I did some experiments a while back with annealing PLA in a water bath

To my understanding, what’s happening is that the PLA is increasing in crystallinity due to the annealing. That makes it behave more solid and less rubbery at elevated temps.

Polymers are weird.

I think it should be possible to do this over a low stove. Depending on your heat output, you could start the water at 60 or 70 and let it rise over an hour, or start the water at 90 and let it lower slowly over an hour.

The difference between the 70C and 90C were minimal, and I think I annealed the garage door opener holder closer to 70C to reduce deformation during the process.

One thing to note is that even at 70, the part will still be soft and will likely change shape somewhat. If you have an opening that needs to be a precise size, you will probably need to have some kind of jig or shim to gently hold the important dimensions in place while annealing.

Nice thing about doing it in water is that gravity is less of an issue for sagging.

Depends on the size of you item, and its structure. I’ve played around with reshaping PLA prints with hot water, and they feel like freshly cooked pasta while the plastic is hot and wet.

I made my daughter a custom pla cup holder for a large water bottle- it shrunk in the sun! I expected it to melt and deform, but not shrink!

@Brook_Drumm how much did it shrink?

I’ll check. But I think around 10%

Beware, you might be shortening the life of your product. PLA is a poly-esther, and all polyesters are susceptible to degradation by hydrolysis. That biodegradability we seem to love about PLA, is actually hydrolysis. The more water is in the plastic and the warmer it is, the faster the hydrolysis goes. You can see this in action after printing PLA: the remaining PLA in the nozzle drips out after a print, because the viscosity changes/lowers with hydrolysis. Degraded PLA has a lower viscosity (don’t know about the melting point) and results in more brittle prints (when you print is too slow&hot).
In your case you are also changing the crystallinity of the plastic: while printed PLA is almost totally amorphous, if you cool it slowly like you did you give it plenty of time to form large crystals. That makes you parts shrink (denser packing of molecules), and they will be more brittle.
I would be very interested in some impact resistance and yield strength measurements before and after your treatment!

Yes, the tests from the papers I referenced before I began the experiment showed that while the heat deformation temperature increased, the test parts did become more brittle. It was initially concerned that might not be suitable for my visor clip (which required the part to deform slightly and maintain tension), but it turned out to be okay

As far as hydrolysis goes, I think that 4 hours of immersion may be insufficient to cause significant change. PLA is fairly difficult to compost (thus it’s use as a more eco-friendly replacement for PET has been disputed, because PET is easily recycled for net profit, whereas PLA composting is hard enough that it comes with a net cost). Your mention of hydrolysis did give me some new ideas for search terms and I found this interesting paper:

http://www.expresspolymlett.com/articles/EPL-0001793_article.pdf

While this paper focuses on experiments done with a PLA+Rice Hull combination for improved biodegradability, they do some comparative studies with 100% PLA at various temperatures. Their overall finding is that at 23C, material properties are relatively unchanged after 30 days of immersion. At 69C (which get us into annealing range) there are significant changes after 9 days of immersion (in particular, a significant decrease in glass transition temperature. the reverse of the results I found at a lower amount of time immersed). This probably means that one shouldn’t go overboard with slow ramp up/downs in temperature if annealing with the parts in water directly. Longer multi-day ramps (which are probably unnecessarily long) would need to be done in air or at least in a bag.

My cup holder was a throw-away piece, so no worries. I reprinted in xt (high temp pla that prints at 250. I painted it and am coating it in epoxy tonight. Should hold up well!