Prusa I3 - Burn marks on print

Question

This is an issue that's started happening recently. This is a Prusa I3 using a 1.75mm pla filament

The settings for this print are as follows

Layer height: 0.2mm
Line width: 0.4
Infill: 60%
Infill pattern: Triangles
Nozzle temp: 190C
Plate temp: 60C
Retraction distance: 8.5mm
Retraction speed: 60mm/s
Print speed: 50mm/s
Cooling fan speed: 20%

This happened on a previous print, but not as much. The only difference is the other print was printed 10mm/s faster. I'm thinking it has to do with too much time in a certain area since it seems to happen in pretty small areas that don't required infill and print as two walls back to back

Answer 1

I have seen this after I reassemble the extruder heat block and then leave it too loose during the print.

Is your extruder above the nozzle coated in a brown or black film?

What happens is the liquid plastic oozes through the loose connection, travels around the outside of the block, where it is heated longer and hotter than normal turning it black/brown. Then it drips, or flakes, into the print where it is spread around giving it a diffused look that you see.

The fix is to tighten the heating block. Do this while it is at operating temperature (be careful!) because it fits differently when cold.

Another cause: I made a tiny sharpie mark on my filament. I did not realize how concentrated it would come out in the print. Perhaps you have ink or adhesive or something on your filament that you can't see that turns brown when heated.