On the Topic of Disposing Dirty IPA Washing Bath From Resin Printing

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Resin printing is wonderful. I love it – it’s fast and detailed compared to FDM printing. Also, the materials are getting better and better. However, compared to FDM printing is a really messy process. The resin frequently drops where you don’t want it to, and you have to wash the models in an IPA bath.

And here comes the problem ­– how should you safely dispose of a dirty IPA bath? The resin before it is cured is toxic, especially to aquatic life. The best way is to probably bring the bath to a facility for hazardous waste disposal. Nevertheless, the idea of recycling IPA is appealing so you see a lot of people on YouTube and Facebook trying to clean it and recycle it.

Here I bring my 2 cents on the topic. Note that what I present here is more of an idea or a proof-of-concept rather than a complete solution. Also, I am no chemist, so I cannot guarantee that the presented procedure actually yields safe waste. If you have some insight or ideas, please, let me know in the comments!

Double bath approach

First, let me start with this: If you don’t use a double bath (dirty/clean) for cleaning, start now! Double bath cleaning makes your cleaning process more efficient (I can clean 10–20× more printed resin using a double bath than with a single bath approach). If you want to learn details, see my post on this topic.

In short, you should have a dirty bath where you do your first round of washing. This will remove most of the resin and should you leave with a model covered in a thin film of the dirty bath. Then you clean it in a clean bath. The principle is that you almost don’t care how much dirty your dirty bath is, the only goal is to remove the excessive resin. Then your clean bath lasts much longer.

What have people already tried?

Back to the topic. Let’s start with a brief recapitulation of what I found people on the internet are doing in order to dispose of or recycle the IPA bath.

Some people simply put dirty IPA in a container and let it evaporate in the sun. Once the IPA evaporates, the resin cures and solidifies. Most manufacturers claim that cured resin is a safe waste. Most people report good experience with this. I tried it on a small volume and I run into two issues: first, my dirty bath was so dirty, that it ultimately blocked light within half a millimeter (see explanation) so I was left with a solid resin cylinder with half-millimeter walls that was full of dirty IPA. The second thing where I have doubts is whether is actually good for the environment to let IPA evaporate in the air.

Another approach is to let the resin sit in a dark container for a long time so the dirty part settles. In my experience, what settles are only the pigments and fillers, but most of the components of the resin are left in the IPA. When you put paper in a bath cleaned in such a way and expose it to light, it will actually harden. So, this approach didn’t work for me. You are still left with a dirty bath that will only make your prints sticky.

Another approach is to let the resin sit in the sun. The resin forms a white gel. Some people claim that after a long time IPA separates from the gel, but I wasn’t able to verify it. For me, it never separated.

There was also an approach of using aluminum sulfate to clean the resin. Aluminum sulfate is often used as a coagulation agent that’s added to water to clean it. I haven’t tried this, so I cannot make any claims about it.

The last approach is distillation. I haven’t tried this as it’s relatively dangerous when done wrong, has high input costs, high running costs (come on, 1 L of IPA costs 2€!) and also, I am not sure if the boiling point of IPA is below the boiling point of all the volatile parts of the resin. If somebody knows, please, let us know in the comments.

What seems to work as a disposal method is soaking paper towels in the bath and curing the towels. However, this is extremely messy and laborious. But the resin solidifies nicely, so it should be safe to dispose of.

Another spin on the resin disposal

The solidified resin should be safe to dispose of. However, how to cure the resin in the solution of IPA? If we shine through it, it will not cure though due to the pigment in the dissolved resin blocking the light. And we also risk that we only form a white gel from which we won’t be able to extract the resin.

I came up with the following device – it is a slowly rotating paper roll forming drum, that is by 1/3 submerged in the dirty IPA. On top of the devices are high-power UV LEDs. The whole device is closed in an air-tight box with the IPA. As the paper tube slowly rotates, a thin layer of the dirty IPA forms on it. It should have enough time for the IPA to evaporate. Once it reaches the top, the UV LEDs cure it. In this way, we simulate what a printer does ­– curing thin layer by thin layer. After a while, we should get all the reactive components from the bath onto the roll in a cured form.

I made a quick & dirty prototype; to verify the hypothesis. The prototype was printed on an FDM printer with a 0.8mm nozzle. This is why the print is a little rough. It is powered by a small DC motor. The LEDs on the top have 10W in total. Everything is wired “as is” and it is powered by a lab bench power supply.

First, I tried the design with only 1 LED attached. To my surprise, it ended up well. After a minute, you could see that there is a layer of resin forming under the LED. I let the device run for about 25 minutes. The dirty bath got notably cleaner (though, not completely clear) and the paper tube under the LED was covered in a thick layer of hard resin. I washed the roll and cut it in half. It seemed that the device worked as expected. The resin on the tube was darker than the dirty bath, and it was rock solid throughout the whole depth. When you look closely, you can see the individual layers. You can also see that the layers get lighter and lighter towards the outside ­– as we extracted the dirtiest part of the bath.

The first prototype in action. There’s unfortunately not much to see, as the UV light completely confuses the autoexposure of the camera.

Given this success, I designed a second prototype with an array of LEDs and some small improvements. This time I switched to another motor, that I thought would be more suitable. The second experiment turned out as a partial success: the LED arrangement was great but the motor wasn’t suitable. It wasn’t able to spin slow enough, so instead of nice hard cured resin I got hard (but dry) jelly – probably a lot of IPA got captured in the resin structure. Also, this time I used a magnetic stir bar and magnetic stir board. This has shown as a good idea as it cleaned the resin more evenly.

What I learned from these experiments:

  • I need to switch to another motor; either with a high gear ratio or a stepper. It seems that the optimal rotation speed is about 1 turn in 1–3 minutes. If you go quicker, a lot of IPA gets trapped in the cured resin and you get a hard rubber, not hard resin. There is the time needed for the IPA to spontaneously evaporate so only mostly resin stays on the drum.
  • The LED power seems more than sufficient; I actually used only 2.5 W of the original 10W of power. However, a more even pattern is needed. On the other hand, when I rotate very slowly, the pattern might not matter that much.
  • The hubs for the paper roll need to be coated in mold release otherwise they stick really hard.
  • The hub needs openings so that when IPA gets inside the roll, you can pour it out.
  • Overall, the idea seems to be working ­– it just needs further tuning so it is actually convenient.

My idea is that it should be something you put inside an air-tight and light-tight container and let it run for 1-2 days. During this time, it should cure all the resin components on a disposable roll that you can then trash.

Do you have any ideas on improving this method? Do you see some flaws in it? Let me know in the comments!

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