Less density for some layers

Vertex 3D Printer Vertex - Issues while printing
1

Hello,

I have a K8400 for a few days.
I have made a few prints already, all of them were good until yesterday when a part came out with some issues.

Some layers are not good looking. It seems that the density is not good there.

An image being better than a thousand words, see for yourself:


(preview of images doesn’t work but if you clic, you can see the images on a google drive)

Any ideas why it came out like this ?

S.

2

One possible cause is the filament being tangled on the spool. It blocks the feed of filament until the traction is enough to untangle the filament and then the print proceeds normally.
Another option is a poorly tightened hobbed pulley on the extruder motor shaft. It slips for a while and then grips again.