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I've heard I should store filament in sealed container, preferably with a desiccant.

But let's say I let a spool get a little old on the printer, or I purchased a filament spool that was old or improperly packaged. How would I know? How would this impact prints (what kind of symptoms would I see)? What things could I do (perhaps in the slicer) to correct for this and prolong the life of marginal filament?

And the corollary... in a typical environment, how long can filament be left out without suffering from the exposure?

I'm thinking mainly about PLA, but responses for ABS and other materials are useful, too.

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  • $\begingroup$ If it's PLA it will snap easily. I had a roll for 3 years open to the air. It snapped like macroni. Nylon will expand and not print well. You'll also hear it popping an hissing. $\endgroup$ – user77232 Sep 6 '18 at 2:23
  • $\begingroup$ Have you found and fixed the problem? If so, have the answers below led you to the solution? Please vote to accept an answer (using the tick button next to the answer) so this question is not bumped up once in a while and can be removed from the unanswered question list. You may even add your own solution and accept that after 48 hours! If you have not been able to address the problem please update your question. $\endgroup$ – 0scar Sep 23 '18 at 14:46
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The primary issue with long-term exposure of filament to the environment is that it will absorb water moisture from the air. When a filament that has absorbed water is passing though the hot end of a printer, the water will turn to steam and cause problems with extrusion:

  • Small bubbles of steam can form, causing extrusion to sputter - you might hear a sizzling noise and have poor consistency.
  • Large steam bubbles can cause significant oozing followed by no extrusion.
  • Extreme cases can cause mysterious jams that seem to clear themselves (the extruder cannot overcome the steam pressure).

In short, this will cause terrible print quality and failed prints. As the effects are not consistent, there is nothing that can be done by slicer settings to "recalibrate" for filament that has absorbed water.

This can be avoided by storing filament in an air-tight container with desiccant to ensure low humidity. Some people use "dry boxes" that allow the spool to be mounted inside while filament can be passed to the printer, so there is minimal exposure even while the spool is in use.

If you do suspect that your filament has absorbed moisture, you can dry it out, by placing the spool in a warm oven or in a food dehydrator for a few hours. If you weight it before and after, you should find that it weighs several grams less afterwards. WARNING: It is important that the temperature does not soften the plastic at all, or it can become distorted or bind on the spool. Most ovens will peak well above the set temperature as the thermostat cycles. Of course, fully melting a roll of filament could destroy your oven or cause a fire.

It's hard to say how much environmental exposure is too much, as every filament and environment is different. When I started out, I had several spools of PLA that I stored in the open for months. I didn't think I was having any problems, but I was also learning much and improving my printer settings at the same time. After getting PETG, it became unusable with oozing and jams after about two weeks but a few hours in my oven was a miracle cure! I then dried some PLA as well, and I found that print quality did improve, but not amazingly so. I have not used ABS, but in theory it is less hygroscopic than PLA, so it is probably not very sensitive to exposure.

I set up a dry storage box, and I am careful to always store PETG or my "good" PLA when I'm not actively using it. I have a couple rolls of PLA that I don't like as much anyway and generally just use for draft prints, and I don't really worry about it that much.

Note: An object that has been printed will also absorb moisture, but in general this isn't a problem.

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  • $\begingroup$ So I could run a dehumidifier in the room with the printer and spool, and probably be "good enough" if I always run through an entire spool before opening the next? $\endgroup$ – Joel Coehoorn Aug 31 '18 at 13:42
  • $\begingroup$ @joel a dehumidifier will be a very slow way to dry the spool. Putting it an oven at a temperature well below the softening point, e.g. 120 C, will work much faster. I"m not convinced that modern ovens have overshoot of more than a few Kelvins. $\endgroup$ – Carl Witthoft Aug 31 '18 at 14:57
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    $\begingroup$ I'm still on my first full spool for my first printer. Right now I only have the one spool (may order some more this weekend), but I've left it on the printer and it's been about three weeks. No sign of any problems yet, but I want to know what to watch for. $\endgroup$ – Joel Coehoorn Aug 31 '18 at 16:17
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    $\begingroup$ Great answer. We've actually made a filament dryer by taking a deep fryer, adding a spindle, and putting a pneumatic fitting in to run feed tubing (4mm OD) to the print head, so the filament is preheated, dried, and never exposed to the general air. You might try this to salvage older filament, but use caution. Don't set the temp to be too close to the glass transition temperature of your filament. $\endgroup$ – Davo Aug 31 '18 at 16:51
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    $\begingroup$ Having lots of fun :) My eventual plan is to use gallon-size freezer bags and just keep them in a nearby cabinet that doesn't otherwise seal well. $\endgroup$ – Joel Coehoorn Aug 31 '18 at 17:27
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Moisture creeps into the filament when PLA is exposed for a long time to a humid environment. This is audible like popping bubbles when heated in the hotend and does worsen the print quality. The moisture causes a noticeable property as it breaks the long molecular chains of the thermoplastic material. Once this has happened, its effect is irreversible.

The effects of this moisture breaking the long molecular chains results in a more brittle filament; it can even snap during feeding, leading to unpleasant results like unfinished prints. If the filament snaps/breaks when you bend it, instead of folding, your filament has taken up moisture.

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