Filament storage: how to keep your spools dry and print-ready

Filament picks up moisture from the air constantly. Leave a spool open on your shelf for a few weeks and you will notice it. Leave a spool of nylon open for a few hours on a damp day and you will really notice it. Good storage is one of those things that seems optional until it suddenly is not.

Why moisture ruins prints

When filament absorbs water, the moisture sits inside the polymer. At printing temperatures, that water turns to steam. You hear it as a faint popping or crackling in the hot end. You see it as rough, bubbly layer surfaces, excessive stringing, inconsistent extrusion, and more frequent jams.

PLA is relatively forgiving. PETG and ABS absorb moisture faster. Nylon and TPU are the worst — nylon can go noticeably damp in a few hours on an open shelf on a humid day, and wet nylon prints badly enough to be unusable.

Signs your filament has absorbed moisture

The print will usually tell you before anything else:

• Popping or crackling sounds while printing
• Rough or bubbly surface texture on layers
• More stringing than usual between features
• Inconsistent line width or blobs mid-print
• More frequent clogs or partial jams

If the tip of the filament at the nozzle looks foamy or has tiny bubbles in the melt, that is moisture. A crackling sound that disappears after the first few minutes sometimes means only the surface is damp and the core is still fine. A spool that crackles all the way through a long print is properly wet and needs drying before you use it again.

Which materials absorb moisture fastest

Hygroscopic materials draw in water actively. From most sensitive to least:

1. Nylon (PA) — hours on an open shelf 2. TPU and other flexible filaments — a day or two 3. PETG — a few days to a week 4. ABS — similar to PETG 5. PLA — most tolerant, but still affected over weeks

PLA seems fine until it is not. A spool that has been open for six months may print cleanly at first and then start stringing and popping halfway through a long job.

How to store filament properly

The goal is keeping the humidity around the spool below about 15% relative humidity. You do not need anything exotic to do that.

Airtight containers with desiccant

A large airtight food storage container — the kind with a rubber gasket and clip-down lid — does the job well and costs a couple of pounds. Drop in one or two silica gel sachets and seal it up. A 15-litre container holds two or three standard 1 kg spools comfortably.

The critical thing is a proper airtight seal. Snap-on lids without a gasket do not work well enough. Press on the lid — there should be some resistance from the seal, not just the plastic tabs clicking into place.

Vacuum storage bags

Vacuum bags remove most of the air around the spool, so there is very little moisture to absorb. They are cheap and stack well in a cupboard. The practical downside is that you need to re-vacuum them every time you open one, which gets tedious if you switch materials often. They are best for long-term storage of spools you will not touch for months.

Dry boxes with passive or active drying

A dry box lets you print directly from a sealed container while the filament feeds through a PTFE tube to the hot end. Passive dry boxes use desiccant inside a sealed enclosure. Active dry boxes add a small heating element to drive the humidity lower and keep it there.

For PLA, PETG, and ABS, a passive dry box with desiccant is usually enough. For nylon, TPU, or any long print in a damp environment, an active heated dry box makes a real difference. Commercial options from Sunlu, Bambu, and eSUN sit around the £30-50 mark and are a clean solution if you print every day. You can also build a basic version from a large airtight container with a PTFE coupler through the lid and a spool sitting on a bearing. It works well.

Drying filament that is already wet

Storage prevents the problem. Drying fixes it. You cannot dry filament by leaving it in a dry container — you need heat.

A food dehydrator is the best tool for this. Set it to the right temperature and leave the spool in for four to eight hours:

• PLA: 45-50 °C
• PETG: 65 °C
• ABS: 65-70 °C
• Nylon (PA12/PA6): 70-80 °C
• TPU: 45-50 °C

Do not exceed these temperatures. PLA spools can warp or fuse at higher settings, especially the cardboard centre insert.

Kitchen ovens work but the element cycles on and off, which can spike the temperature above what you set. Most ovens are not accurate at 50 °C. If you use one, keep an oven thermometer inside and leave the door slightly ajar. A dedicated filament dryer is more reliable and sits on your bench without tying up the kitchen.

After drying, move the spool straight into a sealed container while it is still warm. Leave it to cool on the bench and it starts absorbing moisture again within minutes.

How long desiccant lasts

Silica gel sachets saturate over time and stop working once they do. Most colour-indicating silica gel starts orange or blue when dry and turns pale or white when saturated. When it changes colour, regenerate it: spread it on a baking tray and put it in the oven at 120 °C for an hour. Regenerated desiccant works as well as new.

A rough guide: one 50 g silica gel sachet per 10-15 litre container holding two or three spools. Check the sachets every few months — more often if you live somewhere humid or if you are storing nylon or TPU.

How long does sealed filament last

A properly sealed spool with fresh desiccant should stay printable for a year or more. PLA is the most stable. Nylon and TPU are the most demanding — even sealed, the desiccant will eventually saturate and moisture will creep back in, so check and regenerate more often for those materials.

If a spool is old, print a small test piece first. Dimensional accuracy and surface finish will tell you quickly whether it needs a drying cycle before you commit to a long print.

When to get it printed instead

If you would rather skip the spool management entirely and just get a part in hand, we print to order from our UK workshop. We use dry, freshly opened filament for every job and run both a Bambu A1 and a Creality K2, so we can match the machine to what your model needs. Upload your file at /printing.html, get an instant quote, and we will turn it around within a few working days. No need to worry about whether your PETG spool has been sitting open since last winter.