Filament Drying preliminary results

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Wow! This is quite a project!

I’ve been trying something very similar using the Sunlu V2 dryer and a small air pump. I haven’t got as far as using desiccants or cryogens yet :slight_smile:

I performed a lot of physical measurements on my system and found that enclosed dryers like the Sunlu can’t possibly be practical for drying filaments.

For example, I understand that a PLA can absorb up to ~1.0% of its weight in moisture. For a 1kg spool, that’s 10g of water. At a drying temperature of 50°C the water vapor saturation concentration in air is 83 mg/L. If water desorbed from the filament instantly, it would take 10000/83 = 120 liters of dry air just to remove it. The internal volume of air in the Sunlu dryer with a full spool inside it, is about 3.5 L and pumping fresh air through it will only exponentially dilute that moisture and it would take about 5x the volume of fresh air to get out the moisture. You’re going to need hundreds of liters of air to dry things this way. Finally, the distribution of water between the air and filament is a thermodynamic process - it just doesn’t all come out at once, it partitions, which means that here’s another factor that slows down the extraction.

I did perform some experiments to verify this and found that when I had a full spool of medium wet PLA filament in the Sunlu dryer at 50°C, it took 7 full days pumping 1.7 L/min fresh air continually into the dryer in order to dry the filament (or at least stop the humidity from building up in the air inside). A bucket of desiccant will be needed for this. Don’t trust the Sunlu humidity reading - it’s way off, particularly at low levels. I used a BME280 sensor to monitor the internal temperature, pressure and humidity. %RH is rather meaningless in this context - the critical metric is absolute humidity (or concentration of water vapor in the air) - that’s what the thermodynamics care about and which drives this whole process.

I don’t mean to be critical here but the laws of physical chemistry are working against this approach. I think dryers like the Sunlu are best used to keep filaments that have been already dried from getting wet again.

I would like to reach out and share a new approach I’m working on - and I’m finally getting some success. Without going into too much detail, rather than dry the whole spool of filament, why not dry just a small part of it as it’s being fed to the printer. Ok, this is not a new idea but the ones I’ve seen don’t look as if they’ve really considered the science behind it. I’ve been able to fully dry a 1m section of PLA filament at 50°C with an air purge flow of about 25 mL/min in about 15 minutes - less time than it takes the extruder to pull it through. The heating and the air purge only need to be active while printing. Desiccants become a real possibility for this.

Let me know if this approach interests you.

PS. I was a research chemist for many years before taking retirement. I sort of understand the physiochemical relationships involved in these wetting and drying processes - which has helped.

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:+1: Very much so. Yes! Please do say more.

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Reading this made me remember a vid I saw:

and then this product:

and this overly expensive product:

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Thank you for the positive responses. My family and friends think I’m a bit weird for working on stuff like this. It’s good to find people who are interested and doing much the same :slight_smile:

The video was crazy - baked bean cans are far too big!

I was aware of the very expensive Drywise but the Thordsen was new to me. It takes time to dry filament and so for an online dryer this time is related to the heated bed length. The Thordsen bed looks to be far too short to be effective. However, the desorption time is much reduced at higher temperatures and drying PLA at 50°C is probably the worst case. Note that Thordsen totally omits the mention of drying PLA in their blurb - even though it’s probably still the most popular filament. I note that they mention that a patent is being sought for their design. I did find their application online - Bambu won’t let me post links, so search for patent # US20240083115A1 on the Google Patents site. It looks like they are using a sheath of desiccant around the filament as it’s dried - this looks a bit too over-engineered to me. I’m not even convinced that a desiccant is needed at higher temperatures. In any event, a granted patent doesn’t prevent amateurs from working on the same stuff - we just can’t commercialize it.

Drywise doesn’t say much about how their product works - but it’s notable for its sheer expense.

There’s also the FilaDry - if you want to learn how not to do this at a high price, take a look at a couple of videos on YouTube - search for Chris’s Basement FilaDry review and 3D Kundry’s teardown.

How do you want to proceed? I think it’s important to have some understanding of the scientific processes involved in drying to get an effective dryer and I’m happy to share my take on this but I don’t want to bury you too much in equations etc. I can also share some of my ideas for a viable dryer design.

Let me know what you want me to do.

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I took a look at the patent application US20240083115A1 in the USPTO portal (I am a patent agent by profession, but this is all public information). It looks like they are on track to have this patent allowed with fairly broad claims as claims 2-4 and 11 were found to be allowable in the first Office Action which was issued on 12/9/2024. Claims 12-17 were withdrawn from consideration for being to a separate invention (a technicality) and could be put into another patent application should the applicant decide to do so.

