Ender 3 atomic petg warping

[nhyrum]

I've got a few large prints that the back left corner peels up the build-tek mat. It doesn't peel OFF the mat, the mat itself lifts. Print head is at 250c, bed between 65 and 80c, I've got a binder clip in that corner, but it still lifts the magnetic mat. I'm thinking about getting a glass bed. There isn't any airflow in the room, and the room is warm. I'm printing at 45mm/sec with 20mm/sec wall/first layer speed. Any ideas?