Filament dryers explained: what they do, what RH means, and which to buy

Filament absorbs moisture from the air. Most plastics do it slowly, some do it quickly, and a few — Nylon especially — will pick up enough in a single humid afternoon to cause visible problems. A filament dryer keeps your spool at a controlled temperature and low humidity, either before or during a print, so none of that moisture makes it into the hotend.

Signs your filament has picked up moisture

The clearest tell is sound. When wet filament hits the melt zone, water turns to steam almost instantly. You hear a soft popping or crackling from the hotend — it ranges from occasional pops on a slightly damp spool to a steady snapping on something that has been left out for days.

Beyond the noise, quality drops. Surfaces come out rough and matte where they should be smooth. Stringing gets worse because the moisture disrupts viscosity at the nozzle. On translucent filaments, you often see a cloudy or frosted finish instead of a clear one. In bad cases you get small bubbles or pits in the layer surface.

Not every material reacts the same way. PLA can sit out for a day or two in an average UK room without much drama. PETG and TPU start to degrade noticeably within hours in a humid environment. Nylon and PVA are the worst: leave them unsealed overnight and you can go from a good spool to an unprintable one by morning.

What a filament dryer actually does

A filament dryer is a heated enclosure sized to hold a filament spool. It circulates warm air across the plastic for several hours, which causes moisture to migrate out of the filament and vent away.

That's it. No magic. It's a gentle convection oven purpose-built for spools, with a hole in the side so filament can feed directly into your printer while it dries.

The things that separate a good dryer from a mediocre one are temperature accuracy and airflow. A dryer that claims 65°C but only delivers 50°C won't touch Nylon. A dryer that heats air without moving it will have hot spots and cold corners where the spool barely warms up. Better units have a fan that keeps air circulating properly around the whole spool, not just the outside layers.

What the RH reading means

RH stands for relative humidity — the percentage of water vapour the air is holding relative to the maximum it can hold at that temperature. Because warmer air can hold more vapour, heating the enclosure causes the RH reading to drop even before the filament has fully dried out.

The useful signal is the trend, not the number. When you load a damp spool, the RH inside the dryer rises as moisture leaves the filament. Once drying is nearly done, the reading levels off and stops climbing. That plateau is your cue that the spool is as dry as the dryer is going to get it.

Most materials print well when the RH inside the dryer is below 15–20%. Nylon benefits from pushing below 10%. If you hit that floor within an hour or two, the spool was probably already fairly dry. If it takes eight hours to plateau, the spool was seriously wet.

Some dryers show RH on the front display; others don't show it at all or hide it in a submenu. It's genuinely useful rather than just a marketing feature, because without it you're guessing when to start the print.

Which filament dryer is worth buying

For most home printers running PLA and PETG, a budget dryer does the job. The Sunlu S1 and S2 are widely stocked in the UK and work reliably for everyday materials. The S2 improved the RH and temperature display compared to the older S1 — that's the main reason it gets recommended more often.

If you run Nylon, carbon-fibre blends, or other engineering-grade materials regularly, you need a dryer that can reach and hold 70–80°C without drifting. Budget units often struggle above 55°C. The eSUN eBOX and PrintDry Pro are worth looking at for a wider temperature range and better build quality.

Things to check before buying:

• Maximum temperature: it must reach the drying temperature for your materials. Nylon needs 70–80°C.
• Feed hole design: check the filament path fits your spool type before assuming printing-while-drying will work.
• Spool capacity: most dryers take standard 1 kg spools. 2 kg spools won't fit in smaller chambers.
• RH display: not essential, but useful if you regularly print moisture-sensitive materials.

Filament dryer vs food dehydrator

A food dehydrator works on the same principle and plenty of people use one. The appeal is that larger tray models handle multiple spools at once and cost less per spool than a dedicated dryer.

The drawbacks are real. Dehydrators aren't designed for round spools, so you end up improvising support. Temperature calibration can be 10–15°C off from what the dial shows, which matters when you're trying to avoid melting the spool hub while still drying properly. And there's rarely an RH display.

If you already own a dehydrator, try it before buying anything else. Set it to 45–50°C for PLA and PETG, stand the spool on the tray, and run it for 4–6 hours. Use a separate thermometer probe to check the actual temperature if you have one.

A kitchen oven is not a good substitute. The minimum setting on most domestic ovens is around 70°C or higher — too hot for PLA spools and most cardboard or plastic spool hubs. Ovens also cycle on and off rather than holding a stable temperature, and those swings can warp the spool or degrade the filament before any useful drying has happened.

How long does drying take?

These are rough figures for a spool stored open in an average UK room:

• PLA: 4–6 hours at 45°C
• PETG: 6–8 hours at 55°C
• TPU: 4–6 hours at 45°C
• ASA: 6–8 hours at 60°C
• Nylon (PA12 / PA6): 8–12 hours at 70–80°C
• PVA: 6–8 hours at 45°C

A spool stored in a sealed bag with fresh desiccant only needs an hour or two, even for sensitive materials. A spool that has been open for weeks in a damp garage may need longer than the figures above, and the RH plateau is your best indicator that it is actually done.

Keeping filament dry after drying

Drying a spool and leaving it on an open shelf defeats the point. Most materials begin reabsorbing moisture within hours in a normal room.

Keep dried spools in resealable bags or airtight boxes with silica gel desiccant. Colour-indicating gel is worth using over plain white — you can see when it is saturated and needs recharging. Recharge it by baking at around 120°C for an hour, and it will pull moisture again for years.

If you print moisture-sensitive materials often, a dry-box setup is worth the small effort: a sealed container the spool sits in, with filament routed through a PTFE tube directly to the printer. The spool stays dry throughout a long print without needing the dryer running the whole time.

When to get it printed instead

If you're working with a demanding material — Nylon, TPU, carbon-fibre composite, or anything engineering-grade — and you'd rather not invest in a dryer, dry-box, and storage system, we can take care of the print for you. We run a mail-in 3D printing service from our UK workshop using calibrated machines and freshly-dried filament. You send us the file, we print and post the part back. No equipment to buy, no drying schedule to manage. Find out more on our printing page or get in touch with the details of your project.