I was noticing on a print I had just done that the quality was not up to typical snuff. I had just started using a roll of PLA filament that I had been keeping on a shelf without a wrapper for a couple months. How long can you store filament before it gets too hydrated from the air to print? I expected more than a couple months but perhaps I am wrong?
In theory, most filaments don't go bad.
It is however always a good idea to store filament dry. To enforce this, some use racks in a well-heated room, others are blessed with very dry weather overall. And others are forced to use dryboxes. Dryboxes can keep the filament reasonably isolated from the surrounding air and so prevent moisture interacting with them. It is also a good idea to store them out of direct sunlight, as UV light might destroy color and/or the plastic.
More information on why to use them is for example at the question Which filaments actually do need to be stored in a drybox?
But fear not: most filaments - PLA included - can be freshened up again! Just bake them at a low temperature or store them in a dehumidifier. For PLA, keep the temperature at below 80°C. A couple hours should get all the moisture that has seeped in out again. The Quality might not get back to that of fresh filament in all cases, but you might at least regain reasonable to good printability.
The answer is that it depends on the climate you are in or the climate in your house. The more humid, the faster the moisture is taken up in the filament.
I have obtained a spool of filament that has been left in the garage for a few months, this spool has taken up so much moisture that it has become very brittle (unspooling will break the filament). Note that once the filament has taken up moisture, you could de-hydrate it, but it will never reach the same properties anymore as the moisture will change the molecules (shorten them), once broken they will not fuse by hydrating. It will however get rid of the water content so that there will be no steam bubbles when you print the filament.
Note that there are many types and brands of PLA, each with their own formula. The quality of the PLA has improved much over the years, implying that the moisture intake also depends on the brand and time of purchase. It is therefore very difficult to answer your question with a specific time frame, it could be months, but it could also be a week.
My largest printer is located in a room where occasionally laundry is being dried, and although there is only a little ventilation, the large spools of PETG do not take up the moisture of the laundry. Basically, the moisture saturation is also depending on the type of filament.
To answer your question: it depends on the relative humidity. Generally, a few weeks in a semi-arid environment. But, it'll still print really well. If you're looking for really tight filament diameter tolerances, a week or even less could change the diameter .001 or more if there is moisture in the air. I've used PLA that has been exposed to air for a few months and it's been fine, but with small issues.
If the filament has absorbed too much moisture, you'll typically hear popping coming from the hot extruder as the water is burned off at the nozzle. Sometimes steam comes out of the nozzle when printing as well with "wet" filament. Usually, you can still get great results even if it has absorbed moisture. If the filament diameter has been affected significantly, (if it jams, or the water keeps the plastic too cool when it's coming off the nozzle) you can dry it out in an oven for a few hours like @trish suggested. I keep my filaments in a plastic bag with desiccant inside a big storage bin. Probably overkill but my water softener is in the same room and introduces moisture to the air, and my a/c blows right at the setup.
Dust is probably worse because it can accumulate much faster and cause jams.
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