How to Tell If Filament Is Wet, and the Best Ways to Dry It

Wet filament causes stringing, pock marks, rough surfaces and even nozzle clogs. Here's how to tell if your filament has absorbed moisture, and how well different drying methods actually work. I tested each one by drying a wet 10 g sponge for an hour at 60°C.

How to tell if filament is wet

• The snap test: bend it. If it snaps easily, it's absorbed moisture. It isn't fully reliable though; filament can bend fine and still cause problems.
• The extrusion test (more accurate): extrude a little and watch and listen. Popping, sizzling or small blobs mean moisture.
• Check the packaging: new spools are usually vacuum sealed. If the bag is loose, moisture has probably got in.

Drying methods compared (10 g sponge, 1 hour at 60°C)

• Fan-forced oven: about 80% of the moisture removed. The fastest, but the riskiest. Put the spool on a rack over a tray, start low, and check often. Times vary by oven; weigh the spool before and after (start with around 2 hours at 60°C).
• Eibos Polyphemus dryer: 35.7%, rising to 37.6% with desiccant bags inside.
• Sunlu S2 dryer: 31.7%.
• Printer bed with a cardboard box on top: nearly 20%. Free, but it ties up your printer. Bambu publishes a drying guide with times and temperatures.
• Airtight container with desiccant: 1.4% (versus 0.3% without). Slow, but great for keeping filament dry over time.

Sunlu S2 vs Eibos Polyphemus

Sunlu S2 (around $40, I bought it myself): dries one spool, has a fan and preset times, three filament exit holes and a 360° heater, so no rotating is needed. Downsides: a dark lid hides how much filament is left, it can tip over from printer vibrations, and its humidity sensor was inaccurate, a common complaint. It still dries well.

Eibos Polyphemus (around $160, sent to me free, which doesn't change my opinion): dries two spools (with a raised enclosure for bigger ones), keeps humidity constant for storage, rotates spools automatically, has adjustable fans and desiccant slots, and includes a Bowden tube to feed your printer directly (print a PTFE coupler and use three PC4 connectors). It was also about 10 dB quieter than the S2 (around 38 dB vs 49 dB). Downside: spools can occasionally get stuck if filament sticks out or a cardboard spool is damaged. For a deeper sensor test, see this Polyphemus sensor testing video.

Tips for drying filament

• Keep relative humidity inside your dryer below about 10%.
• Follow the temperature guide: too hot damages filament, too cold won't dry it.
• Use your own humidity sensor. Mine has Bluetooth, so I can check dry boxes from my phone.
• Store filament in airtight containers with rechargeable desiccant. Recharge desiccant bags carefully: low heat, spread out, checked every 30 minutes, especially in a gas oven.
• Want to build your own dryer with a custom temperature controller? PCBWay makes custom PCBs and parts ($5 credit with this link).

Which should you choose?

If you print regularly, a filament dryer is worth it for better quality and fewer clogs. The S2 is great for occasional use, the Polyphemus suits heavier use, and for casual printing a careful oven dry or a simple drying box does the trick.

What I use

Questions? Ask in the video comments, or join the community.

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Common questions

How can I tell if my filament is wet?

Extrude a little and watch and listen: popping, sizzling or small blobs mean moisture. A filament that snaps easily when bent has probably absorbed moisture too, and loose packaging is a warning sign.

What is the fastest way to dry filament?

A fan-forced oven removed about 80% of the moisture in my one-hour test at 60°C, but it's the riskiest. Use a rack over a tray, start low, check often, and weigh the spool before and after.

Is the Eibos Polyphemus better than the Sunlu S2?

In my test the Polyphemus removed 35.7% (37.6% with desiccant) versus 31.7% for the S2, dries two spools, rotates them automatically and was about 10 dB quieter. The S2 costs around $40 and is great for occasional use.

What humidity should a filament dryer keep?

Keep relative humidity inside your dryer below about 10%.

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