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Diagonal Scars on Overhang

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I've tried to print the overhang test from this benchmark https://www.youmagine.com/designs/make-2015-3d-printer-shoot-out-test-geometries

The print went relatively well after some adjustments but I haven't been able to get rid of diagonal "scars" on the overhang. The scars consist of bits of the outermost shell that curled up. The shell is always printed counterclockwise, and the scars move clockwise along the shell. Adjusting temperature, cooling and speed doesn't notably change the number of scars, but reduces negative effects of the scars on the overall print quality.

This images show the best I've achieved so far:



Apart from the scars the quality looks very good on the 30 and 40 degree overhang and acceptable on the 60 degree overhang.

I'm printing ABS on an Ultimaker 2 (with added front cover and housing). Temperature is 240°C (I've tried 210-260), bed temp 100°C (tried 80-100), fan speed 40% (tried 0-60), speed 50mm/s (tried 20-100), layer height 0.1mm, 0.8 shell thickness and top/bottom thickness. The temperature in the build chamber is at about 50 to 60°C. I've also tried to print without front cover and housing.

Any thoughts on how to get rid of this scars and/or what might cause them are greatly appreciated.


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I don't print much ABS but in PLA the fix is to use more fan. MUCH more fan. Add window fans, turn on a vacuum cleaner backwards and blow a hurricane at the thing:


Now with ABS, it shouldn't need nearly as much fan. ABS has problems with layer adhesion - you need the current hot layer to be hot enough to melt the layer below slightly so that you get a good solid bond. It's very easy in ABS to print too cold or to have too much fan and result in bad layer adhesion. So with ABS you want the highest possible fan but still get good layer bonding.

Older slicers work better with ABS and they will turn the fan on during overhang passes and off on the rest of the layer. That's like on and off every few seconds. This is ideal. Cura is written in the "age of PLA" and so doesn't have this feature (overhang fan).


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Well... more cooling seems to work. At least for the benchmark-object.

Following settings worked: 100% fan, 230°C hot-end temp, 100°C bed temp, no housing, no front cover, rest as above.

I guess that I've just been scared by hearing that ABS does bad things if it cools down to fast. Yes, it wraps if you cool it, but the heated bed seems to do a good job at limiting the negative effects and a 20 line brim gave enough adhesion to hold the object. I will test if this still works for bigger and more solid objects, as they have more problems with wrapping in my experience.


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Netfabb. But I think it's very buggy for retractions. I don't think they have updated for UM output for a few years!

That's the most likely one. Slic3r, kisslicer might also do the fan thing.


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