Black stains in prints  

RE: Black stains in prints

Wisps of filament are being extruded as the nozzle is moving along, they get caught on the nozzle where they cook and eventually fall off into the part. Try reducing flow 3 to 5 percent, see if it changes. If you see an improvement, try doing a fine calibration of the extrusion ratio.

RE: Black stains in prints

I get PETG stuck on the sides of the nozzle, especially when printing objects that are prone to generating strings (the filament makes a difference too, I have some cheap PETG that produced a very hairy treefrog full of black inclusions). It builds up and blackens there, and then randomly gets snagged and drawn into the print. I've found it helps to remove any buildup when the nozzle's preheating before a print. Also make sure that the ooze that happens during preheating hasn't curled up and started a new blob.

RE: Black stains in prints

Hi Tim,

If I understand you correctly the problem is related to small pieces of filament that stick to the outer part of the nozzle and that fall on the part after being charcoaled by the temperature of the nozzle.

Regarding the fine calibration of the extrusion ratio, do I have to do this for each extruder nozzle, or even for each spool? (I guess the latter is a bit of overkill).

RE: Black stains in prints

Also inspect your hotend. It's not uncommon for stray filament to collect around the nozzle and entire hotend, especially if you're not aware of it. If you've got some build-up, you might try cleaning the nozzle and hotend using a soft brass wire brush, being very careful not to damage the thin wires. If you're using a coated nozzle (you will have paid extra for it) you can use a strip of cardboard to clean the nozzle off.

I finally started using a silicone sock to reduce the build-up and it's made a big difference.

RE: Black stains in prints

As Bob pointed out, not letting it stick and bake in also a good option.

Measuring filament diameter and setting it in the slicer is a good idea for repeatable print quality. It let's you adjust for real variables in life. The extruder ratio only needs to be set once.  As for nozzle to nozzle, it hasn't been my experience changes are needed: the volume extruded is controlled by the motor and Bondtech gear speed.  Larger diameter filament means more extrusion; Smaller diameter means less extrusion.  And the motor speed is controlled via gcode generated by the slicer.  And these:

Total flow rate = Flow multiplier in firmware (M221) x Extrusion multiplier in PrusaSlicer.

https://help.prusa3d.com/article/d9j1xdg7vj-extrusion-multiplier-calibration

RE: Black stains in prints

Having a similar issue with Sunlu white PETG.  Started with the generic PETG PrusaSlicer settings and modified based on reading forums and running several calibration prints:

• lowered perimeter / infill printing speeds
• reduced extrusion multiplier from 1 to 0.98
• increase temp by 5°C to 235°C/245°C (and same with bed temp)
• increased retraction distance to 2mm and speed to 40mm/s
• increased Z-offset from 0 to 0.05 

My understanding is that white can be more challenging to print on, but I haven't been able to finish anything other than a small print without a burn mark.  What are my next steps?  Tweak the above settings, further reduce multiplier, silicon sock?

I also have 3 colours of Prusament PETG arriving this week.  Interestingly, the PrusaSlicer settings for Prusament have the multiplier set to 1.  

RE: Black stains in prints

I did a cleanup of the nozzle (that was slightly dirty). I also cleaned up the internals by using the "cold pull" method and I extracted many black fragments. I guess that the black fragment were coming from within the extruder.

Checked also the multiplier. I didn't change it as both the visual and the precise method gave good results.