I’m new to using PVA support filament, and have read it is especially prone to absorbing moisture out of the atmosphere. People made it sound like it starts to happen quickly, and if you aren’t supposed to leave it on the machine overnight, what’s the difference between that and doing an overnight print with the roll?

I’m building an enclosure for the printer (BCN3D Sigma, filament lives in the build space), expect the ambient temperature when printing PLA to be ~35 °C, unless I add an active heating element (100 W lightbulb possibly). The question is, do the hygroscopic materials continue to absorb moisture in a warm, toasty environment? And if so, do I need to construct a dry box for the filament to live in as it prints?

A frugal attempt at research brought back this: As noted by the Sciencing.com website article "How Temperature & Humidity are Related":

As air temperature increases, air can hold more water molecules, and its relative humidity decreases. When temperatures drop, relative humidity increases.

After adding an inexpensive thermometer + hygrometer, I can report my enclosure, heated passively with the heat bed, isn’t very effective at raising the temperature and even less impressive at lowering the relative humidity. With the bed at PLA temperature (65 °C) for 2 hours, the temperature raised from 22 °C to 28 °C, and the relative humidity lowered from 32 % to 29 %.

I may go for an active heating element and will report back on what happens when the chamber is hotter.

  • $\begingroup$ FWIW, I''ve got a spool of generic PLA that's been open for three weeks plus a couple days, and as of last night it still printed well. I live in humid North Carolina, though it's been drier than normal recently. $\endgroup$
    – Zeiss Ikon
    Jun 2, 2021 at 13:02

1 Answer 1


The concept that relative humidity decreases with an air temperature increase runs consistent with the concept of using a food dehydrator to purge moisture from filament spools. As the filament heats up, energy is imparted to the water molecules within. The dryer air around the spool will accept the moisture and "distribute" it to the environment. A warmed enclosed chamber will not have much humidity to endanger the filament.

I have a Sigma R16 and have moved the spools to the outside of the enclosure, but I don't plan to print hygroscopic materials in the near future. I suppose I'd have to build some form of drying enclosure around the spools, particularly for nylon and PVA.

My Qidi3D X-Max has both internal and external spool mounts and it is recommended to use the internal mount for nylon, as even an hour's exposure to humidity can deteriorate print quality.

PLA is safe for days, ABS is safe for perhaps less, but PVA is not a good overnight exposure material and nylon is definitely an inside-the-box condition.

I suspect that having the filament inside your Sigma, especially if you plan to fully enclose it, is not going to be problematic. I created a front panel for my Sigma to reduce ABS warping and the internal temperatures reached 37 °C. That's warm enough to keep the water clear of the filament, unless you have an extremely humid environment.

Before you add an additional heater, consider to place a perforated container of color-change desiccant inside the build chamber and observe over time the color change. Obviously when not printing, seal the desiccant from collecting non-test humidity, but I think you'll find it's pretty dry while you're printing.

  • 1
    $\begingroup$ Thanks for your notes. Desiccant in the build chamber is interesting. I’m ordering a thermometer+hygrometer (to measure humidity), I’ll report back what happens. I could see the crucial variables being how much the enclosure heats up, and the local humidity. $\endgroup$ May 17, 2021 at 7:13

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