How I finally solved warping issues with enclosure PLA

I ran into the same thing last month with a big flat print that kept lifting at the corners. I had the door shut and the top cover on because I was also printing some PETG that week, and the PLA jobs suffered every time. Once I started leaving the top cover off for PLA, the corners stopped lifting completely. My garage sits around 68F, so the room itself was never the problem, it was just the trapped heat building up over a three hour print.

What helped me was watching the chamber reading on the printer screen during a test print. With everything closed it kept climbing the whole time and never leveled off. With the cover cracked open it settled after about twenty minutes and stayed put. That was enugh for me to just leave it open for PLA and closed for anything else. One thing I woud add is to check your drafts too. My printer sits near a window and on cold nights the door side would get a chill even with the cover open, which caused its own warping on tall thin parts. I moved it a couple feet away from the window and that solved that. So it can go both ways, too warm inside the enclosure or too cold hitting one side of the part.

Been printing since the days of open-frame i3 kits where chamber heat was never even a thing, so I still catch myself running with the lid propped open out of habit. I keep a chunk of scrap foam wedged under the front edge of mine, just enough to let the hot air bleed out the top. Cheap fix and it works. Only quirk is the first layer takes a bit longer to stick on cold mornings, so I bump the bed temp a touch if the garage has been sitting cold overnight. Works fine after that.

I stuck a cheap stand-alone thermometer inside mine for a week just to settle an arguement with myself, and the placement mattered more than I expected. The air up near the top of the chamber ran noticeably warmer than the shelf level where the part cooling fan actually pulls from, so whatever the printer screen reports does not tell the whole story about what your layers are feeling. The other thing I noticed is that most of the heat soak happens in the first stretch after a fresh preheat, when the bed and hotend are dumping warmth into a closed box. These days I crack the lid from the start on PLA rather than waiting for curling to show up, because by the time you see lifting, those layers have allready set that way.

Update: sorted it out. Ended up trying the foam trick from the second reply, just a scrap piece propping the lid so hot air can bleed out at the back corner. Chamber stays cool enough now that my PLA corners stopped lifting on the last two big flat prints. Also stuck a cheap thermometer in there like the third poster did and confirmed the top of the chamber was the hot spot.