Clearing a Blocked Extruder Nozzle

A blocked extruder nozzles apparently happens from time to time.  I was four hours into a five hour print and the job suddenly stopped printing.  The extruder filament drive gears were making an awful intermittent graunching noise as they chewed the sides of the filament.  Hardly any PLA was being extruded from the nozzle.

Everything about the print, up until the point of failure, was just fine with no perceivable thinning of the extruded filament.

The first thing to try when this happens is to reverse feed the filament from the extruder (with the extruder still at operating temperature), cut the filament on an acute angle to a clean section (no melt or surface indentations from the extruder drive), increase the extruder temperature by about 15°C above normal and re-feed the filament in 5 mm increments to see if this will clear the obstruction.  This will also test the extruder filament drive gears are working and clear of obstructions.

Cutting the filament on an acute angle helps with the initial feed.  Manually straightening the first 25 mm or so of filament by gentle flexing or heating with a butane lighter will also help.

In my case this didn’t work.  The extruder filament drive gears started graunching again after about 25 mm of feed and hardly any PLA being extruded.  At this stage we can be reasonably sure that the extruder nozzle has blocked or is critically obstructed.  We need to clear it.

Reset the extruder temperature to normal.

The nozzle is best removed from the heating block while it is hot, otherwise there may be significant resistance from any PLA that has set in the extrusion path.  Wear some leather gloves and place a cushioning surface on the bed to protect it in case you drop something.  Use a large spanner to hold the heater block still and a second smaller spanner to unscrew the nozzle.

Note that MakerGear now recommend that the nozzle should not be removed without first removing the extruder.

With the extruder nozzle removed and the extruder still hot, re-feed the filament in 5 mm increments.  This should confirm that the extruder filament drive is operating and clear any contaminated filament from the heater.  If there is still undue noise then the extruder filament drive should be opened, inspected and cleaned.

Reverse feed the filament clear of the extruder, wipe any excess filament from the base of the nozzle thread, and cut back a clean section on an acute angle.  Now turn off the printer.

There are several techniques for clearing a nozzle.  Here’s what I do.

In a well ventilated space, place the nozzle in ethyl acetate (nail varnish remover) in a Pyrex beaker covered with aluminium foil or a watch glass.  Heat this gently on a water bath to boiling and maintain for about five minutes.  Do not use an open flame for heating.  The hot solvent will soften any PLA in the nozzle.  While you can use cold solvent and an extended soak time, solubility increases at elevated temperatures.

Now we need a ridged wire smaller than the orifice diameter to clear the orifice.  Copper wire is useless because it bends too readily.  I use a disposable micro-fine insulin syringe (available from most pharmacies or chemists).  The needle is just 0.2 mm in diameter and has a very sharp tip.  Gently rotate the needle through the extruder nozzle orifice working from the exterior of the tip.  This pushes any obstructions back the way they came as opposed to forcing them into the orifice.

  • Clear a blocked nozzle by softening any residual filament in the nozzle with hot solvent, clear the orifice using a disposable micro-fine insulin syringe needle, polish and inspect the nozzle.



Figure 1.  Disposable Micro-fine Insulin Syringe


With the orifice clear its time to polish the nozzle inside and out to remove any remaining residues.  I use metal polish on a clean rag for the exterior and on a cotton bud for the interior.  Inspect the orifice to make sure it is clear and circular by looking through the orifice at an incandescent lamp.  The orifice will be magically magnified by the pin hole effect.   If it is orifice or badly scratched then it may still be usable but it should be replaced.

  • A nozzle and extruder heater (hot end) are useful spares for your M2.



Figure 2.  Inspect the Exterior...



Figure 3.  and the Interior...



Figure 4.  and the Orifice


Some folk recommend soaking a blocked nozzle in acetone for about five minutes, then heating the nozzle in a propane flame for 1 to 2 minutes, followed by using a steel wire to clear the orifice.  While this may work combustion can cause carbon residues that may be more difficult to remove than the original obstruction.  Ask anyone who has burnt dinner in a pot on the stove!

Now reassemble the nozzle onto the heater with the printer off (and still with something in place to protect the bed).  Re-heat the extruder and re-feed the filament.  At this stage the extruder should be working as good a new.

Finally you will need to re-set the bed level and zero the height.  This is also a good time to check your filament drive tension.

A question that is worth asking is what caused the blockage?  I don’t actually know but there is no evidence of a gradual build-up in the orifice and this is a relatively new spool of filament.  I keep the printer under a plastic dust cover when it is not in use and wipe it down before every print job.  So  I suspect that either there was something in the filament, on the filament, or otherwise found its way into the extruder.

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