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Like mentioned in #328, printing overhangs with edges tends to cause upward curling. In my case, this even happens with a print speed of 20mm/s and full ventilation when printing the small extruder gear (which basically only consists of edges) for the @reprappro Mendel. I observed this issue a while when printing things, and I think the core problem here is that the extruder pulls the still hot filament with it in its travelling direction, causing edges to be "shortened out". But since the filament is still attached the the lower layer, this causes tension inside the print, which in turn causes the edge to curl upwards.
So, as a possible solution to this problem, I propose to print the perimeters in clockwise and anti-clockwise direction alternatively, in order to eliminate this effect. Since I haven't tested this in practice (no favor for crafting my own GCode ;-)), I currently cannot say anything about the effectiveness of my solution.
As in, printing one layer's perimeters in one direction, and then using the other direction in the upper layer?
@nophead, do you think it would be a good idea to make this the default behavior, without any option to disable it? It would force people to eliminate backlash from their printers.
@alexrj I think this should be default behavior when not using spiral vase mode. I noticed same problem with my print and "invented" same solution as @rohieb.
@mrbi11 There's usually also the infill to print which gives some time to cool down. The benefit would be reduced curling in overhangs, especially when doing sharp corner over half-empty space. When doing perimeter always same direction, sharp corner slowly "travels" (and curls) to the direction of nozzle movement if it hasn't completely solidified yet. More cooling, yes, but at least in my test piece it would require stopping printing for a while between layers to allow even more cooling what my fan can achieve..