I’ve been storing spools in sealed boxes with desiccant, and I keep an eye on the hygrometer. But I’m not sure what numbers actually matter.
Mine sits around 20-25% RH now. Is that “dry enough”? Or do I need to get it lower? What’s the threshold where moisture starts causing problems?
Also, does the reading inside the box actually reflect what’s going on inside the spool? Or is it just the environment around it?
Just curious what numbers people actually target.
That’s not a black-and-white question. Humidity matters as a function of relative humidity, exposure time, temperature, and the specific filament.
For example, PLA is only moderately hygroscopic, so leaving it at >50% RH for some period of time may not produce any noticeable change in print quality. PETG is more moisture-sensitive and can absorb enough moisture to affect printing more readily.
Time is the other major factor. If I start with a very dry spool of PETG and print the entire spool during an 8-hour period, even in relatively high humidity, it may not absorb enough moisture during that time to produce a noticeable difference in the print.
So RH by itself is not a universal pass/fail number. The material, starting moisture level, temperature, and duration of exposure all matter.
Since I asked nearly an identical question, and got no definitive answers, I target the lowest I can get.
I’m presently with a dry cabinet that is doing well to hold 24-25%RH. Then I additionally dry in one of my AMS2 Pro’s from which the filaments are served to my P2S printer.
I have ordered up a Sunlu S4 to use as my “go-between” because I like the specs on it, and because it can handle 4 spools of filament.
So the Sunlu S4 will hold exactly the same number of spools as one of my AMS’s. How novel. My plan is to be able to put 4 spools in one of my (2) AMS2 Pro’s and run through the jungle.
So no matter what, I’ll have done my best to feed dry filament. Presently, my AMS2’s are doing great holding <10% including desiccant holders I printed for them. When my RH creeps up to ~20% I do a drying cycle and regenerate my desiccant from the holders.
So my answer is to do what you can and are comfortable with to provide dry filaments to your printer(s). I find I need to service my AMS’s about weekly with my RH% hovering between the 30"s and mid-40’s ambient, so far.
Hope that helps. I can say I don’t have any problems I can relate to wet filament. So I’m happy. I don’t run much exotic or specialized filaments. Mostly PLA, PET-G, ABS, and TPS.
Bottom line from me would be <10% to ~20%. Other people’s milage may vary.
If your filament is in sealed boxes with desiccant (or without) at 20-25% RH, that’s the absolute limit of what I would print with without touch-up drying and I would expect filament moisture issues of some degree.
That measurement but without desiccant is how I know if filament needs touch-up drying. Since you are seeing that with desiccant, the desiccant should also be regenerated. Or at least I would regenerate it. Typically I would do that measurement without desiccant since dry desiccant skews or obliterates a filament humidity test, but it sounds like your box has bled up and both desiccant and filament have the same humidity value where that doesn’t matter.
You can tune out water effects to some degree so you may or may not see the same issues at that filament humidity.
You did get an answer. I gave it back in your drybox thread.
I’m big on maintenance, and I group filament storage in that same category. When I go to use something, I want it to just work. In my mind, dessicant is cheap, and my frustration and time isn’t, so I try and be generous with the silica, and keep my readings at 10 (I think that’s as low as they’ll read, I’m not sure), and with PLA I’ll sometimes letbit get to 20, but everything else, at 15 I swap some fresh silica and it goes back down to 10. I’m a ■■■■, I know, but keep your stuff happy, and so will you be.
Thanks for breaking it down by material. That makes sense — PETG is the one that actually gave me trouble, so I focused on that. PLA definitely forgives more.
Nice setup. 24-25% RH with the dryer cycling in makes sense. That’s basically what I’m aiming for.
Same boat — small space, limited room. But even a few sealed boxes at 20% RH made a way bigger difference than I expected. Worth it.
