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As I understand it, putting filament in a box and heating it increases the air's ability to hold water, and this draws water out of the filament.

But when the air cools, it will not be able to hold as much water, and that water can go right back into the filament, for a (near) zero net effect.

It would seem the proper way would be to remove the filament when it's hot, move it to a different box, and leave the dryer box open to dry.

But I don't see anyone recommending this, and people using the filament from the (cold) dryer box seem to be getting good results.

So my question is whether I'm missing something, or if leaving it in the dryer while cooling is "good enough", but it would be a barrier to remove it hot.

I'm asking more about when the filament is still in the dryer. All of that water is still in the dryer when it turns off, so the relative humidity goes way up as the air cools off. Some of the videos I saw even had dew on the dryer's lid.

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  • $\begingroup$ If you left the dryer box open, that allows humidity from the air to be absorbed into the filament, which would defeat the entire purpose of drying it in the first place. That's how the filament absorbed enough moisture to need drying in the first place - by absorbing it from the environment in which it's being stored. $\endgroup$
    – Ken White
    Commented May 23 at 2:24
  • $\begingroup$ I was referring to leaving the dryer open after moving the filament to a different box. The dryer would possibly have dew, or at the very least very humid air in it. If you didn't leave it open, that would stay in there until the next use. $\endgroup$ Commented May 23 at 3:40
  • $\begingroup$ Leaving the dryer open would allow any moisture remaining in it to be released back into the environment, which would increase the relative humidity in the room. But if you've got moisture remaining in the dryer, it's not a decent dryer and you should investigate other solutions. $\endgroup$
    – Ken White
    Commented May 23 at 3:46
  • $\begingroup$ As far as I can tell, that's every dryer on the market. They are a sealed box with a heating element. Unless you can provide some example of one that isn't. That's why I'm asking the question. $\endgroup$ Commented May 23 at 15:51
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    $\begingroup$ I use a modified food dehydrator, which by design includes vents to exhaust the moisture-laden air. I've noted the condensation in some dryer boxes and realize that it's typical for warm moisture filled air to condense on cooler surfaces. Such results certainly indicate that the filament was far too wet for practical use, but there should be means to remove it. If it's condensed, it's no longer in the filament, right? $\endgroup$
    – fred_dot_u
    Commented May 23 at 15:58

2 Answers 2

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I believe that the issue is that when you heat the dryer, you're driving moisture out of the filament and into the air. The heated air can hold more moisture, so you end up with dryer filament.

If you were to leave the filament in the dryer for an extended period of time (and allow the box to cool) the exact same amount of moisture in the air would then be reabsorbed by the filament, leaving you right back where you started.

However, most people will dry the filament, then put it to use after a (relatively) short period of time. The filament will start absorbing moisture from the atmosphere immediately, but at a much lower rate than that at which it was driven out. Thus you have dryer filament to print with.

Even if the box has completely cooled and there's condensation in the bottom of the box, that condensation consists of water that was in the filament (pulled out by the drying process) but has not yet been reabsorbed by the filament. Thus, you can use the cooled filament from the cooled box with the drops of water in the box because those drops are no longer in the filament.

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After one dehydrates a spool of filament, one typically removes the spool and secures it in a manner to prevent the return of moisture.

Using a dehydration device increases the speed at which the moisture is removed. Once the spool is dried, there is a longer period of time involved to "allow" the moisture to return to the filament. Even a simple sealed plastic bag will eliminate the return of moisture, although experience has shown me it's not fully eliminated.

I place my spools in a slide-lock bag with a vacuum port, along with a container of color indicating desiccant and remove the air. In a matter of months, the desiccant indicates exhaustion, although I don't know if it is due to removing more moisture from the filament or from penetration of the plastic components of the bag.

For particularly moisture-sensitive filaments such as nylon, one uses an active heated enclosure for the printer and for the filament spool, as even one hour of exposure will cause the nylon to present undesirable printing artifacts if these measures are not implemented.

PLA is far less sensitive, ABS and PETG a bit more sensitive than PLA, less than nylon. Of course, the working environment is a factor. Our house at fifty percent relative humidity will create more complications than that of a house with RH of thirty percent.

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  • $\begingroup$ I'm asking more about when the fillament is still in the dryer. All of that water is still in the dryer when it turns off, so the relative humidity goes way up as the air cools off. Some of the videos I saw even had dew on the dryers lid. $\endgroup$ Commented May 23 at 0:09
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    $\begingroup$ I've seen those dryers too. That shows a good example of a poorly designed device. There should be the means for the heated air carrying the water to the outside of the device! Consider to add your comment to the original question and to correct filament spelling. $\endgroup$
    – fred_dot_u
    Commented May 23 at 0:34
  • $\begingroup$ Thanks, I've made those changes. $\endgroup$ Commented May 23 at 1:20
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    $\begingroup$ Here to second what @fred_dot_u said. A sealed dryer that keeps the moisture inside is a scam product that does not do anything useful. A functional dryer needs to be constantly cycling the moisture-laden air out for new air that's maybe 40-60% RH at room temperature but extremely low RH once heated, so that it has capacity to absorb more moisture out of the filament. $\endgroup$ Commented May 24 at 17:15
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    $\begingroup$ @R..GitHubSTOPHELPINGICE Therefore, commercial food dryers are excellent to use for drying filament, the hot moist air is driven out, I've been using such a dryer for a while. After drying I put filament in sealable, vacuum-able bags. Or print directly from the dryer. $\endgroup$
    – 0scar
    Commented Jun 8 at 8:26

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