What causes this?  

RE: What causes this?

Is this caused by the same problem?

RE: What causes this?

Hi Wilco,

I would suggest to check if the Bondtech gear is still tight on the extruder motor shaft. This gives unregular extrusion which causes lines in the print and retraction issues.

RE: What causes this?

Thank you for your reply!

I took the little bolt almost out of the Bondtech gear (it seemed to still be stuck tight) and then retightened it.
Did two test prints, but still was met with blobs and random strings.
I guess it's something else. 🙁

RE: What causes this?

Problem two is simple adhesion issues, and it "multiplies" as the extruder moves upwards since the prior layer is missing.  it's pretty common on supports to see the ends not be as perfect as you'd like.  Variations of a couple mm maybe, but more than that seems extreme.  

If the set screw on the extruder gear isn't aligned properly on the motor shaft flat, it can still spin. Generally, when it starts spinning, it doesn't recover on its own, but could allow better results as the print speeds decrease, as they do for external perimeters and top infill.  Kind of fits what you are experiencing.

Also, there is a known issue with the axle of the Bondtech idler gear - it is too short and doesn't fully connect with both ears in the door, so must be carefully adjusted mid-way - so both ends connect just enough.  A quick test is pressing on the idler gear: there shouldn't be any give or flex.

And one other common issue: MK3's have two springs and adjustment screws, MK3S's only have one, and need more turns to get a good grip on the filament.  

I presume you are printing PETG ... maybe someone with more PETG experience can comment, but the range of temps needed to get good prints seems quite wide. I've printed between 220 and 250 ... and honestly each filament I've played with has different temperature needs.  Higher nozzle temps and slower cooling fan may help things like the support adhesion issue; and might help the underextrusion issue in the middle of the part. 

And there's always a chance for hard debris making it into the nozzle. Either printing ABS left a small bit - or even something coming along in the filament itself (chunks of hard plastic are rare, but it does happen).   I presume you've done a few cold pulls to eliminate common nozzle plugs.

RE: What causes this?

Thank you for your extensive answers!

There is no flex in the bondtech idler gear. I've indeed done several cold pulls, but also replaced the entire nozzle with a brand new stainless steel one. I will take the bondtech gear screw out once more and see if that is indeed sitting on the flattened part of the motor. The smaller prints that I have made these last couple of days seem to be working as they should, but it never hurts to make sure that all things are in order and working as planned. I have a box full of MMU2S parts that I am eager to start installing and using, but I want the MK3S to work flawlessly first.

And yes, I tend to use PETG for most of my prints.

RE: What causes this?

Okay - the hardened nozzle is a different subject altogether.  Especially as nozzle temperature increases.  I can't help much with how to tweak for it, but the general sense I get from reading the forums is that you must increase the nozzle temp and slow down print speeds over settings that normally would work with a brass nozzle.  The amount of adjustment is finicky... each post having an opinion for what's best.  All I can suggest is to start reading, maybe something will click for your set up.