Key elements of claim 2 (which is currently found to be allowable) are 3 nested tubes where the filament passes through the inner tube, a heating element is located between the inner and middle tube, and desiccant is contained between the middle and outer tube. Vent holes allow air to pass through both the inner and middle tubes.

Claim 11 requires it to be built into an FDM 3D printer, but does not require that there be 3 nested tubes.

You are technically incorrect whether you can use a granted patent for personal use. A granted patent DOES mean that building or using an infringing device is not allowed, even for non-commercial use. But, the reality of it is that you are unlikely to be sued for doing so, as there are minimal damages to collect if you are not commercially selling a product. This makes it uneconomical to sue you, and patent infringement in the US is a civil matter, not criminal, so you don’t have to worry about being arrested for doing it.

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The guy in the YouTube video has another video about a second version (which doesn’t work - his statement, not mine)

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Hi Youngbru, thanks for the clarification on this patent - and for correcting me on the amateur usage. I have had a bit of exposure to patents (I hold over 50 of them) and understood that no one would be concerned about people building their own stuff - as you say this could be true but it doesn’t make it legal.

I don’t think anything I’m doing will be affected by this patent. I don’t want another patent myself, so I’ll want to publish my ideas and hopefully establish a prior art record.

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Whatever you’ve got that you think may be interesting to at least one other person in the universe. If you’re worried about dilution, just start a separate thread, but be sure to link to it from here so that we’ll know it exists. Of if linking doesn’t yet work for you on the forum, give the thread name and one of us will supply a link to it.

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I’ve speculated that this might be the case for some dryers where the sensor in mounted on a PCB that straddles the exterior and interior, especially if the board is insulated with a shroud on the inside. The sensors are designed to have the body at the same temperature as the measured air and not be mounted on a relatively cool PCB.

There are at least 3 or 4 people on here who are interested and personally I’m into the equations side, so feel free to go full nerd.

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Redrying filament…

My AMS #2 had crept up to a 2 humidity on its own as it doesn’t have spools swapped in that often where the spools act like desiccant too. So I decided to run the spools through the dryer again and replace the desiccant.

With the AMS showing 2, all spools that were in that AMS are showing 32-33% RH when put in a poly cereal box with a hygrometer. The spools are also taking a full drying cycle to get the S2 filament dryer chamber back to 19% again.

I had hoped/thought the spools might just need a “touch up” but the drying times are taking as long as new spools.

I also had a spool of PETG HF that I had to dry as quick as possible with the pump turned up to max flow. By the time it had to go into service it was showing 21% RH in the filament dryer when I normally dry to 19% yet the print showed some water issues. After the print when I pulled it to finish drying, it took the filament dryer RH up to 26% with the pump backed down around where I usually run it.

What it feels like is trying to dry fast can skew the apparent humidity level. It may also say drying just takes time for the water to come out of the filament and high flow rates can mask filament moisture content. It should speed up drying some but it can affect the reliability of drying to a humidity level.

The photo shows where the PETG HF spool was changed out at a filament hair above and to the right of the center left hole. First spool used in the print was dried to 19%. Above that was the spool dried to 21% but dried “fast”. I don’t think the 21% was a true 21% since on putting it back into the dryer it went to 26% at my regular flow rate. For whatever it’s worth.

IMG_4863
IMG_48632872×2524 1.34 MB

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Ideas for Drying Filament. Part 1 - Theory

Okay, here goes with my first take on drying filament…

Some solids, which include the materials used to make 3D printing filaments, have the ability to retain water - which affects their printing performance. This retention of moisture will occur on the material surface (adsorption) and within the matrix of the material (absorption). I don’t think we need to be concerned with the adsorptive effects as they will be minimal and shouldn’t affect the properties of the filament. Absorption on the other hand will be the major process that causes moisture to pass into the filament. This absorption is actually a two-way process - water molecules will be passing in both directions in and out of the surface of the filament. This is very similar to the situation with molecules moving in and out of liquids which is called partitioning. There are molecular energy levels associated with the presence of water molecules in both air and in the filament material. As the concentration of molecules is increased, so does the energy level. The energy level is also dependent on the chemical properties of the material (air or polymer). More molecules will tend to move from the higher energy phase into the lower one - so, in time the energy levels will become the same and we say that this process is now at equilibrium. This means that the ratio of the concentration of moisture in the air to that in the filament will be fixed and if the total amount of water present is known, we can predict how it would become distributed between the air and filament. This ratio is called the partition coefficient and is represent by Equation [1].

