No need to be worried. I have those purge line marks too. Once you see a purge line slipping even after a very thorough, repeated cleaning with a clean brush, you may want to try using a fine steel wool pad to try to extend the plates life.
See the white marks there? That’s bubbles or voids forming between PEI coating and steel surface. Yes, PEI coating starts delaminating off the steel plate. Next time I print on top of it, that piece will flake off.
The PEI plate above has gone through more than 10kg of ABS.
You should decide for youself when you need new plate. I just want to chip in that build plate for me is very much consumable and could last only 10-20 rolls of filament. Just like you pay extra 3-5$ for each filament roll to get a new plate after 10 rolls and always have the best bottom surface each print.
But of course, if you only print PLA, that plate is going to last much much longer than printing only ABS like me.
I’ve got around 4k hours on two printers and still using the original build plates. I flip them over once in a while and clean them but that’s it. I print all PLA
