I always get these little dark spots in my work from over cooked plastic.

3D Printing
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1

I always get these little dark spots in my work from over cooked plastic. I have two theories on the cause:

  1. I am over extruding causing the nozzle to drag through the work in the first few layers.

  2. The nozzle needs to be cleaned inside and out. I once ran a 6 hour print with the extruder malfunctioning so it seems reasonable that there’s junk in the nozzle.

Any thoughts on other causes I should look into when I have the machine apart?

Prusa i3, 0.5 jhead, abs @ 220. These pictures are 0.25 layers but I see it at all heights so far.

92c2aa8ab333a55f1d47ef5f08e032ca.jpeg
92c2aa8ab333a55f1d47ef5f08e032ca.jpeg1936×2592 1.32 MB

3216325f029061a94777959d0d25ecb5.jpeg
3216325f029061a94777959d0d25ecb5.jpeg1912×2560 1.1 MB

2

If your print curls a bit, it can wipe the nozzle just past where the extrusion is happening and either build up crud that does this, or it wipes off said crud.

3

It looks like the pictures were taken after doing some post work (nice shiny and smooth). Fume bath maybe? Anyway it would be nice to see the print before any post production. I’m thinking the print bed may be too close causing the nozzle to hit the already printed plastic. It then build up on the nozzle getting a bit over heated and scorched until it finally get attached to the print leaving a dark spot.

4

Do you have a real j-head or a cheap knock off? Cheap versions can melt filament high in the chamber and it can ooze out. I sometimes get the exact same behaviour with my j-head from ebay.

Also, do you have a fan on your j-head to cool the top?

5

I also shared in an experience similar to @Evan_Gillespie

6

@Evan_Gillespie , it was part of a makerfarm kit so it should be legit. I do have a 40mm fan, but it is pointed more towards the tip (no fan shroud) to improve overhangs.

@Michael_Spano_Jr_Ama , yea acetone bath. Before the bath it was more pronounced. I can probably find a better example, but I am fairly sure the larger pieces are from outside the nozzle. I’m not sure how to clean that.