Strange artifact on one side of 3d printed cylinder
I created a replacement knob for a stereo I own in Fusion 360, which is basically a short cylinder that was shelled from behind. I wanted to get the walls of the cylinder printing more smoothly, so I turned down the "External perimeters" speed from 35 to 20 in Slic3r PE. The slice preview looks fine on all sides. The print proceeds normally for a while, and then at some point one area of the cylinder side gets some really odd and ugly 'rash'. The first time it happened I suspected that something had gotten stuck to the side of the nozzle and it was scraping the print on that side. But just to be safe, I regenerated the gcode and started again, and got the same result.
I've printed this model many times either as a shell or a hollow solid, and most prints have been successful. It seems like the one speed config change was the only difference. I'm printing again at default speed to see how it goes. Any ideas?
My best guess was that this particular movement was causing the spool to rock back and forth in such a way that it was pulling on the filament on that part of the perimeter and under-extruding.
Bad part:
Good part:
