Vertical lines at different speeds, VFA / Belt artifacts?  

Vertical lines at different speeds, VFA / Belt artifacts?

I've been noticing some fine vertical lines on my prints. The printer is a mk4s built from a kit about 2 months ago. It's really a minor visual issue, I have even noticed these same lines on the printed Prusa parts. But my Mini+ that I used before this didn't have this. So if it's fixable I'd like to try.

I saw them sometimes in my prints, but it wasnt consistent. I thought it might be cheap filament. When I was doing a little speed test, I noticed the pattern and these artifacts consistently show up at certain speed ranges and directions.

My speed test print, in prusament pla. It is a single perimeter, the external perimeter speed setting is changed every 4 mm. The speeds are from the bottom up 40, 80, 120, 160, 200, 240 mm/s. This was the left side of the print, the heatbed was moving to the back of the print along the Y here. You can see the artifacts clearest at 80 mm/s. The other side of the print, where the heatbed was moving to the front was almost clear of artifacts.

This was the back of the print, where the extruder was moving to the left along the X axis. Here the artifacts show up at 120 mm/s. Again the front of the print where the X axis was moving to the right was clear.

The pattern seems to match the notches on the belt.

So I was thinking it must be the related to the belt. But it al seems perfectly fine. The belt is not grinding against the pulley. I tried tightening and loosening the belt and I had exactly the same pattern.

Any insights to what might be causing this, and how to fix it?