First layer adhesion, under extrusion - work around found
Hey All,
So i was having the issue that i've seen a few others having where i was not able to get a good first layer. It just wouldn't stick. I have a mk3s and an MMU2S which was upgraded from a fully working mk3. My issue looked like under extrusion, though as it was suggested that it could have been a non-clean bed, i did clean it as well as i could.
The strange part is, whilst the filament change happens, or the initial purge happens (if i have disconnected the mmu2) it get a reasonable amount of filament out, but then once it swaps to the print (or first layer calibration wizard) the amount of filament movement drops, 
and the side by side layers never stick to each other - and peal up from the bed. Also makes it impossible to get a good first layer height, as there is just not enough filament coming out. 
I originally had a print head which had had a failed print result in plastic sticking to the hot end - i tried to replace that with a new - pre-assembled - hotend but got the same results.
I even swapped to a brand new prusament filament in case it was a filament issue. and again got the same error.
I was able to get a successful print (the same one which has failed consistently since since upgrading) by going into slicer and setting the extrusion multiplier to 1.1. I suspect that this value is higher than i truely need, but it was the recommended max, and i was wanting to see if it would even achieve anything.
Now that i'm 99% sure that my issues seems to be associated with under extrusion - and that the nozzle isn't blocked (and 99% sure the extruder is assembled correctly) could there be something with the firmware (such as it upgrading from a mk3, to a mk3s) or otherwise which could have a bad value for the extrusion or any other issue within the firmware?
If others doubt it is under extrusion - i have replaced the hotend, used prusament (old hot end and filament was working right before the upgrade of the print head - i printed a part for the mmu2) i've disconnected the mmu2, cleaned the heat bed and even tried tweaking the tension of the filament idler.
RE: First layer adhesion, under extrusion - work around found
If your filament is passing through a lot of external PTFE (Bowden runs), then you could be having friction problems. Does the under extrusion still happen if you load filament directly into the extruder (remove Bowden tube and feed filament directly).
RE: First layer adhesion, under extrusion - work around found
As part of my testing I reduced the PTFE tubes more and more - I am now using only the small bit that came with the filament buffer as part of the mmu2 upgrade - also, feeling the filament as it gets pushed into the head doesn't feel like it's being impeded.
Something I noticed though - when the machine goes from the purge to the print process the move seems to stutter and when that happens the display shows it is loading files from the SD card which seems like an odd thing to do.
RE: First layer adhesion, under extrusion - work around found
Can't work out how to edit the original post to fix the images - but i did have the following included - the transition of the filament from good to under in the first layer print: 
and the overall image of the first layer process: 
Hopefully these work compared to the ones in the original post.