How does expiring resin affect the print? Does expiring resin tend towards overcuring?

Looking at the forum posts it seems most resins have a 12 month shelf life and several months in the tray. But I didn’t see anything about what actually happens to prints as the resin expires.

I typically print in surges - with months in between prints. I’ve noticed that I have a large amount of variation in print behaviour regarding curing - meaning a lot of prints are heavily overcured - where holes as large as 1mm will actually be closed up, and larger holes will be “constricted” by up to 0.3mm per wall.

Previously I didn’t pay attention to resin age, and put the overcuring down to firmware changes combined with machine variability, combined with resin type variability. Since different resins seemed to behave so differently,

Then I started getting good dimensionally accurate results with Grey V4 and stopped worrying about overcuring, until now. On the latest print surge I think I may have experienced a fair amount of variability printing with the same cartridge.

It looks as though the resin sitting in the tray for almost 2 months (in summer 22degC+ temps with a couple of 30+ heatwaves) since the last print surge may have been overcuring when compared against prints that came from the fresher resin near the end of the cartridge.

Is that what happens to resin as it nears and passes expiry - it overcures?

I don’t have detailed knowledge, but I’d guess that the different chemicals in the resin age differently, affecting the resin properties. How resin sitting in the tray compares to freshly dispensed resin with the same age I don’t know, also mixing the resin thoroughly might affect the outcome.

Which resin is it?

Grey V4 was the resin that seemed like the tray resin was overcuring and the effect lessened as it was replaced with resin from the cartridge.

But it is a general question. My assumption is that the general purpose resins (clear/grey/white/black) should behave similarly as they age - not exactly the same - but the general effect should be similar.

I’ve had a lot of problems previously with Clear V3 (and above) overcuring and I found White to be even worse.

I’m wondering if some or all of my overcuring problems could have been resin age related.

I’d be curious to hear what someone from the materials team here has to say. What follows is just my observation with printing, not official formlabs recommendations.

Last week I printed with Grey Pro that had been sitting unused in a late-prototype LT tank since January 2018. It worked surprisingly well. I had no issues with the print or resulting parts. I’ve had similar experiences in the past. I suspect the expiration date is set for two reasons. The first, like water bottles, is that it may actually be the tank that is at risk of degradation when exposed to resin for too long. The second is that the photoinitiators in the resin degrade such that they no longer offer the same cure properties—perhaps resulting in dimensional inaccuracies or increased failure rates. These are my guesses.

@CraigBroady to be clear what I mean by “overcuring” will almost never result in a failed print, quite the reverse in fact - Instead it’s the dimensions that are off (expanded by up to 0.3mm all around and closed up small holes) and the contact points are so hard they damage the print on removal.

Understood. I’m not actually sure whether expired resin would result more in over-curing or under-curing.

@CraigBroady well I think your experience rules out significant undercuring - since that causes dramatic print failures .

if you didn’t post cure before separating part from supports …

How did the part feel when separating from the supports - did if feel harder to separate than normal?

Or if you were using snips to separate rather than pulling it off like velcro - were the supports more brittle than usual?

I’m not thinking of one instance in particular in terms of over-curing or under-curing. For the part in Grey Pro that had been sitting in a tank for 18 months, I didn’t post cure, but I also didn’t inspect the supports enough to get an idea for dimensional accuracy.

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