Attempting to Automatically Clean a 3D Printer Nozzle Using a Wire Brush

I want to print more lamp shades using UMMD with clear ABS or PETG filament.  The only problem with these very large, long prints is that eventually, charred crud that builds up on the extruder nozzle deposits itself in the print.  I had a small, dense wire brush for nozzle cleaning from an old Stratasys printer, so I ran a couple experiments to try to use it to clean UMMD's nozzle.

After a couple quick measurements on the printer, I printed a bracket to hold the brush:

Printed bracket to hold the brush near the front edge of the bed.  The screw allows the height of the brush to be adjusted.

I heated up the nozzle and manually moved it back and forth over the brush.  The brush height is adjustable so I played with it a little to see if there was any particular setting that was better than any other.  It didn't seem to make much difference if I really buried the nozzle in the brush or just gently touched it.

Then I went into PrusaSlicer and modified the printer settings by adding some custom gcode that would run on every layer change.

The "After layer change G-code" tells the nozzle where the wire brush is located.

I ran multiple test prints (all ABS) to see if I could get the brush to clean the nozzle but found it just left blobs on the nozzle that eventually found their way into the print.  I played with it quite a bit and couldn't find any combo of zig-zag path and brush height that would get the nozzle clean every time.  Sometimes it did manage to dislodge a blob of molten plastic, but the nozzle picked it back up again on its next pass over/through the brush.

Here's some slo-mo video of the nozzle going through the brush.  You can see blobs of plastic on the brush and nozzle before it goes into the brush, and one gets picked up by the nozzle as it leaves the brush.


Better video of 3D printer nozzle encounter with a wire brush. from Mark Rehorst on Vimeo.
Nozzle brush in slow motion from Mark Rehorst on Vimeo.

A little of the plastic sticks to the brush, but that blob may get picked up by the nozzle on a future pass through the brush, and end up on the print.

Not my idea of a clean nozzle...

I don't know how or if Stratasys actually makes this work, but I can't seem to get it to work.  Maybe the shape of the nozzle is the problem- the brush came from a stratasys printer that had very shallow, rounded nozzles, not the sharp, pointy type that fit the E3D hot-ends.

Maybe a motorized spinning brush will work if it spins fast enough to fling the plastic it scrapes off the nozzle away so that it doesn't just redeposit on the nozzle and eventually the print.

Maybe I'll come back to this later, until then, score one for entropy...

If anyone has any success with this sort of thing, I'd like to hear how you make it work.