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Coral heads growing next to verical surfaces

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I have noticed that I often get what I am calling 'coral heads' growing next to vertical surfaces, particularly columns. It's very 'organic' and looks like coral to me!



This is a particularly good (or bad) example, but there is often a single point column growing up and away from the base of the vertical surface. It's not a problem, it breaks away very cleanly, but is an interesting phenomenon! Is anyone else familiar with this? Is there a specific reason for it? Can it be cured?




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The general problem here is that retraction isn't working, so that you are stretching out strings of filament as the head moves from one place to another. There are small imperfections in the height/density of the string, and then as the head moves over it again and again, you get more plastic pulled out of the head as it crosses the thicker part, and the structure gradually grows and evolves. You can probably prevent it happening (at least to this extent) by enabling/tweaking your retraction settings.

But you're right, there's a related issue that I've seen sometimes where just a single column grows away from the base of a surface. I think it's mostly still retraction related - that while retraction is working well enough to prevent threading, there is still a slight ooze from the head during a longish move. The ooze gets wiped on the base of the print at the end of the move, providing a seed that wipes the ooze on the next layer, and so on up, such that the 'growth' grows away from the solid print, but opposite to the direction of head travel at that point.

Another example of this is not related to retraction. It happens when printing the low-density grid infill in the latest Cura builds, especially at high speeds. This was a 150mm/s test print:

New Cura Grid Infill - Failure at crossing points


The head prints a full grid on each layer, but traveling in opposite direction from one layer to the next. It prints continuously, but seems to stutter as it crosses over the perpendicular lines that have already been printed. This causes a gap, and a blob, which then grows in much the same fashion you showed above.


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I would also lower the temperature to get rid of this coral, maybe up the speed a little bit.

I usually have this when i print the hollow pyramid, which i use to find the sweet spot for filaments.

Like a calibration model.

I start of with lets say 210°C at 45mm/s, and see some of this coral rising. Then i wipe it off with a screwdriver or something, tweak the temperature and speed and see if it stays gone for a couple of cm. Often i end up with a unrememberable combination of 117% (based on 45mm/s) at 212°C or something. So i write it on a piece of tape and put it on the reel of filament.

Usually i only do this for long prints or when high quality is required.


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Thanks for all the replies! I was really just curious about what was causing it and if I should be worried, both questions have been answered I think.

This example occured with a bad batch of PLA filament, the same print with a good batch had only a single 'growth' at each corner, not enough to really worry about.

Many thanks,



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