hotend leaving burned debris on print

I just started using a J-head Mk. V-BV (had to rebuild it after the temp regulation went bad and destroyed the PEEK and damaged my Melzi last December). The print bed is as level as I can get it (there is a dip about .005", or .127mm, in the middle of the borosilicate glass), and I have the Z height set. I noticed the nozzle seems to pick up material (I'm using ABS), scorch it, then leave the burned lumps on or in the print. What would cause this, and how can I fix it? NOT using a Chinese clone, BTW, it's a real US made J-head with a .4mm orifice. I'm printing on a Mendel90 "sturdy", but I am using herringbone gears instead of the standard Wade's gears (less backlash and they seem to work more smoothly).

EDIT: Should have mentioned, I used the PID autotune on both my hotend and my heated bed. I am using a temp of 220 for the hotend and 110 on the bed (using ABS juice).

Edited 1 time(s). Last edit at 02/22/2015 03:43PM by Rich K..

Does your ABS smells like PLA? I've had a roll of filament (USA branded) that smelled like ABS and PLA mixed together and had the same problem as you, though I used a higher temperature. It clogged up the nozzle in the end so I got rid of it and bought different brand ABS, which smells as normal ABS.
Also make sure that the nozzle is clean on the outside and that it doesn't leak (if it's a screw on nozzle). My E3D hot end gets some filament on the sides of the nozzle which deposits on the print sometimes, but that's rare. If burned lumps, blackened specks or bubbly expanding debris comes out of the nozzle, it could be some old residue onside the nozzle that has been heated for quite a while. It may not melt properly. Best way to fix it is to completely clean out the nozzle.

No, it smells like ABS. I started with a completely clean nozzle (burned all the old ABS out with a torch before I screwed it into the new PEEK barrel; the inside was nice and shiny). THe nozzle/heater block is screwed in as tight as it is capable of going. It's almost as if it is wiping up some of the material and depositing it on the outside of the nozzle, where it then "cooks" and flakes off.

Over-extrusion?

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Over-extrusion?

Don't know. How would I check for that? I calibrated my extruder by marking the filament sticking out above my extruder at 100mm, then telling Pronterface to extrude 100mm. I noted the difference, then made the change thus in Marlin:

// Mendel90 hobbed bolt and 39:11 Wade's gears
#define E_STEPS_PER_MM ((3200 * 39.0)/(11.0 * 6.75 * 3.142)) * (100.0/91.0)

Where it extruded 91.0mm when told to extrude 100mm.

Only thing that might make a difference is, instead of using Nophead's gears, I am using the herringbone gears from this thread:

Mendel90 Herringbone gears

However, they are also supposed to be 39:11, so I don't know if that is the problem or not.

Run your finger over a solid layer. If it's rough or the infill is higher than the perimeters, chances are you're over extruding. How much you're extruding is also dependent on the filament diameter, even if your e-steps per mm is correct.

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Definitely over-extruding, then. All of the above-mentioned symptoms are there. I didn't have this problem withthe CHinese J-head clone I was using, but when it broke (long story) and I switched to the genuine J-head (which supposedly has the same nozzle size, 0.4mm), I started having this problem. My prints are coming out crappy, and I have wasted a lot of filament.

I have my filament diameter in Slic3r as 2.85mm, which was checked with a micrometer. What should I do to get it to extrude less, and where should I start?

Try lowering the extrusion multiplier (under filament settings) in steps of .02.

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