I don't really get much of a z seam on my parts probably because I tend to print slow - usually 35mm/sec.
The Z seam used to be where the printer changed layers and there was a pause while the Z axis moved. But now the Z axis is moved on inner layers only (infill or inner shells). But we still have a "Z seam". It's the place where you go from outer shell to inner areas.
The problem is identical to why you get large rounded corners on a cube.
The problem happens anytime you have to speed up or slow down the extruder. When you come into a corner the printer has to slow down for the sharp corner and the extruder slows down an equivalent amount but it's too late as there is already a high pressure in the print head so the extruding doesn't really slow down at the Z seam spot even though the X and Y axis do slow down. So you get overextrusion (and of course a corresponding amount of underextrusion right after although that tends to be spread out on a larger area).
So one solution is to increase acceleration and jerk settings on the firmware because now the print head doesn't spend so much time at the corner. The other solution is to lower print speed such that, again, the normal speed and corner speed are about the same. Other solutions include fancy features such as "advance" which is disabled in Ultimaker firmware but it is part of Ultimaker Marlin. You can turn it on in Configuration.h if you make your own version of Marlin. It's not the best fix. It's a bit of a hack and it underextrudes the wrong amount on corners and needs to be adjusted if you later adjust printing temp (which changes viscosity) or if you change filament color or type or print speed and so on. Even then it's a bit of a hack.
Anyway, the UM answer I suppose is to basically slow down the outer most printed shell.
Anyway this issue is much less pronounced on my UM2 than most printers because it has a reasonable jerk value (20m/sec) and a high acceleration value (default is 5000mm/s/s) compared to most printers. And again, I tend to print slow. I'm usually not in a rush and I have several printers.
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Ricky 5
It is Cura's fault. Tweak z-seam option in Cura and check the planned path for the spiral object.
Check out my bench test: https://www.thingiverse.com/thing:2890860
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