HI folks,
I’m having a problem with all prints that have an internal horizontal plane.
I get this line, it’s bonded fine, no clogs, single filament, no obstructions, belts tight, cleaned rods, calibrated filament and printer.
It only happens when there’s an internal horizontal plane.
All other surfaces are smooth and butter as you can see.
Does anyone have any experience with this?
Thanks,
Harrison.
Everyone gets that. Which some searching probably would have shown. 
You can mitigate it to some extent by doing outer/inner/infill on the wall order.
But I haven’t seen any definitive explanation for why it happens.
Given it happens on many people’s printers, I don’t think it’s a mechanical issue or some people would have it much better or worse than others, and we’d see evidence of the mechanical play in other areas as well. This is consistently just the layer that corresponds to the top of an internal plane/surface.
HI,
I did search, but turned up nothing. But also difficult to know what to search for.
What do you mean by outer/inner/infill on the wall order?
Thanks,
Wall Order is a slicer setting. Change it to print the outer wall first, so that hopefully that wall will be cool and solid before anything else gets printed that might force it outwards.
To kind of clarify the physics that creates this phenomena, filament shrinks as it cools. So if you have let’s say a floor butting up against a wall, that floor has more filament in the X and Y axis than the upper layers of the wall which have nothing but air. So as the filament shrinks, the floor is going to tug much greater than the space on the wall that only has air behind it.
The setting can be found in the quality tab. You can do it either as a global setting for all objects or click on the objects submenu and navigate to the quality tab and apply the setting to a sole object. By forcing the model to be built from the outside in, change the order of how the wall is constructed. What this will do is allow for the outer wall too cool before the inner floor is laid down. So as the floor shrinks, the wall is “stiffer” as a result of having had time to cool.
One other trick you can do to mitigate this phenomena is to thicken the walls of that model. Again, it can be done globally or on a per object basis. By thickening the walls, even when the floor does shrink, there is more wall material to combat the tug of the shrinking floor.
Thank you Olias for your civil and perfect response. Have a good day.
I’m not sure that’s what’s going on. The phenomenon occurs regardless of the thickness of the wall that separates the inner “floor” from the outer surface. If it was driven by filament cooling, it would decrease as wall thickness increased.
Also, the defective line tends to bump out, not in, which suggests it’s not driven by shrinkage. And my previous FDM printers didn’t do this… maybe speed related, I suppose. I haven’t tried a test with a lower speed to see if it makes a difference.
Yes, you are correct and I should have reread my response. However, the physics still stands as I understand it and from what I’ve tested, it’s just the opposite of what my words stated. 
Sorry for the confusion.
The issue is that you have a dissimilar amount of filament behind the edge of the wall. Whether it’s expansion or contraction, the physical principle remains the same: different expansion and shrinkage rates due to variations in material thickness. When I first encountered this, I found a YouTube video suggesting this to be the cause. The author of the video provided no proof, just theory. So I tried it out on some troublesome parts, and lo and behold, the theory proved to be correct: the lines reduced dramatically when I changed to outer/inner, and combined with increased wall thickness, they even disappeared.
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