Specific Garvity of IPA

I don’t have a form wash and therefore no Formlabs hydrometer. What is the specific gravity of spent IPA?

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It depends on how “spent” it is.

Understood. I want to know the threshold SG when it’s time to change it…

If you stir it up and you can’t see the model because the IPA is so cloudy, it’s time to change it. But the resin settles to the bottom of the IPA if left for a few days. You can decant the clear stuff and then dispose of the sediment and small remaining amount of IPA. Or filter it through a coffee filter. And exposing the IPA to UV cures the resin that’s mixed in to the IPA, making it easier to filter and/or usable for longer without filtering it.

Thanks for the info. That said, Formlabs offers a hydrometer so somebody should know the SP of spent IPA.

I’m sure they do, but in >5 years of running Formlabs printers, the way I describe has always worked great.

In fact, I have a new trick. I put a paper towel down on the wash bucket basket after I’ve finished rinsing a print. Over the course of a couple of days the resin settles out and the IPA is crystal clear again. Then I slowly pull out the basket and throw the paper towel, with all the sedimentary resin deposited on it, away. I haven’t actually tossed a bucket full if IPA in as long as I can remember…

Hi Randy_Cohen
That’s actually a cool trick. Mind sharing how exactly you put the paper towel? Do you lay it down flat on top of the bottom of the basket?
Also, does that really suck up the resin after a few days?

I just lay it down, torn to size, in the bottom of the basket. Let it sit for a day or two, slowly draw out the basket, dispose of the paper towel. It’s not perfect, because it’s hard to go slow enough to keep stuff from leaking over the edges. But I would guess I’m pulling out 80% of what’s collected on the towel.

I don’t print a huge amount. I think I’ve owned the Form3 for about 300 days and I have barely used 2L of resin, so the IPA buckets sit around for days, undisturbed, which is what gave me the idea. The first-wash bucket would be so cloudy I could not see the print once things got stirred up, but after a couple of days it’d be clear as glass except for a layer of sediment on the bottom. Can’t reach in and vacuum the sediment out with a straw, unfortunately, so this seemed a good alternative.

Thank you Randy_Cohen, I appreciate the explanation.
It’s a good thing for me to try out since the alternatives are somewhat messy and / or annoying.

If the spent IPA is put back into the containers it came in and is kept in a dark cupboard for a couple of weeks there will be about 5-10mm of grey sludge in the bottom and the rest will be clear. Putting the container into the sun for a couple of days will turn the whole of the contents into a solid gel, showing that the active ingredients of the resin, or at least the monomer, and the crosslinker, are dissolved in the IPA.

The reason why the ‘trick’ of putting spent IPA in a glass jar to clean it appears to work is that glass is partially opaque* to UV, so not enough UV is transmitted to trigger the curing of the dissolved resin constituents. The only way of removing these chemicals is by distillation.

*There is not enough UV in sunlight transmitted through glass windows, for instance, to give people tans.

I haven’t tried it myself, but appropriately sized bag filter socks might work better than a paper towel though the cost would certainly be higher.

While window glass does a great job of blocking UV B and UV C wavelengths, which are all much shorter than the wavelengths required to cure Formlabs resins, UV A passes through untreated window glass just fine. The UV A band goes up to 400nm. 405nm is the frequency of UV required to cure resin. That’s close enough to 400nm to not matter much. Sunlight through a window will do a fine job of curing resin.

Yeah, maybe only 50% of the UV gets through. But that’s 50% of a gagillion-watt light source, so it’s still plenty.