Storage & Prevention

Drying filament is only half the job. If you return a freshly dried spool to an open shelf, it starts re-absorbing moisture immediately. Proper storage keeps your filament ready to print without re-drying every time.

How Quickly Does Filament Re-Absorb Moisture?

Re-absorption speed depends on the material and ambient humidity. A rough guide:

The key implication: for Nylon and PVA, sealed storage is not optional — it's essential. For PLA, a loose storage habit will cost you in print quality over weeks or months.

Desiccants

Desiccants absorb moisture from the air inside a sealed container. No desiccant lasts forever; all eventually become saturated and need to be recharged or replaced.

Silica Gel

The most common desiccant for filament storage. Available in loose granule form, sachets, or large packs.

• Absorbs approximately 30–40 % of its own weight in water at typical conditions
• Rechargeable: spread on a baking tray and bake at 120 °C for 2–3 hours until dry (watch for colour change if using indicating type)
• Readily available and inexpensive

Colour-indicating silica gel: Granules contain a dye that changes colour when saturated.

• Classic blue (cobalt chloride) → pink when saturated. Note: cobalt chloride is being phased out in many countries due to health concerns; many modern products use alternatives.
• Orange → green when saturated (cobalt-free, more common now)

Molecular Sieve (3A or 4A)

More aggressive than silica gel — achieves lower residual humidity inside a container.

• Can reduce humidity to below 5 % RH (silica gel typically maintains 15–25 % in a reasonably sealed container)
• Better choice for Nylon, PVA, and other very hygroscopic materials
• Rechargeable at higher temperatures (200–300 °C) than silica gel
• More expensive than silica gel

Recommendation: Use molecular sieve for Nylon and PVA storage; silica gel is sufficient for PLA, PETG, and ABS.

Clay Desiccant

Cheap, disposable, and less effective. Fine for short-term protection but not recommended for sensitive materials or long-term storage.

Storage Containers

Sealed Storage Boxes

The workhorse of filament storage. An airtight clip-seal box with enough desiccant is effective for most materials.

• Good options: IKEA 365+ (various sizes), Sterilite clip-top boxes, Really Useful Boxes, Sistema containers
• Holds 1–3 spools depending on size
• Easy to open and reseal
• Check the seal quality — not all "airtight" boxes are truly airtight; add a thin silicone bead around the lid if necessary
• Add a small hygrometer inside to monitor humidity (inexpensive digital versions cost £3–8)

Vacuum Storage Bags

Excellent for long-term storage or materials you won't use for weeks or months.

• Removes most of the air along with ambient moisture
• Significantly slows re-absorption even with a marginal bag seal
• Works well combined with a small desiccant pack inside the bag
• Reusable with a hand pump or vacuum cleaner

Zip-Lock / Resealable Bags

Adequate short-term protection (overnight, a few days) but not truly airtight. Better than leaving a spool open on the shelf, but not suitable for Nylon, PVA, or long-term storage of any material.

Humidity Monitoring

Knowing the actual humidity inside your storage containers tells you whether your desiccant is working and when to recharge it.

Target: < 15 % RH inside sealed storage. For Nylon and PVA, target < 10 % RH.

Hygrometers: Small digital hygrometer/thermometers (often sold as "mini hygrometer" or "cigar humidor hygrometer") are accurate to ±3–5 % RH and cost £3–8. Place one inside each storage container.

• When the reading creeps above 20 %, recharge or replace the desiccant
• Readings above 30 % mean your container seal has failed or the desiccant is exhausted

Print-While-Dry: In-Use Dry Boxes

Even with proper sealed storage, removing a spool and mounting it on an open spool holder exposes it to ambient air during the print. For sensitive materials and multi-hour prints, a dry box or in-dryer printing setup solves this.

Option 1: Print Directly from a Filament Dryer

Several dedicated filament dryers are designed for this:

• The spool stays inside the dryer throughout the print
• A small filament exit port allows the filament to feed directly to the extruder
• The dryer keeps the spool at the target temperature, actively preventing re-absorption
• Good models for this: EIBOS Cyclopes (rotates the spool for even drying while printing), Sunlu S2, most other dryers with a filament outlet

Option 2: DIY Dry Box

A sealed container with a feedthrough for the filament path.

What you need:

• An airtight container large enough for your spool (standard spools are 200 mm diameter × 70 mm wide)
• Desiccant inside
• A small hole with a PTFE tube pressed through it — the filament threads through the tube and into the Bowden or directly to the extruder
• Optional: small hygrometer inside to monitor conditions

Tips:

• The hole should be small enough to press-fit the PTFE tube snugly — this prevents air exchange
• Route the PTFE tube so there is minimal friction on the filament
• Check desiccant regularly during long print sessions

Option 3: Commercial Dry Boxes

Products like the Polymaker PolyBox, eSUN eBOX, and similar are purpose-built dry boxes with filament path outlets, built-in hygrometers, and sometimes heating elements. They sit between your filament storage and the printer and maintain a low-humidity environment around the spool during printing.

Maintenance Schedule

Desiccant management is the key maintenance task. A suggested schedule:

Storage for Specific Materials

Nylon and PVA — strict protocol required

• Seal immediately after drying, before the spool has time to cool in open air
• Use molecular sieve desiccant if available
• Store in a vacuum bag or genuinely airtight container
• For PVA, consider a dedicated sealed container that is only opened immediately before printing

PLA — practical storage

• Sealed storage with silica gel is more than adequate
• Old PLA that has become brittle on the spool has likely undergone hydrolytic degradation (polymer chain scission from moisture over time) — drying will help with print quality but cannot reverse the brittleness entirely

PETG, ABS, ASA — standard storage

• Sealed storage with silica gel
• Reasonable tolerance for being left open for a typical printing session (a few hours)
• Sensitive enough that overnight or multi-day exposure is worth avoiding

What to Do With Old, Questionable Filament

If you find a spool that has been left open for weeks or months:

1. Dry it first at the appropriate temperature for the material
2. Test print a small object and evaluate surface quality and any sounds during extrusion
3. Check for brittleness by bending a short length — PLA that has been wet for a long time may snap rather than bend even after drying, indicating hydrolytic degradation has occurred
4. If quality is acceptable after drying, seal it with fresh desiccant and note the date it was dried
5. If quality remains poor after a full drying cycle, the polymer may be permanently degraded — at that point, reserve it for non-critical prints or discard