How do I know if my filament is wet?

Three signs: popping or crackling sounds at the nozzle during printing, surface stringing and 'fuzz' on otherwise smooth walls, and prints that snap easily under light pressure. PETG and TPU show problems first because they absorb moisture fastest. PLA can sit open for a few weeks before symptoms show.

Does PLA actually need drying?

Most of the time, no. PLA absorbs less water than PETG, ABS, or nylon, and most spools last a month or two open without issues. Dry it if prints suddenly look fuzzy after a humid week, or if your spool sat opened in a garage all summer. Otherwise don't waste the energy.

Can I leave filament in a dryer overnight?

For dedicated filament dryers like the Sunlu S2, yes, that's what they're built for. For ovens, only if you're confident in the thermostat (some home ovens overshoot by 10-20°C, which would melt PLA). For food dehydrators, generally yes, but check after the first hour to make sure the temperature is stable.

How long does dry filament stay dry?

In open air at typical humidity (40-60% RH), maybe 2-4 weeks before reabsorbing enough water to print poorly. In a sealed dry box with active silica gel, several months. In vacuum-sealed bags with desiccant, almost indefinitely.

What about the microwave method?

Don't. Microwaves heat unevenly, can melt small spots in the filament, and can damage the spool's plastic core. The 'a few seconds in the microwave' tips you see on Reddit are bad advice. Stick to ovens or dehydrators.

How to dry filament: 5 methods compared, with temperatures and times

Wet filament is the cause of more failed prints than people realize. The symptoms look like printer or slicer issues: stringing that wasn't there last week, fuzzy walls, weak layer adhesion, popping sounds at the nozzle. The actual problem is the spool absorbed moisture from the air, and now the water flashes to steam during extrusion.

The fix is simple in concept, fiddly in practice: heat the filament to a temperature that drives moisture out without softening the polymer, hold it for several hours, then store it in a sealed environment with desiccant. Different methods cost different amounts and give different results.

Drying temperatures by material

The numbers below are conservative. Higher temperatures dry faster but risk softening the spool. Lower is always safer.

Material	Temp	Time	Notes
PLA	45°C	4 hours	Glass transition is 60°C, do not exceed 50°C
PETG	65°C	5 hours	Most common candidate for drying
ABS / ASA	80°C	4-6 hours	Watch for spool warping above 80°C
TPU	50°C	8 hours	Slow to release moisture, needs longer time
Nylon (PA)	80°C	8-12 hours	Most hygroscopic; often needs drying mid-print
Polycarbonate	85°C	6-8 hours	Very hygroscopic, dry before every print

Method 1: Kitchen oven (free, works for most filaments)

The cheapest method if you already have an oven. Set to the temperature for your material, leave the spool on a baking sheet for the time listed above, then move to a sealed container.

Risks: many home ovens overshoot the set temperature by 10-20°C, especially when first heating up. PLA at 60°C softens enough that the spool can deform under its own weight. Use an oven thermometer to verify, or use a different method for PLA. Gas ovens are worse than electric here.

One trick: preheat the oven, turn it OFF, then put the spool in and let it cool slowly. This avoids overshoot at the cost of taking longer.

Method 2: Food dehydrator ($40-60, best cheap option)

A food dehydrator like the Presto 06300 or Cosori CFD-N051 holds steady temperature in the 40-70°C range, perfect for most filaments. Single-tray models can hold one spool at a time. Stackable models can dry several spools simultaneously, which matters if you go through filament fast.

Cost vs benefit: a dehydrator pays for itself if you print PETG, ABS, or TPU regularly. For a PLA-only printer, an oven works fine and a dehydrator is overkill. Dehydrators run very low power (around 250W), so leaving one on for 6 hours costs about 25 cents at average US electricity rates.

Method 3: Dedicated filament dryer ($60-150)

The Sunlu S2 (single spool, $60), Sunlu S4 (four spools, $130), and Eibos Polyphemus ($150) are purpose-built. They go up to 70°C reliably, hold humidity well, and most can feed filament while drying so you can print straight from the dryer for materials that re-absorb moisture quickly.

Worth buying if: you print PETG, nylon, or TPU often, you live in a humid climate, or you want a "set and forget" solution.

Skip if: you print PLA almost exclusively and your filament doesn't sit open for long.

Method 4: Vacuum bag plus desiccant (preventive, not corrective)

This doesn't dry already-wet filament; it keeps dry filament dry. Cheap food vacuum-seal bags plus 50-100 grams of color-changing silica gel per bag will hold a spool at <15% relative humidity for months.

The color-changing silica is important. It turns from blue or orange to pink as it absorbs water, so you can see when to recharge it. Recharge by baking the silica at 120°C for an hour.

Best workflow: dry a fresh spool once after opening, then store in a vacuum bag with fresh desiccant whenever it's not on the printer.

Method 5: Microwave (don't)

You will see this advice on Reddit. It is bad advice. Microwaves heat unevenly. Some spots on the filament melt while others stay cool. The plastic spool itself can warp. Microwaves are built to excite water molecules in food, not to evenly heat a large plastic object.

Even when it appears to work, you've introduced microscopic deformations in the filament that cause stringing and inconsistent extrusion later. Use any other method.

Cost summary

Oven: $0 upfront, ~$0.30 per drying session in electricity
Food dehydrator: $40-60 upfront, ~$0.25 per session
Dedicated dryer: $60-150 upfront, ~$0.20 per session, can dry while printing
Vacuum bag plus desiccant: $30 upfront for bags and 1 kg silica, runs indefinitely
Microwave: avoid; ruins filament regardless of cost

When wet filament becomes a real cost

Wet filament can push your effective failure rate from 5% to 15% or higher. On a heavy-print month, that's real money in wasted material. The Failure Rate Calculator can help you spot this trend over time. If your monthly failure rate spikes after a humid week, drying is usually the answer.

Frequently asked

How do I know if my filament is wet?: Three signs: popping or crackling sounds at the nozzle during printing, surface stringing and 'fuzz' on otherwise smooth walls, and prints that snap easily under light pressure. PETG and TPU show problems first because they absorb moisture fastest. PLA can sit open for a few weeks before symptoms show.
Does PLA actually need drying?: Most of the time, no. PLA absorbs less water than PETG, ABS, or nylon, and most spools last a month or two open without issues. Dry it if prints suddenly look fuzzy after a humid week, or if your spool sat opened in a garage all summer. Otherwise don't waste the energy.
Can I leave filament in a dryer overnight?: For dedicated filament dryers like the Sunlu S2, yes, that's what they're built for. For ovens, only if you're confident in the thermostat (some home ovens overshoot by 10-20°C, which would melt PLA). For food dehydrators, generally yes, but check after the first hour to make sure the temperature is stable.
How long does dry filament stay dry?: In open air at typical humidity (40-60% RH), maybe 2-4 weeks before reabsorbing enough water to print poorly. In a sealed dry box with active silica gel, several months. In vacuum-sealed bags with desiccant, almost indefinitely.
What about the microwave method?: Don't. Microwaves heat unevenly, can melt small spots in the filament, and can damage the spool's plastic core. The 'a few seconds in the microwave' tips you see on Reddit are bad advice. Stick to ovens or dehydrators.