I printed a phone case (for as a test/experiment). It was flat and perfectly fitting my phone after the print.
But, 1-2 days later main surface bent a lot
Is it Pure nylon or is it reinforced with CF or GF? Once it warps there is almost no way of restoring it back to the original shape. My doubt is: Where has it been stored? does the phone heat up a lot while use? Did you wait for the print to FULLY cool down before removing it from the print bed?
Polyamide absorbs moisture from the air for a day or two after printing, but ive only seen that tested in the mechanical sense. What vendors nylon and what PA formulation?
Overture Easy Nylon.
as I mentioned, it happened 1-2 days later, I left it on a table and 1-2 days later it was bent (standard 50% humidity in a house I guess). I don’t think it was with a phone at that time.
It looks like some VERY unbalanced stresses in the material. Some of that may be down to the overall part geometry, but hopefully not to this extent.
First, make sure your filament is properly dry prior to print.
Make sure your nozzle fan is OFF for the entire build. This is what I suspect your main problem is.
Door and lid closed for the whole print. Wouldn’t be bad to preheat the bed before sending the print to warm the chamber somewhat.
Allow to fully cool before removing part.
Anneal after printing.
I always preheat the chamber to 40-45°C for nylon (also ASA, ABS, PC).
If you can’t get a successful nylon print, TPU protects and cushions a phone very nicely.
Nylon was of course dry, I wouldn’t get such a nice surface/print if it was wet, right? 
As I mentioned, that deformation happened 1-2 days AFTER it was printed. do you think changing print parameters would help ? or should I just do annealing
Yeah, I hope someone who has used Overture’s Easy PA can chime in. It seems like something the material itself is doing, maybe not relevant to the print settings.