That explains why mine won’t go below 10%. In my AMS Not-So-Lite, each spool compartment has a pair of dessicant boxes, plus I put another dessicant container into the spool’s middle hole. After that initial period where it needs to dry the air that got in while I was loading it (with a pre-dried spool), the container’s hygrometer gets down to 10% and then will not go lower, even after a few days.
The 25% was the advertised bottom. So I have it set for 25%. In the real world it is achieving 24%, which is fine by me. It beats vacuum bagging and setting in stacks on the floor where who knows what it can or has absorbed.
After beginning to get into worse filaments (ABS, TPU, and even my PET-G) I’ve added a Sunlu S4 because it made sense to have a 4-holer for whatever I want to put in a single AMS2 Pro for feeding the printer. Logical thinker I am. The Sunlu S4 is bouncing along in a FedEx truck somewhere in or about Kentucy as of yesterday’s check. I’m a pitbull about watching my shipments. Anyway, that’s my 3-step plan for my filaments.
First, you should decide what level of quality you are satisfied with. If you will not tolerate anything less than perfect prints, you probably need to keep moisture to a (reasonable) minimum. If you can tolerate small imperfections, there are many more options available to you. You probably do not need to worry about PLA at all.
Contrary to some opinions you may see around here, the humidity in your storage location IS in fact a pretty good proxy for the amount of moisture in your filament, assuming it has been stored in that environment for long enough to reach equilibrium. The moisture in filament always moves toward equilibrium with its environment.
But here are some guidelines that I put together after looking at many data sheets and published studies:
RH ≤ 20% You’ll keep PETG near its lowest achievable moisture content. Drying rarely needed if you maintain this level consistently.
RH 20–30% Moisture uptake stays very low (probably <0.15% by weight). Suitable for most prints; you can skip pre-drying unless you need maximum clarity or strength.
RH 30–50% Moisture content approaches ~0.2% (datasheet range). General-purpose prints will still look good, but highly detailed or optical parts may show stringing or hazing—consider drying if you see defects.
RH > 50% Uptake exceeds ~0.2%, and you risk bubbling, popping, and loss of tensile strength. Active drying or sealed storage with desiccant is recommended before printing.
The average RH here is 69%. I store my filament in vacuum bags or storage totes with at least 25 grams of color-indicating silica gel per spool. The totes also have cheap hygrometers that do not read below 10%. The meters read 10% for several weeks with dried filament and fresh desiccant in the tote.
I dry or replace the desiccant when the humidity reaches 20-25%, at which point the desiccant also has a noticeably different color. A single spool in a bag might need fresh desiccant after 5-6 months; the desiccant in a large tote might need to be refreshed in a couple of weeks if I’m in and out of it frequently.
I also record spool weights into and out of storage. A spool gaining more than 2-3 grams during storage is re-dried before use, but that is a very rare occurence.
I can’t prove my filament is scientifically 100% absolutely dry, but I don’t have moisture-related print problems.
Could I delay desiccant renewal until RH reaches 30%? Maybe, but I prefer to not take the chance.
If you are seeing the bags leak up to any value, they are not 100% absolutely dry.
But the part about not having moisture-related problems is key. Once prints are good, that’s all you need.
Are you sure? From other information I’ve read, mostly from the manufacturers, it takes a certain amount of heat, the temperature varying with the material, before the filament releases its bond with water molecules. If that is correct, then the filament can certainly have a higher moisture content than the surounding air. And of course we can only measure the moisture level of that surrounding air.
I’m no expert and I have no absolute way to determine which advise is correct, so in a case like this I prefer to play it safe and believe the manufacturers.
I would be interested in seeing the claims from those manufacturers.
This is a summary that ChatGPT wrote. I will add some citations at the bottom:
Moisture in thermoplastic filament is continuously exchanged with the surrounding air. Water molecules can move into and out of the plastic at ordinary storage temperatures; they are not generally locked inside until the filament reaches a particular “release temperature.”
Assuming the filament has been stored long enough to reach equilibrium with its environment, its moisture content is determined by the filament’s material, the storage temperature, and the surrounding relative humidity. The filament will not necessarily contain the same percentage of water as another plastic stored at the same RH. PETG, PLA, nylon, and different grades or brands of PETG can all have different moisture-equilibrium levels.
