Filament storage: dry boxes, vacuum bags and desiccant

Every spool of filament starts dry and gets wetter. Storage is the whole game: prevention costs a few pounds, and the cure costs the price of a dryer plus six hours per spool. If you buy filament in bundles because the per-kilo price is better — and it usually is — storage is what makes that saving real rather than theoretical.

The three approaches

• Vacuum bags with desiccant. Cheapest per spool and genuinely effective for long-term storage. The downside is friction: a bagged spool is not a spool you can print from, so in practice people leave the current spool out for weeks.
• Sealed boxes. A cereal-style airtight container or a purpose-made dry box with a gasket. Holds several spools, easy to open, and with fresh desiccant will hold low humidity for months.
• Print-from dry boxes. A sealed box with a PTFE port so the filament feeds out while staying enclosed. This is the one that actually changes behaviour, because there is no reason to take the spool out.

The honest recommendation for most people: a couple of large sealed boxes for the library, and one print-from box for whatever is loaded. Vacuum bags if you are buying in bulk and will not touch some of it for months.

Desiccant — the part people get wrong

The small sachet that came in the filament bag was saturated within days of the bag being opened. It is doing nothing. Desiccant needs to be present in useful quantity and it needs to be regenerated.

Buy indicating silica gel — the beads that change colour (typically orange to green, or blue to pink) when saturated. Guessing is the failure mode; being able to see it at a glance is worth the small premium. Regenerate it in an oven at around 100–120 °C for a couple of hours, or buy a rechargeable cartridge with a built-in heater. It is reusable more or less indefinitely.

Use more than feels necessary. A couple of hundred grams for a large box is not excessive; one small sachet in a big container is decoration.

Does a hygrometer help?

Yes, and they cost almost nothing. A small digital hygrometer in the box turns “I think it is probably fine” into a number. Below about 20% relative humidity is comfortably dry; above 40% and your desiccant needs attention. This single cheap addition is what makes the whole system trustworthy.

What does not work

• The original bag, resealed. Zip-top filament bags leak, and most people do not get the air out.
• Rice. It is a folk remedy. It is a poor desiccant and it sheds dust.
• A cupboard. A closed cupboard is at room humidity. It keeps dust off, which is not the problem.

How long does dry filament last?

Properly sealed with live desiccant, effectively indefinitely — people print perfectly good parts from PLA bought years earlier. Left on an open holder in a normal room, PLA is fine for weeks, PETG starts misbehaving within a month or so, and nylon can be a problem the same week.

If a spool has already got wet, storage will not recover it. See whether you need a filament dryer.