Cosplay armor warps because the first layer is a raft you never meant to print
Most failed helmets I see are not “bad STL.” They are a first layer that looked fine for 30 seconds and then peeled.
What actually happens
A helmet or chest plate is a huge, thin footprint. The bed is 2–3° cooler at the edges than the center. PLA lets go. You keep printing. Six hours later the brim is a taco.
Do this before you start a 12-hour armor print
Print a 150 mm square, 0.2 mm tall, at the same temp you will use for the armor. If the corners lift, do not start the helmet.
Slow the first layer to 25 mm/s. Raise bed temp 5°. Add a brim, not a raft.
Split the file at the natural armor seams. Two 4-hour plates beat one 12-hour plate that dies at hour 11.
If you are printing PETG for outdoor cons, dry it. Wet PETG looks like a perfect first layer and then strings until the walls are garbage.
I sell print-ready armor STLs and a separate 1-hour live repair session when the machine is the problem. Files and repair are not the same product — if you only need the files, that is all you should pay for.