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E3D-Online and Make Magazine have written about the potential damage printing carbon fiber and glow in the dark filaments can do to your printer's nozzle.

What I can't seem to find is what clues or warning signs to be on the look out for if your nozzle has taken a significant amount of wear. I've printed a few hundred grams of glow filament personally and have not noticed any change in print quality.

E3D says you may have "unpredictable erratic printing" with a worn nozzle. Can anyone explain or provide examples of what this actually means and when a replacement is necessary?

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    $\begingroup$ Obviously, when you notice your print quality deteriorate... Don't fix it if it ain't broke. $\endgroup$
    – Tom van der Zanden
    Jan 22, 2016 at 7:27
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    $\begingroup$ Somewhat off topic, but if you plan on printing more of such kind of filaments, consider getting a hardened steel nozzle: e3d-online.com/E3D-v6/Extra-Nozzles/… $\endgroup$
    – Tormod Haugene
    Jan 22, 2016 at 8:07
  • $\begingroup$ Where did it say that glow in the dark causes the same effects? $\endgroup$
    – Eric Johnson
    Jan 22, 2016 at 13:00
  • $\begingroup$ It's fairly common knowledge. Depends on the particle size though, some GITDs are more abrasive than others. $\endgroup$
    – Ryan Carlyle
    Jan 22, 2016 at 14:20
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    $\begingroup$ If you want to check directly (say, before doing a big or important print), I'd get a set of tiny drill bits (one of many sources: amazon.com/SE-Drill-Mini-Sizes-82615MD/dp/B0052Z9TJG). If the next size larger than should, fits through the nozzle, then it may be time to replace the nozzle. The bits are also useful in general... $\endgroup$
    – TextGeek
    Jan 22, 2016 at 20:57

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I believe the little experiment made by E3D - the same link you provide - answers your question very well. Several points about wear can be found in this article. After printing only 250 grams of ColorFabb XT-CF20 (carbon fiber filament):

  • The nozzle diameter had increased markedly
  • The inner walls of the orifice (opening) showed deep sharp ridges and grooves
  • The tip of the nozzle had become critically rounded, and shortened

All of these symptoms were found repeatedly for standard brass nozzles.

In particular, I believe the last of these symptoms may be the one most easily identifiable without accurate measuring equipment (and without observing print quality).

With regards to reduction in print quality, these symptoms could be simulated by:

  • Setting the nozzle diameter too big in your slicer
  • Leveling your bed too high (the rounded tip will also reduce the length of the tip)
  • Printing with a partial clog that interruptus normal filament flow (due to the grooves and ridges)

Exactly what this will look like on your printed part is hard to predict, but I would assume you could see blobs, under-extrusion, poor layer adhesion, as well as an irregular surface finish of your top layers.

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One of the things I look for is if you pull the print head a good ways off the bed and have it extrude. It should just squirt plastic straight down. If it bends sharply in one direction, or even curls back on itself, then that is a sign of damage.

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According to

the inner diameter doesn't change much, but, as said by @0scar, the nozzle shortens and at the end you get to the inner cavity.

Check the length and you are done.

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TL;DR You could compare the nozzle to a similar new nozzle to look for wear.

The creators of the Olsson Ruby Nozzle have published some pictures of wear on brass, steel and hardened steel nozzles. From their website the following image is taken:

Nozzle wear

Depending on the amount of filament that is being printed, not only the internal dimensions, but also the outer dimension change considerably. This is clearly visible in the images. Comparing your nozzle to a similar new nozzle might give you an idea of the wear of your nozzle. It is expected that long before the nozzle reaches this state, you need to readjust the bed to nozzle distance and the flow modifier, you will see this in the quality of your prints and the first layer sticking to the bed. So, the more (frequent) you need to tinker to get the print to stick or to print successfully and dimensionally correct, the bigger the change your nozzle has worn.

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You can check for shortened nozzle length using a set of feeler gauges to measure the gap from bed to nozzle. https://en.wikipedia.org/wiki/Feeler_gauge

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    $\begingroup$ A worn nozzle is not necessarily shortened. Also, this seems like a pretty unreliable method unless you (1) used feeler gauges when you first installed the nozzle, and remembered the exact gap between nozzle and bed (2) while using the nozzle, never changed anything to the set up like leveling the bed (3) the bed and nozzle were/are perfectly clean when measuring. $\endgroup$
    – Tom van der Zanden
    Jul 6, 2017 at 5:56

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