Today I was printing some parts and I got these burn marks at the corners:
This is the first time I see this on this printer (Anycubic I3 Mega). I was using more or less the same settings (in Cura) as always, although I was using a new roll of PLA from a brand I haven't used before.
I was printing several parts in one job:
Most of them are somewhat affected:
Printing temperature: 200 °C
Build Plate Temperature: 60 °C
Printing Speed: 60 mm/s
Travel speed: 200 mm/s
In addition to this answer which addresses what is actually the cause (but doesn't explain where the burnt corners come from), I've added some information why this is showing burnt corners.
You are printing PLA at a too high of a temperature and do not have sufficient print part cooling enabled (either in the slicer or limited by the fan duct design). The higher the temperature the larger the shrinkage of the filament is. When printing overhangs like the slanted models you print, the last deposited layer has less surface contact area with the previous layer and therefore the shrink stresses may overcome the adhering stresses (it is also more easily weakened by the heat input of the last layer). This results in curling up, especially in corners as the heat input comes from two sides. When the corners curl up, the change of hitting the sides of the nozzle increases. When the outside of the nozzle is covered in burnt filament, the curling corners may pick that up. This results as if the corners are actually burnt.
There are two problems here. One as was mentioned earlier is that your temperature is too high and the cooling is not sufficient. The fact that one corener is affected more than the other can point to non uniform cooling (try using a cooling shroud like: https://www.thingiverse.com/thing:2004629 for more uniform cooling)
Another problem could be that your flow multiplier is off, this is often more noticeable in the corners. Try reducing the flow multiplier in your slicer settings.
Could be a dirty nozzle and temperature too high. When the head changes direction on the corner, the nozzle wipes off burnt PLA. This might explain why there are no dark marks on the long runs and only the corners.
Heat nozzle up and clean with acetone and then see if the problem persists.