K=Ca/Cf

Where K is the partition coefficient, Ca is the concentration of water in the air and Cf is the concentration of water in the filament.

Why is this thermodynamic process important to us? If we change the concentration of water in either phase, the remaining water molecules will move in and out of the filament until the concentration ratio defined by the partition coefficient is back at equilibrium. So, this means that if we reduce the concentration of water in the air, water will be pulled out of the filament. This is why drying the air with desiccants or applying a vacuum is so effective at drying filament. It is also the reason why water molecules will move from humid air into a dry filament.

However, the major problem with drying filament this way is that the movement of water molecules, especially within the filament matrix is very slow - yes the filament will become completely dry but will probably take weeks if not months to get there. It’s also good news because it will take a long time for a dry filament to get wet.

Fortunately, we can use temperature to speed things up. The partition coefficient K for a given material is very temperature dependent. There’s an exponential relationship between K and temperature. The equilibrium is pushed towards the gas phase at higher temperatures meaning that more water can be extracted out of the filament - and faster. It’s faster because the equilibrium is now way out of balance and because the diffusion of water molecules within the filament matrix is much faster and they get to the surface quicker. The water capacity of air goes up exponentially, too which helps with the extraction. For any filament, the hotter the better but will limited by the tem-erature limit of the filament - we don’t want to melt it while we’re drying it.

Hopefully, this explanation will help with the understanding of how and how much water gets in and out of a filament and help model possible technical solutions for eliminating it.

I’m going to leave this discussion here. Part 2 will discuss the fallacy of using %RH to monitor moisture levels.

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One thing I forgot to mention.

I can’t find any published values for partition coefficients. However, it seems possible to determine these myself (given enough time). If we had a small database of these, we could predict where the water will go in any system at any temperature.

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Equilibrium RH% in air and moisture content are physically related. If you specify one, you specify the other. It’s like a conversion factor.

What you are saying is the equivalent of “it’s a fallacy to use Fahrenheit to monitor temperatures in Celsius.”

A better example would be displacement by force on a spring. For each force there is a displacement and vice versa. For a particular spring when you specify one you are specifying the other.

It doesn’t make sense to say it’s a fallacy to use one to know the other because they are physical relationships/properties of the material.

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I’m guessing he is going to explain adsorption as a function of water partial pressure instead of water activity. It’s confusing to me since I see silica gel plotted against RH while molecular sieves are plotted against Ph20. Why is that?

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Molecular sieves do size exclusion as well as adsorption so maybe something to do with that? Do you mean P (subscript) H2O and the partial pressure of water? I don’t really know the answer on the why. It could be just they are typically used some specific way where that’s a holdover from how measurements were made? I don’t know the answer on that.

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Ideas for Drying Filament. Part 2 - The Use of Relative Humidity

Well, I posted Part 1 in which I tried to show that knowing the concentrations of water in both the filament matrix and in the surrounding air are critical to understanding how a drying process will work. The main response I got back was my use of the word fallacy to describe how RH measurements are often used – and I hadn’t even written this section yet :blush:

My apologies to all, I am a newcomer to this site and the last thing I want to do is stir up disagreement with the established membership. However, I think MZip’s response exemplifies the concern I have about interpreting RH measurements. The example he (she?) gives of using °F and °C is a poor one. They are different units of measurement used to represent the same thing – temperature. Water vapor concentration and RH are both measures of humidity but they don’t measure the same thing. A better allegory would be the length of a rectangle versus its area. Yes, you can use the length to calculate the area but that doesn’t make them the same thing. Like this example other factors are needed to make this conversion and so RH is in no way, on its own, a unit of measurement for concentration.

For instance, I share an anecdote that I’m sure we have all seen when using an enclosed filament dryer. I turn on the dryer and I see that the RH initial reading is 50% - indicating the humidity level in the ambient air. I then put in a spool of ABS filament and heat it to 70°C. A while later, I come back and note that the RH has dropped to 20% Hooray I think – this dryer is working really well - the water level has dropped by a factor of 2.5x! However, when we calculate the water concentrations, we find that it was about 9 mg/L in the ambient air and had gone up 40 mg/L in the heated dryer. This is the fallacy! RH doesn’t really tell us much about the level of water in the air as we change its temperature and in no way does it directly represent the concentration of water present.

Ok, now back to the theory (and I hope MZip will still talk to me).

