Here is how to store filament long term: dry the spool, then seal it in an airtight bag or gasketed box with fresh silica gel and a hygrometer. Hold PLA under 30% relative humidity and PETG, TPU, PC and PVA under 20%, at room temperature and out of sunlight. Polymaker rates an opened, resealed spool at about two years.
The rest is detail that decides whether it works: which number applies to which polymer, how much desiccant a low-humidity box really needs, and why heat does more lasting damage than damp.
What humidity should stored filament sit at?
Under 20% RH covers every common material, and PLA tolerates up to 30%. Those are the maximums in Prusa’s USS Drybox documentation, read on the container’s own hygrometer, and they match UltiMaker’s storage guidance to keep most filaments below 20%.
| Material | Prusa maximum RH | Prusa ideal | Storage priority |
|---|---|---|---|
| PLA | under 30% | short-term tolerance up to 40% | Low |
| ABS | under 30% | under 15% | Medium |
| ASA | under 30% | under 10% | Medium |
| PETG | under 20% | not specified | Medium |
| TPU | under 20% | under 15% | High |
| PC | under 20% | not specified | High |
| PVA, BVOH | under 20% | not specified | Highest |
| Nylon (PA, PA-CF) | not listed | store dry or dry before every print | Highest |
The nylon row comes from Prusa’s drying article, which groups polyamide, polypropylene, PVA and BVOH as highly hygroscopic. The priority column is an editorial ranking built from those two pages, not a vendor figure.
Vendors disagree at the edges. UltiMaker’s PVA page sets the limit at below 50% RH, but that figure is for the storage room, with the spool sealed in a container inside it. Prusa’s 20% is the air touching the filament. For anything sitting for months, use the stricter number and measure it inside the container.
How long stored filament stays printable
The Polymaker FAQ gives three years for an unopened spool kept at room temperature, out of sunlight, in 30–40% RH, and two years for an opened spool returned to its resealable bag under the same conditions. UltiMaker gives a one-year shelf life for its moisture-sensitive grades (PVA, Nylon, Nylon CF Slide and TPU 95A) and up to two years for PP and PC.
Read those as the window a manufacturer will stand behind, not an expiry date. Absorbed water is reversible for most materials: the right cycle from the filament drying chart brings a spool back. Heat damage and chemical breakdown are not reversible, and those are what long-term storage protects against.
PLA also changes slowly when kept dry. In Orellana-Barrasa et al. (Polymers, 2022), PLA strands stored in PET zip bags with silica desiccant at 19 ± 3 °C showed a glass transition temperature that rose from 53.9 °C on day one to 61.0 °C by day 240. Fewer of the aged tensile samples necked before breaking than the frozen controls. That is physical ageing, not moisture damage. The strands were 400–550 µm printed filaments rather than 1.75 mm spool stock, so take it as a mechanism, not a timeline. Freezing at −24 °C stopped the process.
The same paper notes that hydrolysis, water chemically cutting PLA’s chains, needs temperatures around 50 °C. A cool closet never gets there. A summer attic, a shed or a parked car can.
A long-term storage setup, step by step
1. Dry before you seal. Prusa is blunt about it: the Drybox does not dry filament, and a moist spool must be pre-dried. A sealed container preserves whatever moisture goes into it. Run the material’s drying cycle, let the spool cool to room temperature, then seal.
2. Choose the container by how often you open it.
- Untouched for months: a vacuum bag with desiccant inside. UltiMaker recommends vacuum sealing for rarely used filament, and Polymaker’s good-practice page calls vacuum-sealed storage with desiccant the minimum requirement, adding that airtight DIY containers with silica packets can be just as effective.
- Weekly stock: a gasketed tote with a hygrometer, UltiMaker’s own DIY suggestion. One tote per material group means you never open the nylon box to fetch PLA.
- The spool on the printer: a feed-through dry box. The USS Drybox product page quotes about 10% RH with 50 g of silica gel, stable for weeks, with a sealed PTFE exit.
3. Size the desiccant for low humidity. Silica gel holds less water the drier you ask it to keep the air. Delta Adsorbents’ DelSORB spec lists a water capacity of at least 11% of the gel’s weight at the 20% level, 22% at 40% and 33% at 80%, and says silica gel works best at 60–90% RH. A box held at 15–20% RH runs at the bottom of that curve, so 100 g of gel holds roughly 10 g of water before it stops pulling humidity down. Every lid opening spends that budget. Use more than the factory sachet, and use indicating beads so the colour change shows when they are spent. Regeneration is covered in the drying guide.
