Odd flow rate issue after upgrade
Hello,
here is my primary issue with my Prusa mini:
when printing, the flow rate changes within the printed object from upper left to bottom right corner (on XY).
Background:
I always had intermittend problems with the extruder throughout the years but got ever more issues with only certain filaments I once was able to print in the past, causing clogs in the extruder or not being able to be loaded at all (others worked fine). I cant say what about the filament is the issue as diameter seemed OK but its mostly with brands like extrudr PLA NX-2, 3Djakes in house PLA, Eryone PLA and others.
So I decided to finally upgrade to a dual gear variant with a printables design ( https://www.printables.com/model/294169-prusa-mini-extruder-with-filament-sensor/files).
It was assembled and steps set as intended- When first loading with a known problematic PLA it couldn't load and just clicked in the extruder when the fast ramming happens (as seen here https://forum.prusa3d.com/forum/hardware-firmware-and-software-help/loud-noise-when-unloading-doesnt-unload/). This doesn't happen wtihout filament inserted and as I later saw when loading PETG it also worked fine.
I looked into adjusting the extruder stepper current but I couldn't find the parameter in the firmware source code as the structure nowadays seems to differ from the one in the old tutorial. Kinda frustrated I settled for updating to the newest firmware and just puttin on more lubricant and tried loading some PETG and it worked without issues. Heureka!
I calibrated the first layer (as I also upgraded to a super Pinda) and did a bed level test print where the strange phenomenon was slightly visible. Subsequently I went on to test the flow rate with a test I usually use for that. There it became apparent: in the bottom left and top right corner, flow rate was at optimum while in the other corners it diverged into opposit directions... wtf is going on? I printed a upsized version with 4 instead of 2 walls and after that two objects in sequence and the divergence happens within each single object.
I don't have a clue right now other than some issue with the board electronics? Let me know what you think is going on.
RE: Odd flow rate issue after upgrade
Please read https://github.com/prusa3d/Prusa-Firmware-Buddy/issues/2997 - it is SUPER long.
tl;dr different extrusion due to bowden tube design - when X axis moves then it causes tube and thus different diameter in which the filament moves, and this ends in underextrusion or overextrusion in X axis (depending on the print head direction).
You can minimize it to some extent using this mod https://www.printables.com/model/728174-prusa-mini-centered-extruder-mod-lever-remix
See my GitHub and printables.com for some 3d stuff that you may like.
RE: Odd flow rate issue after upgrade
Sure if it was only in X direction it would make sense. But the effect is both in X and Y and the specimens I printed in the corners of the bed have the same values, even tho being placed at different X positions (at around X=40mm and the other X=140 respectively). And the divergence is huge.
The flow rate print is to be measured at the middle of the walls normally. When I measure around the corners I get the following values for the specimen with the least divergences:
top left FR = 122%
bottom left FR = 104%
top right FR = 94%
bottom right FR = 72%
The % values for the bigger specimen are around the same. So even tho the distance travelled is doubled, the divergence is distributed over that length linearly.
So my guess is it is of a computational origin no?
RE: Odd flow rate issue after upgrade
Maybe.
I suggest to look at the generated gcode and see how the print head moves, notice that any x move will also influence extrusion on Y because it creates positive/negative tension. Also it's not only pure X axis issue because the extruder located next to the Z axis twists the PTFE tube in both directions, the Y will be just much less visible.
See my GitHub and printables.com for some 3d stuff that you may like.