Inconsistent horizonal lines

Hi,

I've been trying to work out what the cause of this problem could be for a while and I have yet to figure it out, I think it's time for me to beg the wise ones for nuggets of their wisdom.

Basically, I'm seeing the horizontal lines on my prints come out very irregularly in some parts of the print like these:




However if I print something that is hollow (has no infill, just perimeters) I get nearly perfect walls:



The irregular lines seem to be chunked together causing some sort of "waves". They also seem to happen at points where the part has a volume change, which must mean something.

I've checked the gcode and it doesn't have any of these (and both cura and slic3r give the same results). At first, I thought it was z wobble, but the hollow prints rule that out, unless I'm missing something. I also thought it might be belt tension, but I've tightened the belts to the point I think they are over tightened and I'm still seeing these lines. I also thought it might be variations on the hotend temperature, but I PID tunned the extruder, and they are still there. My latest theory is that it might be retraction-related because of the points where it seems to happen but I don't see any other retraction artifacts, and current settings are pretty aggressive (1.5mm length @ 70mm/s). Print speed is also relatively slow (45mm/s for infill, and about 20mm/s for perimeters) and acceleration is currently set to 250mm/s^2.

The printer is a prusa i3 variant, with a direct drive extruder (bondtech bmg x2 and e3d chimera+). Happy to give more details about it.

I would appreciate any ideas to help me debug what the problem might be, as it annoys me to no end.

Thanks!

You're correct in that the issue isn't Z-wobble. You seem to experience the same issue as alot of Prusa MK3's out there. I would watch this video, which suggests alot of different causes.

How to improve the quality of your prints

I have a similar problem except that the vertical walls seem to have a regular pattern (Vertical waves or bars). I suspect that this is coming from the linear rail bearings (16 mm rails, homebuilt printer, very heavy duty!) when the recirculating balls re-enter to the rail I think there is a slight "bump". When I move the x or y carriage by hand the movement is not silky smooth so I am going to try some bronze bushes which should yield a good smooth movement. See if your printer does the same.
Les

Also, check the filament feed roll for clogging or slipping etc as feed variations will cause extrusion width issues which will give rise to this sort of thing.
Les