4. Put a hygrometer in every container and act on it. Prusa’s rule is to replace or regenerate the silica gel once the reading exceeds the stored material’s maximum. After you close a lid, the reading can take up to an hour to settle, so ignore the number thirty seconds after shutting it. Inspect the seal each time and avoid unnecessary openings.
5. Keep it cool, stable and dark. UltiMaker’s range for most of its filaments is −20 °C to +30 °C, narrower for ABS at 15–25 °C and PVA at 10–30 °C, and it warns against temperature fluctuations and direct sunlight. Polymaker asks for a low-humidity, dark, temperature-stable environment and singles out climates where ambient humidity exceeds 70%. Daily swings matter twice: a container that warms and cools also breathes room air through any imperfect seal.
6. Label, weigh, rotate. Write the material, date dried and date sealed on the bag. Weigh the spool before sealing; the spool weight to length conversion tells you what is left without opening it, and a heavier reading later on a hygroscopic grade like nylon is a moisture warning. Open the oldest stock first.
Which spools to protect first
Nylon, PVA, BVOH and polypropylene get the best containers, then TPU, PC and PETG, then ABS and ASA, with PLA last. Prusa advises that when dry storage is limited, the more hygroscopic filaments take priority, since PLA does not change properties much with humidity while polyamide is affected significantly. Polymaker says nylon-based grades are hygroscopic and recommends printing them from a filament dryer.
So the box with the lowest reading and freshest desiccant holds nylon and PVA, and PLA can live in a decent tote. ABS and ASA tolerate more than PETG on paper, but Prusa’s ideals of under 15% and under 10% show they are not indifferent; ABS vs ASA for printing covers what else separates them.
Is freezing filament worth it?
No, not for a home or small-shop shelf. Freezing at −24 °C stopped PLA’s physical ageing in the Polymers study, but that was a lab protocol for keeping test batches consistent, run on thin strands in zip bags. On a 1 kg spool the practical risk is condensation: open a cold bag in a warm, humid room and water forms on the filament.
If you freeze anything, leave the bag sealed until it is back at room temperature, then plan on a drying cycle before a critical print. A closet at stable room temperature does the job without that risk.
How to check a stored spool before a long print
Work cheapest first:
- Read the container’s hygrometer. Above the material’s maximum, dry the spool and skip the rest.
- Extrude 100–200 mm at normal print temperature. Polymaker flags popping or cracking during extrusion as the sign of excess moisture; UltiMaker adds bubbles in the filament.
- Bend a length from the outer wraps. Exposure is worst on the outside. If the strand snaps with little bend, cut a few wraps off and test further in.
- Print a retraction or temperature tower on the same profile the spool ran before storage.
Success criteria: no popping, stringing no worse than the spool’s earlier prints, the same surface sheen, and layers that survive a hand snap test on a small part. If it fails, dry and retest. If it still fails after a full cycle, moisture was not the cause; work through the wet filament symptoms to rule it out properly, then look at the nozzle, the profile or the batch.
FAQ
can you store filament in a garage
Yes, but only if the garage stays inside roughly −20 °C to 30 °C, UltiMaker’s general storage range, and every spool is sealed with desiccant. Summer heat is the bigger risk than winter cold: PLA’s hydrolytic breakdown needs temperatures around 50 °C, and daily temperature swings pump humid air past tired lid seals.
is a vacuum bag enough to store filament
A vacuum bag is enough only with desiccant inside and a dry spool going in. Polymaker treats vacuum-sealed storage with desiccant as the minimum requirement. The vacuum just shrinks the air volume the desiccant has to dry; the film and the seal block new moisture, and a pinhole or a worn zip defeats both.
how often should you replace desiccant in a filament box
Replace or regenerate the desiccant when the box’s hygrometer reads above the stored material’s limit, not on a calendar. That is Prusa’s rule, with limits of under 30% RH for PLA and under 20% for PETG, TPU, PC and PVA. Indicating silica gel adds a visible cue: the beads change colour as they saturate.
does unopened filament go bad
Not quickly: unopened filament lasts about three years by Polymaker’s figure, if it sits at room temperature, out of sunlight, in 30–40% RH. UltiMaker gives its nylon, PVA and TPU 95A only one year. The factory bag and sachet slow moisture rather than stop it, so test-extrude an old sealed hygroscopic spool before a long print.
