absorb water from the air (they are hygroscopic). When the filament reaches a nozzle at 200 °C or more, that water flashes to steam, pushing out bubbles. The extruded line has voids in it, breaks up, and bonds poorly to the layer below. For some materials, notably Nylon and PC, moisture also damages the plastic permanently (hydrolysis), and drying does not fully bring it back.

Symptoms of damp filament

From the Prusa Knowledge Base and the drying guides, in roughly the order you notice them:

• Surface quality drops on a profile that used to print smooth. Usually the first sign.
• More stringing with the same retraction settings.
• Weak layer adhesion, parts that snap along the layers, and blobs appearing on the surface.
• Popping, hissing, visible bubbles or steam at the nozzle. The clearest sign; sometimes with a little smoke.

The simplest check: dry the spool and reprint the same file. If it improves, you have your answer. Another quick test is to snap a 10 cm length: dry filament breaks cleanly, damp filament bends rubbery and shows fibres at the break (works for PLA and PETG, not for TPU, which is meant to bend).

Which materials get damp fast

Rough figures from the printpal guide, for a spool left open in a room at 45% RH. A non-air-conditioned room here is usually damper than that, so expect it to be faster.

Material	Time until symptoms	Sensitivity
Nylon (PA)	hours	very high; dry before every use, print from a dry box
TPU	3–5 days	high
PETG, PC	1–2 weeks	medium
PLA, ABS/ASA	2–4 weeks	low, but not zero

Prusa’s advice when storage space is limited: protect the moisture-sensitive materials first. PLA changes little when damp; polyamide (Nylon) changes a lot.

How hot, how long

The “Prusament” column is what Prusa specifies for its own filament. The “typical range” column is compiled from the drying guides. Always follow the label on the spool you are using, and never exceed the upper limit: the strands soften and the whole spool fuses together.

Material	Prusament	Typical range	Notes
PLA	45 °C · 6 h	40–50 °C · 4–6 h	Silk / PLA+ soften easily; stay under 45 °C
PETG	55 °C · 6 h	60–70 °C · 4–6 h	damper than you think, even new
ABS / ASA	ASA 80 °C · 4 h	65–80 °C · 2–4 h	absorbs little, dries quickly
TPU	60 °C · 4–6 h	50–60 °C · 6–12 h	soft TPU deforms above 60 °C
Nylon (PA)	PA11-CF 90 °C · 6 h	70–90 °C · 12–16 h	print straight from the dryer
PC	PC Blend 85 °C · 5 h	80 °C · 6–8 h	degrades badly when wet; never skip drying

Notice the two columns disagree. Prusa gives lower temperatures that are safe for its own spools; the general guides go higher because most consumer dryers do not heat evenly. When unsure, start low and dry longer. That is safer than high and short.

Too much heat destroys a spool in a minute. Leftover moisture only gives you an ugly print. If you are going to be wrong, be wrong on the cool side.

Which drying tool

Method	Temperature it reaches	Pros	Watch out for
Filament dryer	about 40–70 °C depending on model	controlled temperature; some let you print through the box	many do not reach 80 °C for Nylon/PC; check the spec
Food dehydrator	usually up to 70–80 °C	cheap, even heat	the dial can be 5–10 °C off; measure with a separate thermometer
Kitchen oven	most bottom out at 60–65 °C	you already own one	cannot go low enough for PLA; overshoots on heat-up; needs a thermometer and a cracked door to let moisture out
Microwave	—	—	never: uneven heating, and some spool cores contain metal

A dryer with a built-in hygrometer that reads down to 10% is telling you it has hit the bottom of its scale, not that the filament is fully dry.

Worked example: one PETG spool from damp to ready

Say the hygrometer in the storage box reads 35% RH and the latest print has started stringing. The printpal guide treats anything above 30% inside the box as damp.

1. Dry the PETG at 60 °C for 6 hours (inside the 60–70 °C range from the guides and below the point where PETG softens). If your dryer only goes to 55 °C, use Prusa’s 55 °C for 6 hours, and extend to 8 hours if the spool sat open for a long time.
2. Regenerate the desiccant at the same time. Colour-changing silica gel that has changed colour goes in the oven at 100–120 °C for 1–3 hours until it changes back (Polymaker says 100–120 °C for 1–2 hours; printpal says 120 °C for about 3 hours). Use a heat-resistant container.
3. Test by printing the file that had the problem and comparing strings and surface.
4. Store in a sealed box with the regenerated desiccant and the hygrometer inside. Wait an hour and read it; it should be under 20%.

If the box climbs back above 30% within a few days, the problem is a leaky box or too little desiccant, not the filament.

Storing it so it stays dry

• Sealed box + desiccant + hygrometer. Target under 20% RH for PLA, PETG and ABS, under 15% for Nylon, TPU and PC. Colour-indicating silica gel tells you when the desiccant itself needs drying.
• Vacuum bag + desiccant. The cheapest option for spools you rarely use; the printpal guide quotes 6 months or more with fresh desiccant.
• A storage box is not a dryer. A damp spool in a desiccant box stays damp for a long time. Heat it first.
• Print Nylon and TPU straight from a dry box. Route the filament from the dryer or desiccant box into the printer, because they start absorbing within hours to days.
• Dry before a long print. Four hours of drying costs less than an 8-hour print on a spool you are not sure about.

Common mistakes

• Drying PLA in a kitchen oven. It will not go below 60 °C, and the whole spool fuses.
• Trusting the dial on the dryer without measuring. Food dehydrators can be 5–10 °C off; a cheap separate thermometer costs less than a spool.
• Putting a damp spool in a desiccant box and waiting. The box keeps filament dry; it does not pull water out of filament that has already absorbed it.
• Placing the hygrometer right against the desiccant. It reads lower than the air the spool actually sits in. Move it away.
• Tuning retraction and temperature before suspecting moisture. You spend an evening calibrating, and the values are useless once the spool is dry. Go back to the previous lesson: dry first, then tune.
• Forgetting that desiccant saturates. Even non-indicating silica gel fills up. If the hygrometer creeps upward, regenerate it.

Safety

• Dryers and ovens run for hours. Put them on a heat-resistant surface away from anything flammable, and do not leave the house while a kitchen oven is drying plastic.
• Some spools are made of plastic that softens below 60 °C themselves. Check that the spool can take the drying temperature; cardboard spools should not go into an oven hotter than the PLA range.
• Regenerate desiccant at 100–120 °C in a heat-resistant container and let it cool before handling.
• Damp Nylon releases steam and a little smoke at the nozzle. Keep the room ventilated.

Next: Bed leveling and the first layer

Sources

• Prusa Knowledge Base: Drying filaments
• Prusa Knowledge Base: Stringing and oozing (moisture as a cause)
• printpal: Filament drying guide
• The Crafty Catsman: Filament drying temperature and time chart
• Polymaker Wiki: PolyDryer (reference humidity values and desiccant regeneration)