The photo shows serious warping (or curling) of my print from my old PEI sheet which was printed on my MK4 with my old heatbed. All four of the corners warped (only the worst corner is shown).

This has been a persistent problem for the last couple of years. Smaller models printed closer to the center of the bed are fine. Only bigger prints which approach the edges of the bed curl up.

I recently upgraded my MK4 to the MK4S. While I was at it, I replaced the heatbed and I bought a new PEI sheet. And even though the sheet was brand new, I did wash it thoroughly with warm water and Dawn dish soap before using it.

After running through the full calibration and reslicing my model for the new high-flow nozzle using the latest stable release of PrusaSlicer, I reprinted it with high expectations.

Unfortunately, I was disappointed. One of the corners warped. It wasn't quite as severe as before, so that's a slight improvement. The corner that did warp happens to be the same one shown in the pic.

Print Settings: Prusa MK4S printer (no enclosure) 0.4 mm high-flow nozzle 0.2 mm resolution 15% infill Prusament Army Green PLA

-- Edit --

Wow, there are lots of good ideas here. Thanks everyone!

The things I'm going to try (in no paticular order).

• Covering the printer with a cardboard box or insulation sheets to see if an enclosure would help.
• Cutting a tiny relief across the sides on the first layer.
• Adding a brim.
• Slow down the first layer.
• Spraying 3DLac on the corners

-- Final Report --

I tried all of the above suggestions. Spraying 3DLac works! I will definitely keep this stuff in stock. Thanks again everyone!