Creality CR-100 Curl checker.

thingiverse

This is a test for 100mmx100mm printing beds. Tired of curling issues on designs that take up most of your print area? Use this simple test to check your setup. If you have a corner lifting, it will more likely start to curl closer to the extreme edges of the bed. I designed this to use minimal filament, no supports, rafts or brims, and for symptoms to appear quickly. This way, you can easily stop, clean the bed, adjust it, reboot and reprint. Once you achieve good results, you'll have more confidence in your prints. Arduino cases and drill battery brackets have been difficult to get right. Using a different filament means tinkering with settings all over again! It doesn't help if the bed hasn't been leveled properly! I hope this helps you as much as it has helped me.