You've got a jam. Filament is dried and burnt up in there. PETG is a lot more fluid / lower viscosity when hot, so it's flowing around the crusty. Take off nozzle, remove PET tube. You should have another PET tube in the spares bag that came with the printer. Clean out the extruder well.
this post was submitted on 19 Nov 2023
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I did have nozzle cleaned but my ptfe tube is totally melted and i lost the spare so i flipped it and it did helped a lot i have managed to print something without a fail but still only at high temperature 235C so i had a lot of strings so tomorow i will try to find the replacement i hope it will fix it 
Its weird for all metal hotend. Not sure how assembly looks exactly, but make sure your hotend fan is working properly.
At least you found a problem
Yeah, that's bad.
What extruder are you using that has PTFE tube that close to hotend? and want to print that high temperatures?
I wouldn't go above 220C unless its a proper all metal hotend.
cut it with a sharp knife ?
My best guess would be that you have some petg left in the nozzle that for whatever reason isn't being melted and pushed out, what you could try is doing a cold pull
If you haven't done it before, you basically heat up the hotend, feed in some plastic, then you turn off the heat and let it cool down until the plastic is partially hardened, then you yank on the filament
If everything goes right, the plastic down in the nozzle will be yanked out, and pull with it any impurities, leaving the nozzle clean and unclogged
Alternatively buy a new nozzle that you only use for PLA, i have heard a lot of people having issues like this when switching back and forth with the same nozzle
Yes, I have tried that but it didn't help then I tried to burn away everything in a nozzle with a flame so the nozzle was very clean and I think it helped a little but after a few minutes of printing PLA, I still heard grinding of filament.
just came to my mind that I can try to check ptfe tube that goes to hotend I know those are a bit sensitive to heat even tho in Prusa Mini the tube shouldn't be in a melting zone but mine is very old. It is the one that my printer comes with so maybe I should replace it. I will update you guys if that is the case.
The Ptfe tube is likely the answer, if you can replace I recommend trying to. Make sure it is seated correctly and properly clamped in place when you’re putting it back together.
Good luck!
You probably have a petg jam higher up in the heat break.
Take the Bowden tube and nozzle off and see if you can use some filament or a needle to clear the jam.
I tend to also think you have some kind of petg clog in there. Can you still print PETG just fine?
Yes PETG is printing just fine. I have switched to PETG also from Prusament to finish the print which bugs me because I like the colour of the PLA I have better for this one.
Purge some pla at higher temp than you print petg at, or even better use different nozzle for pla. If hotend and ptfe tube are clean try changing tension in extruder spring. Unlikely, but wrong tension can also cause grinding
AFAIK the Prusa Mini does not have a tension spring, but maybe the extruder wheel is dirty?
Thanks for the tip the extruder wheels were dirty from all the grinding but there is a tensioning spring and i have it quite loose because i had an issue with PETG because i have my printer in enclosure and with bed at 80C in enclosure was so warm that my filament which is inside the enclosure as well was so soft that extruder squash it a little bit and it git stuck inside that longer ptfe tube which is between extruder and hotend