Discolored grey prints on Form2

I need some help identifying the cause of discoloration in parts printed in grey resin.

I recently purchased a Form2 for work, we are using it to print some parts for an installation at a museum. I have been using the grey resin almost exclusively, and there is some odd discoloration on some of the prints. The grey is turning a milky white, it only seems to be happening on areas that are parallel to the slice direction, and the more area of each slice that is exposed, the more pronounced it is, if that makes sense. It’s difficult to photograph them in a way which does not look like the white areas are highlights. in the last photo I sanded away some of the area which really shows the contrast to the true grey underneath. In a couple spots you can also see even whiter splotches which look like an artifact from a splash of liquid or something.

All these prints are at .1mm layer thickness, washed in isopropyl and then cured for 1 hour in a UV oven. They have been kept inside, but have been handled and played with a fair amount.

Never seen that before. It’s actually kind of cool. The only thing I can think is that the resin wasn’t mixed completely and pigment was settling out. If I’m looking at your examples correctly, it’d be the first layers that get printed that have the discoloration.

The density of the pigment and the density of the resin are not matched. The pigment is denser and will settle out over time (in as little as a few hours). The wiper will keep the resin mixed if it’s already mixed but it won’t do a good job of re-mixing the resin if it’s been sitting for a while. LIft the lid and run the putty-knife over the bottom of the resin tank to scrape pigment off the PDMS and remix it with the resin, then try another print. When you do this, if pigment settling is the issue, it’ll be obvious when you look at the surface of the PDMS as you sweep the edge of the knife across it. You’ll see streaks of pigment being left behind. Keep sweeping until you can sweep and not leave any obvious pigment still on the PDMS.

Alternatively, you could shoot the prints with some grey primer and just forget about what’s hidden beneath.

I’ve seen water do that before the part was fully cured. Was the part exposed to water at all?

Yes! This must be it, I rinsed them off before curing them in the UV oven. Not sure why I did it exactly, I don’t think I read it anywhere. Thank You.

Randy, thanks for you response. I was thinking about your pigment settling theory but I’m almost certain that the response below has the answer, I rinsed the parts off with water before curing them in the UV oven.

Those parts are probably over cured, if they were closer to the UV light source while curing.

Also possible, the oven I built is small, about 12" x 12" x 12" - Should I reduce the cure time, or only cure one part at a time in the center of the oven? I built a turntable in it, and I had been curing up to 5 parts at once in there.

What I do is cure for like 5 minutes submerged in water, then take it out and dry it off, after dry then I cure it for like 30 minutes

I’ve seen discoloration after a prolonged immersion in IPA; as in forgot about it, came back two days later.

That would definitely do it

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