Isn’t that contradictory?
If you loose vacuum, you are taking in air which will have more moisture than the air inside the bag.
This. According to the hygrometer on my desk, it’s 70% ambient humidity today. If the bags leak and take in air, they’re taking in water with it. At that point permeability is not the main problem I face. 
Even if they would take in 1L of air due to leakage, the amount of water carried in with that air is negligible compared to the water that comes in over months or even weeks through the plastic.
And 1L would be already bulging the bag.
at 25°C, 50% RH, 1L of air contains 0,0115g of water.
my molecular sieve in the bags is saturated after typically 2 months when I don’t open the bags. It can hold roughly 15g of water before it is saturated.
So in those 2 months, more than 1000x the amount of water enters the bags through the plastic compared to the water than 1L of leaked in air would carry.
That is why I care more about permeability of the plastic than about holding a tight vacuum.
Dalton’s Law of Partial Pressures is why. With a vacuum you are removing some or most of all the gasses in the bag - N2, O2, the traces, and water. But in gasses, they each act individually because they are largely isolated molecules in the gas phase. O2 and N2 don’t affect filament but they are what makes vacuum bagging look like it’s helping. But vacuuming does remove a lot of the small amount of water from inside the bag - at first.
Water is a smaller molecule than oxygen or nitrogen and that’s part of why you can have a bag that looks under heavy vacuum yet has had lots of water leak in straight through the walls. See also new spools of filament in vacuum bags with too much water to print properly.
Vacuum bagging is more show unfortunately than useful. Without impermeable bags (expensive), it’s a temporary band-aid because water does leak in through the plastic.
The points about how much water leaks in are really good. It’s driven by dry filament and desiccant. For stuff to move like water through the walls of a plastic bag, it takes a gradient. Outside the bag is an unlimited reservoir of water vapor in the air. Inside the bag is much more dry thanks to desiccant and filament (which acts like a desiccant). As water molecules bounce around inside the thin plastic film, they bounce more in the direction away from the high concentration and move to lower concentration. It’s a no moving parts pump of sorts. The flow rate of water is proportional to the difference in concentration inside and outside.
Unfortunately the dryer it is in the bag, the more water tries to get in.
Ah, that is good data to have; now I understand your position. Thank you!
So in application, it seems that vacuum bags can be useful for short term storage in a humid environment (mine), but will fail for long term storage.
Re my environment, it’s humid AF in July here, plus I face water intrusion issues in the basement of my house. I’ll accept some water permeating through the plastic; it almost has to be slower than filament can grab water from the ambient air.
That depends though. As long as you keep desiccant dry in the bag, the only real difference between bags and thick-wall cereal boxes for example is the water leak rate and how often you have to regenerate the desiccant. If you are mindful of water, bags can be fine for storage especially if kept in plastic tubs that also have desiccant to keep humidity in them down. But you have to monitor and control humidity close if using a leaky system.
Where the problem comes is for those who don’t realize how leaky bags can be. If you dry a spool and seal it in a bag, you have to monitor the humidity in the bag and replace desiccant as needed to keep the humidity in the bag below where water problems appear.
The wrong way is to seal a spool in a bag and leave it to leak up with water. It doesn’t take long to have all the time invested in drying be largely wasted.
Maybe this is why CNC Kitchen’s attempt at vacuum drying was such a failure. He just pumped it down once and then was dismayed when it seemed to make little difference in desiccating his filament. Maybe if he had pumped it down over and over he would have noiticed more of an effect.
On the other hand, it was also unheated, and heating something evenly in a vacuum isn’t as straightforward..
Anyone tried activated alumina? I have a jug of it, but when I poured some out it was so dusty I thought “this can’t be good.” However, I’ve since read that such dust is fairly typical from getting jostled around during shipping, and once you clean the dust off really well it largely stays gone. Activated alumina spheres seem like ithey might be a kind of a middle ground between silica gel and molecular sieve in terms of ease of regeneration (lower temperature than molecular sieves) and maybe effectiveness.
Alumina works very well. It performs better for getting the lowest moisture content. It is considerably faster than silica (i cant speak to the molecular sieve).
I dont use it anymore mostly because its not indicating and it doesnt last as long between recharges.
I’m thinking a good use for molecular sieve would be to put it in my blast oven when I’m drying a filament, after I close the exit ventilation hole. Assuming I could also shut the input air holes, then in theory that would make the oven less susceptible to ambient humidity than constantly flowing new air into it. It also wouldn’t tax the molecular sieve capacity much, because the total humidity emitted by a typical spool of filament isn’t all that many grams anyway.
Well, if the equation requires me to take off my shoes, then it’s too high in numbers to get hung up on.
Naw, I just use the dryer function of my AMS2. Besides, I live on the Westward Spir of the Mojave Desert, so moisture isn’t the issue for me that it is for a lot of folks.
I think some blame for folks being confused has to be the “Hydroscopic” scare of the filaments. Not enough specifics from the manufacturer’s. Around August here the RH drops into the teens. So not likely I will need to dry anything. But lately the RH has been in the mid to upper 30’s. Right now is showing 38-39% RH in the air.
My AMS is reading 20%. Does anybody really know what time it is?
I plan to do the same inside the HT. Just get the humidity to zero inside while drying. Or you follow @MZip 's setup. That gets you 0% humidity inside your dryer while maintaining the ease of regenerating Silica gel. I would be tempted when I had the space.
Fortunately, my setup for regenerating molecular sieves works really well and easy now. Most of the time I keep it in the basement. I currently have ~4 liters and regenerate 1-2 L maybe 3 times a year. Then it sits in the kitchen a few hours and goes back to the basement when I’m done.
I use the box that the filament comes in. I have the filaments on a shelf, side by side. I purposely cut the box so that the label of the filament … manufactuirer and color … shows. I currently have 14 filament boxes that are side-by-side on a 45-inch long shelf.
I think that you have totally missed the point of the question.
It happens. Especially with partial spools you hope to “maybe someday” use up whenever the need for that particular color arises (and then never does).
That’s why I have high hopes the CMYK approach will solve this. At least it can maybe cut down on all the leftovers. Yet, I doubt it will ever give an orange as bright as the one in your photo.
Whatever works, I suppose.
I’d at least put it in something to keep the dust off.
Have any of you run across any filaments that you couldn’t revive to “good as new” with a good drying? I’ve read some filaments get permanently destroyed if they uptake too much moisture, but I can’t say that I’ve ever experienced that, at least not to such a degree that it’s indisputable.
Or maybe I have. I think embrittlement is the term. I have had to toss a few spools that were brittle before drying and still brittle afterward. It’s not obvious that its due entirely to moisture though. Maybe it’s plasticizers that have evaporated away over time or something.
I gave up on some amazon grade wood-filled PLA. It may even have been overdrying, but it ended up too brittle to do anything with. When it fed, it did print relatively fine
I’m thinking the TPU and TPE over molding on power tools is the classic plastic failure, where it becomes all gooey and disgusting, allegedly from heat and humidity and because the plasticizers migrated to the surface over time. I wonder if the same thing happens with 3d printer filament.