Let’s start with some definitions. I’ve made a case for monitoring the concentration of water in both the filament and in the air. At equilibrium, the concentration of the water in the air can be used to determine the residual water left in the filament – that was all in Part 1. The concentration units we’re talking about are unit mass per unit volume – like g/m3 or mg/L. This metric is also known as the Absolute Humidity (AH) which is not displayed (as far as I know) on any filament dryer system – and it should be.

Relative Humidity on the other hand is actually the dimensionless ratio of two pressures as shown in Equation [2]. This ratio is expressed as a percentage for convenience.

RH(%)=100∙Pw/Ps

Where:
Pw is the water partial pressure in air
Ps is the water saturation pressure in air (or the vapor pressure of water)

Once known, the partial pressure Pw can be used to determine the AH using Dalton’s Law of Partial Pressures, the Ideal Gas Law, and the molar mass of water to give a final result in g/m3.
To calculate Pw, we’ll need to know the RH and the vapor pressure Ps. The RH will come from the sensor reading but the water vapor pressure needs to be calculated from one of the following: a look-up table, an Antoine Equation, the Goff–Gratch equation, the Arden Buck equation, or from the Magnus-Tetens relationship (which I have used here).
I’m not going to fill up this posting with its derivation (but let me know if you want it) but here’s the final formula to convert RH to AH (sorry, Bambu won’t let me post formatted equations):

AH(g.m-3) = RH.6.112.exp(17.67.Ta/(Ta+243.5).18.02)/(8.3145.(Ta+273.5))……… [3]

Where:
Ta is the temperature of the air being measured

Equation [3] is not something you would be able to process in your head but is very easy to implement in software. I use an Arduino type of processor to acquire and process the signal from a Bosch BTE280 sensor (which is truly excellent, by the way) which gives me readings for atmospheric temperature, pressure, and RH – all from the same small chip so the readings are well correlated in time and space. I calculate and display the AH in real time and I find that it gives me a much better indication as to what is going on inside the dryer.

That ends my Part 2. Hopefully, some of you will find this useful. Further topics could be how to optimize practical systems for water removal. I could also talk about desiccants and trap design. Let me know.

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It was a poor analogy which is why I added the spring one that is a better fit. But the relationship between spring force and spring length is from the physical properties of the spring and if you know one, you know the other. Same for equilibrium RH and moisture content if you know the curve.

The thing is using RH to get a handle on moisture content is no fallacy. It works and is reliable. I just skip the filament moisture content step and make my decisions based on RH. It works.

RH actually does tell you the amount of water in the air. You need to understand that if you want to talk about this subject. The number itself doesn’t. But combined with temperature you do know the water content of the air.

I’m not sure what you have shown by part 2 besides some equations. Your premise seems weakly defended. Equations but no explanation of why even relevant or what they show.

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If the BTE280 is truly excellent, you’ll find the SHT45 reality-shattering galaxy-shifting celestially epic.

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Okay, I think I’m done here.

I was merely trying to promote the use of Absolute Humidity as a better metric to monitor water movement in a dryer and to provide the means to calculate it.

I see that I failed miserably.

I disagree totally with MZip’s take on things and the only thing that I’ve learned from him is that I’m wasting my time here.

Thank you for your time.

Bye

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The problem with that is few here will do the calculations. Meanwhile, filament dryers and cheap hygrometers in wide use are all relative humidity.

And relative humidity is fine for most purposes.

You came out swinging saying how you were going to prove that using relative humidity for filament drying was a fallacy. You left it as a provocative and aggressive tease without stating what you were apparently after - absolute humidity is better.

But what you left hanging there was using RH was a fallacy without any qualification. Only in your final post did you clearly state what you were getting at. In your second post you buried your actual premise in backhanded insults and equations with no clear explanation of why.

If your premise was that using absolute humidity is the better metric, you could have saved us both a lot of trouble by stating that and not the unqualified using RH is a fallacy which is still incorrect. As to absolute humidity being a better metric, I agree with you because relative humidity can be deceptive.

What I was told when writing a paper is tell the audience what you are going to tell them. Then tell them. Then tell them what you told them. Instead, you came out with a provocative premise with zero support or clarification, threw a bunch of stuff on the wall, pointed to it and said “see”, and then got upset when I didn’t. You didn’t tell me what you were trying to tell me until you were slamming the door.

Sorry if I offended but you could have presented your case much better had you properly described the problem you were solving, and then showed us the solution, and then fitted that into current understandings.

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