Warped corners on PLA - bed surface issue or something else?

I ran into this exact issue when I started using my Ender 3 Pro a few months ago. I figured it was the print surface, but the real culprit turned out to be the temprature in the room. I’m in a basement, and even in summer, it runs about five degrees cooler than the rest of the house. When I first started printing, my parts would reliably lift around the fourth layer til I changed things up. I tried swapping to a brand new PEI sheet, but the problem persisted even with the fresh adhesive. The change only happened when I got the thermostat up and into the low seventies while the print was running. The warmth seems to keep the plastic flexible enugh to adhere properly for longer periods. It isn't just about keeping the room warm for comfort, it's about keeping the nozzle hot enough for the plastic to stick before it cools down. If your window is open, that draft is probly why it is failing only in the evenings or during warmer months. I found that a consistent ambient temperature is just as critical as the bed leveling itself.

I'm seeing this issue more often than I'd like on the Elegoo machines, and honestly, I think the culprit is probably the nozzle height, not necessarily the room temperature or the bed coating. When I had this problem cropping up on my platform, I thought it was always the ambient heat but it turned out to be something much finer. The nozzle was just too high, even though it seemed perfectly leveled at the start of the print. I went back and did a full 10-line test to verify the distance between the print head and the build plate, since the printer software calibration seemed okay. I discovered that even after cleaning the plate with isopropyl alcohol, the Z-offset needed a careful manual adjustment. I adjusted it in tiny increments, maybe by a hundredth of a millimeter, until the first few lines merged together without any gaps or excessive squishing. Once I got that initial layer looking perfect, I watched the next few layers. By ensuring the initial contact pressure was exactly right, the adhesion held much better as the print progressed. This made a big difference in keeping those corners from lifting and dragging, even when the print got quite large. It’s always about the contact pressure, I think.

It’s definitly not just about the surface itself, but how accurately the printer hits that surface right from the start.

I don't buy the whole-room heating thing. Cranking a thermostat to warm a basement seems like alot of effort for something that's probably just that open window. I had the same lifting issue on mine and all I did was close the door and shut the window during prints, no thermostat change at all, and the problem went away within a week. A draft blowing right across a hot bed is way different from a room that's a few degrees cooler. Try just blocking that window off for a few prints before you touch anything else. If it still lifts with the window shut, then yeah, maybe look at temps or the surface.

Before you touch the room at all, try a brim. I had the same thing on bigger flat parts, first layers looking perfect then a corner peeling right as the part got wider. Adding a 5mm brim to my slicer profile fixed probably 90 percent of my lifts overnight. Big flat footprint is exactly where corners curl worst because the whole piece shrinks as it cools and the corners are the furthest from the middle. Cheap fix, ten seconds in the settings, and it tells you a lot too. If a brim mostly solves it, your bed adhesion is fine and you can stop chasing surfaces.

Ha, kids these days buying new sheets and enclosures. Back when I started we just slathered glue stick on the plate and got on with it. I still keep one in the drawer for the big flat stuff that likes to lift. Put it on after the first couple layers are down, it gives the corners just enough grip to stop the peel without welding the part on. Cheap and fixes it more often than not. Also try bumping the bed temp up a notch on those wide pieces, heat is what keeps the shrink from pulling the corners up.

For me the game changer was a cheap infrared thermometer gun, the kind you'd use for checking a grill or an AC vent. I was convinced my bed temp was fine because the display said so, but pointing the gun at the plate mid print told a different story, especially near the front edge where the window air was hitting. Once I saw how much cooler that corner actually ran, the lifting made total sense. Might be worth doing the same before you spend money on sheets, since it costs almost nothing and settles the draft question in about two minutes.