Burnt filament causing black spots in print...

I am printing using Orange ColorFabb Co-Polyester Amphora.

When the job starts printing over a wide section of lattice infill, tufts of blackened filament occasionally appear in the bridging gaps.

These get smoothed over, but often leave a black spot under the surface of the finished object.

What causes these and how can I prevent them?

I have never seen Amphora turn black (well, at least not at printing temps).

It could be some leftover deposits of other filament (darker color from previous print) occasionally breaking loose and mixing into the Amphora. You might try a “cold pull” to get the nozzle squeaky clean. Here’s a link to a recent post with more info on that:

This might be pretty obvious but worth checking, you’re not over extruding or Z too low and getting excess stuck on the outside of your nozzle which is turning black and scraping off in parts of the print?

I am thinking that this may be a possibility - although I am not sure exactly what you mean by “over extruding” or “Z too low”. I tend to run default settings as my experience is low. Over time, the outside of the extrusion part of the head does get a bit crusty. I have cleaned it once. Perhaps I should increase the cleaning rate in my operations.

Anyway, the problem occurs when a layer is being formed over infill latticework near completion of the top of an object.

I did do some experimenting with cleaning the head internally as mentioned in the first reply (interesting results!) but I suspect that clogging artifacts of this type would appear early in the print cycle.

Thanks so far to everyone who has responded!

I would imagine over extrusion is probably your problem then. Over extrusion is quite self explanatory, it literally means extruding more plastic than you should be, thankfully it’s pretty easy to set right. You need to make sure you measure your filament properly and have an accurate filament diameter set in your slicer, then it’s very worthwhile taking the couple of minutes it takes to calibrate your e-steps (calibrating the extruder motor to make sure it’s delivering the correct amount of plastic to the nozzle). Lulzbots official guide for this is easy to follow but if you search the forum there are plenty of discussions on the topic which offer various tips and techniques to make it easier to get an accurate calibration.

Making sure you have both these settings right will not only help combat dirty prints but will also make a big difference in helping you get cleaner, true to size prints. Try both these things and see how it goes.

I think I figured out my problem!

The extruder had built up a coating of filament around the base. As the extruder tip was constantly going through a heat-cool cycle with daily use, it would occasionally drop a piece of filament debris onto the object currently being printed.


Keep your extruder clean!

I keep my nozzle spotless with a scotch brite pad between almost every print and I get this sometimes too when printing with nGen (also Amphora) in the situation you described in your original post. It seems like the first layer over large infil doesn’t bridge very well and it curls up in some places that the nozzle then rubs against when doing subsequent passes on the same layer, this collects a bit of the filament on the nozzle which discolors and then gets deposited later in the same layer when it brushes another curl. I’ve only had this problem on real large areas.