I do like my filament dry

It's plastic, what could possibly go wrong?

It's not a secret to anybody owning a fdm printer that (most) filament is hygroscopic, which means it tends to absorb water from out of the moisture of the air.

Despite the common feeling that plastic tends to ignore water, that's a problem while printing if suddenly small amounts of water evaporate in the printers heat block or nozzle. 

My printer always was located in the basement of my home, which means during the winter time I have 14-18°C room temperature. Not really the best environment to store filament in the open. 

Back in 2020 there were not really many filament storage solutions on the market and since the topic of humidity in air always was somehow interesting for me, I decided to try something else then the know little boxes with heating elements (if you heat up air, the relative humidity decreases and the air is ready to take up more water).

Some years (read decades) in chemical laboratories however gave me a trace of experience about how to decrease humidity in gases (and liquids).

And that's why I have chosen my not really conservative approach.



As said, my printer is in the basement, and heating up the filament storage to 60°C or so in a room which is intentionally not heated wasn't my preferred option.

So how can I get air dry, if not by heating?

  • Extreme cooling and "chilling" back to room temperature (condense the moisture)
  • Absorb the moisture in a desiccant

Now, years in chemical labs ... guess which way I went.

But stop! A desiccant is the most common way and I told something about "my not really conservative approach".

That's because my solution is somehow special. I'm not using a box/bag/whatever of desiccant placed in the filament storage and let nature work it out.

I wanted something to control the humidity.

Basic idea by a mad scientist

There are some chemicals known to be way more thirsty for water than filament. One of the chemicals is Lithium Chloride (LiCl). As dry substance the air surrounding it will dry down to 11.3% RH (relative humidity). Too low for my taste. Additionally the salt will solve on the surface and create clumps which make everything more difficult. 

The air above a saturated solution will eventually reach 45% RH, too high. 

However, if I pump air through a saturated solution the air reaches below 20% RH (tested by sampling 1l and performing a coulometric KF titration).

Theoretically one would expect to go down to min. 45% since I "only" increase the contact area of the air with the solution. But that's only half the truth. It also swirls up crystals of LiCl and comes in contact with those.  

I eyed for a RH of 25-35% for my filament storage, so great. Pumping the air within the storage through the solution until the RH is low enough.

And if I stop pumping water through it...... the RH will increase to 45%.. f**k.... in theory. But nothing beats theory better than reality.   

That's the point the "Gaswaschflasche" (Gas washing bottle) steps onto the stage. 

A bottle which has an input side reaching below the surface of the washing liquid (near to the bottom) of the bottle, and a small outlet for the "washed" gas. Without changing the pressure within the bottle, almost none of the gas above the washing liquid is exchanged with the surrounding space. Perfect. 

So plan finalized:

Some acrylic plates (I used original Plexiglas, YOLO) glued together are my filament chamber. One plate s not glued but attached to a hinge as door, two neodyme magnets to keep the door closed. Everything else is placed within the filament chamber.

A small hygrometer sensor to measure the relative humidity in the filament storage chamber, a gas washing bottle filled to half the height with a slurry of LiCl, a relay, an ESP8266 microcontroller board and a small air pump pushing the air through the solution/slurry.

RH >= 35%  switch the pump on

RH <= 25% switch the pump off

That's it, isn't it?

No for sure it's not the end. What worked really well for a few hours, to be more precise until the RH went below the 25% and the pump switched off, failed once the RH was back above 35% and the pump switched on again. The slurry became quite solid on the bottom, and the now again air created a small channel into the  upper part of the "solution" sputtering it on the top of the wash bottle and a "few ml" out of the outlet into my filament chamber. Quite a mess.

But lessons learned....and I have a 3D printer.

So I took a second bottle, drilled a couple 2mm holes in the lid, drilled a center hole which fits the hose I use and connected it to the gas outlet of the first gas wash bottle. To disperse the air in the second bottle and avoid the building of big bubbles I used my printer an created an outlet basket with 144 1mm holes.

And at this point my journey towards dry filament was completed. 

 The whole setup is now running for three and a half year and according to my sensor (which logs the humidity via mosquitto) the average relative humidity is around 32%. 


 This is a picture I have taken after 2 years of usage. At this point the LiCl in the lest bottle dissolved completely an I used an cleaned out food can (coated on the inner side) to boil the solution to roughly half the volume. After cooling it down while swirling I ended up with a slurry which almost matched the one I filled the bottle initially with.

Remarks and conclusion:

That's a completely over engineered design. It works well and was a fun to built. And the fact that I either way have an ESP8266 running within the filament storage chamber made the decision to do so even easier.

However: Don't take that as instruction. Lithium Chloride is not a toy. I handled dangerous chemicals for living. I know what I'm doing. 

To quote wikipedia:

Lithium salts affect the central nervous system in a variety of ways. While the citrate, carbonate, and orotate salts are currently used to treat bipolar disorder, other lithium salts including the chloride were used in the past.

One question may remain: 

Why the hell was there already an ESP8266 in a filament box (power cable sealing and so on)?

I'll answer it with a photo.....because that's a story for a different day.