Relative humidity is therefore a useful indication of the conditions driving moisture absorption or loss, but it is not a direct measurement of the water inside the filament. RH also changes with temperature, so both temperature and RH matter. In real storage situations, the filament may take hours or days to reach equilibrium, especially after being moved from a humid environment into a dry box.
Heating mainly speeds up the movement and evaporation of water. It does not usually cross a sharp threshold where previously unavailable water suddenly becomes free. Heating makes the polymer chains and water molecules more mobile, allowing moisture to diffuse out faster. The recommended drying temperature is chosen to remove moisture efficiently without deforming or damaging the filament.
For PETG, drying matters because absorbed water can react with the polymer when it is melted. This hydrolysis can reduce the polymer’s molecular weight and lead to popping, bubbles, stringing, poor surfaces, reduced strength, and inconsistent extrusion.
Thus, if filament is truly equilibrated with its storage environment, a lower-RH environment generally means drier filament—but the RH reading alone cannot tell you its exact moisture content. For practical printing, follow the manufacturer’s drying recommendation when the filament has been exposed to humid air or shows signs of moisture.
Citations:
Water activity and equilibrium
IUPAC Gold Book, “water activity”
Defines water activity in terms of the chemical potential of water. For air in ordinary storage conditions, water activity is approximately related to RH by:
aw≈100RH
This supports the idea that a polymer exposed to air tends toward a moisture content determined by the material, temperature, and water activity—not by RH alone.
IUPAC Gold Book:
https://goldbook.iupac.org/terms/view/W06642
Polymer diffusion and sorption
Crank, J., The Mathematics of Diffusion, 2nd ed., Oxford University Press, 1975.
This is a standard reference on diffusion. It explains why moisture can move through a polymer at ordinary temperatures, while also showing why the process may be slow and why heating accelerates it.
Polymer solubility and diffusion
Brandrup, J., Immergut, E. H., and Grulke, E. A., eds., Polymer Handbook, 4th ed., Wiley, 1999.
The chapters on diffusion, permeability, and polymer–solvent interactions cover the concepts behind moisture sorption, equilibrium concentration, temperature dependence, and diffusion through polymers.
Diffusion in glassy polymers
Vrentas, J. S. and Duda, J. L., “Diffusion in Polymer–Solvent Systems. I. Reexamination of the Free-Volume Theory,” Journal of Polymer Science: Polymer Physics Edition, 15, 403–416, 1977.
This work describes why diffusion depends strongly on temperature and polymer-chain mobility. It is relevant to the observation that drying becomes much faster when a polymer is heated, without implying a sharp temperature at which water suddenly becomes free.
There’s also the question of moisture induced damage to the filament over time, especially if stored for a long time. The damage is irreversible and shows up as embritlement. Not sure what the “safe” level is for that, if there is such a thing, but it might be a different number than the acceptable print quality number. I’ve not seen any discussion around that.
Are those citations yours or from an AI? If yours, then thank you and I’ll peruse them at some point.
Anything from an AI (Artificial Imagination) will be completely disregarded by me, as will anything from Reddit, Facebook or similar.
I am 100% in support of you being you. Do whatever you like. It makes no difference to me whether you believe or disbelieve. And I feel no obligation to convince you of anything. The information is out there. What you do with it is your choice.
It does take a certain amount of heat but even room temperature has heat and a fair amount of it.
It’s why water evaporates without heating it. But it evaporates faster when you do.
If you want to understand why that is, at any non-zero temperature a fraction of molecules will have energies above whatever threshold. It’s called a Boltzmann Distribution if you want to read more on how that works.
But it is absolutely true that given enough time filament will equilibrate with its environment and lose moisture if it started out with more than at equilibrium. It picks it up too but that process is more energy favorable so happens faster.
But @thczv is right about moving to equilibrium.