Resin Printing: cleaning your IPA

I recently got my hands on a Prusa SL1 resin printer, and I am in the process of getting acquainted, and that will be the focus of a separate post. In the meantime, during the pandemic, people seem to be buying Isopropyl alcohol (IPA) by the gallon and raising prices so it can be quite tricky to get hold of some for resin print cleanup. Yeah, there are some good reports with other liquids, but IPA is widely recommended, and so I think it's probably a good idea to see how much mileage I can get out of that. The way it works is after print you have to wash your parts in IPA to get off all of the uncured resin so you can properly cure the parts without shiny spots or wash lines. Even if you leave the new prints in the printer for a few hours, the resin on the prints still needs washing off - it's quite viscous so sticks to everything.

After washing off several prints, the IPA can get quite mucky with all the resin that's floating about in it and you need to replace it. Uncured resin is a hazardous material and needs disposing of correctly. Once cured it is perfectly safe though, so you could pour the mixture on a sunny bit of the garden where nobody visits and retreat to the house. The sun will evaporate the IPA quite quickly and cure the resin. You can then dig it up and throw this in the bin. But that's a waste of a lot IPA that you could reuse. So, here is my process.

First of all, I need to point out that this process is not about making your IPA lab grade clear again. It is about getting it to a point where you can reuse as much of it as possible.

You will need to pour your wash tank out into a tall, clear container to allow for good settling. Oh, I have to say here, DO NOT USE ANYTHING YOU WANT TO USE ELSEWHERE. This is really important. Resin gets everywhere and is a biohazard while a liquid. I use the Sistema 3.25l bakery storage containers for settling and the Sistema 2l Fresh Juice Jugs for IPA storage, both available from amazon. You want a tall settling container because when you pour off the IPA later you will disturb the sludge less this way. The storage container I'm using is just an easy fit for the mouth of the other container, so pouring out is easy.

This was a particularly heavy batch to clean. I started with 1.5l of IPA, I washed 7 prints (200g of Prusa tough orange resin, and 300g of Prusa tough pink) and ended up with just over 1.5l of wash. I thought this was good.

Resin and IPA is an imiscible liquid - that means that like soap and water, it will never truly disolve and will always settle out a fair bit, but there are parts of the resin that will always remain in the IPA because it is a solvent, mostly colour compounds for example, so it can be a good ideea to expose the well mixed IPA/resin mix to a healthy dose of UV light so that any resin can clump and cure taking a lot of the components with it. If you have easy and prolongd access to the fiery ball in the sky this is good, otherwise you can use a gel nail light thingy.

After a couple of hours, you can start to see the liquid separating, and it may go clear and then very cloudy depending on the temperature.

It also appeared that the all-illusive sun was going to stay around for a while, I decided to use the free UV. After a couple more hours outside, the settling was more pronounced. I left the jug out until after dark, then left it overnight on the windowsill ready to process the next morning but the longer you leave it to settle the better your results will be.

EDIT: At this point you can just pour off the clear IPA on the top. The rest you can store for a few batches until you have enough resin to dump and you can settle and decant a little bit more IPA that way as well. When I first documented this process, I experimented and took photos to help others, so the rest of this process is that experiment.

The Clean

the first step here is to prepare. You will make a mess and this is heavily contaminated IPA. It has resin in it, and that resin is really sticky. Lots of paper towels - everywhere. The first step of cleaning will involve a 100-micron paint strainer and a funnel as well as the storage jugs.

Yes, that is the very expensive CW1 being dual used as a prop for the sieve. More on that later.

So just pour the settled mix, carefully, through the paint strainer into the storage jug. I ended up with this.

I now had 1.3l of IPA that is perfectly serviceable... but letting it sit for a couple of minutes I noticed more IPA in the settle jug... so I poured it out... and immediately regretted it. after a couple of goes I got maybe another 20 or 30ml of IPA... but now there was a lot of fine particulates in the storage jug. But ok, I was going to filter it anyway.

Once I'd given up extracting IPA from the mush, I cleaned up by putting the loop outside again - that wonderful sunshine - to evaporate the rest of the IPA and to fully cure the resin. You can see that crap that accumulated in the paint strainer.

Back to the Prusa CW1 support system and a coffee filter paper in the sieve over a collection container... Pour in your juice, and it goes through pretty quickly. You end up with this.

Would it clean better on a second pass... Well, the second pass took nearly 5 minutes to it had caught a lot of stuff in the filter. I ended up with this (please note that the apparent colour difference is just due to a different position of the camera,and the saturation being different).

Hmm, ok, well, it didn't look that much cleaner but it did take over 5 minutes to go through... more stuff must have been pulled out right!! third filtering took 20 minutes. The fourth nearly half an hour. I decided to change the filter paper for the fifth and it went straight through. After 5 (well 4.5) filtering I ended up with this.

If you can tell the difference then you have better eyes than me. I now had a little over 1.2l of IPA so I had lost nearly 100ml doing all the filtering and decanting. So at this point, I called it done. I topped up the container to 1.5l ready for the next cleanup and moved all the tools outside to cure any resin after cleaning things as best I could.

Conclusions

Well, I did this so you don't have to. Do you really need crystal clear IPA - no. after the first wash of any print, the IPA is so much more contaminated than this reused IPA is, so the impurity here is pretty negligible. My process will not use the coffee filter process as I lost a fair bit of IPA at that point. The clean pour initially was good enough for me. If I did get dregs in again, maybe just one filtering. Yes, filtering did help but not when you weigh it against the loss of product and the time it took me.

NOTE: The cleaning above was done when using opaque resin. The transparent red I've just finished reacts very differently. Sitting in the UV where it also warms up makes all the resin go cloudy, settling takes ages and filtering is a real pain in the butt. More storage and settling containers required :), oh and more paper towels. A LOT more paper towels.