[Solved] PrusaSlicer vs. S3D: understanding my flow problem  

Unfortunately I don't think this actually answers your question but here's a very detailed discussion of the 95% flow rate issue: https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-hardware-firmware-and-software-help/why-is-flow-set-to-95-as-standard/#post-122743

But why you would see more overextrusion by going lower I don't know. 

Prusa's start gcode M221 is annoying!

That Prusa M221 line is pure evil, IMO. It's not easy to see in the slicer preview, and causes a lot of confusion when people look at their printer and notice the reduce flow rate. I prefer to remove it and do all my flow rate adjustments using the extrusion multiplier setting in my slicer.

As @fuchsr notes, it's not clear why you're getting over extrusion when going lower. The extrusion multiplier will be applied to any extrusion width, so tweaking the width probably won't help much with the problem you're describing. Save your current  3MF project file, zip it up, and attach it to a reply here so we can see your part & settings and give better recommendations. 

I also started with Simplify 3D. Life is much simpler with PrusaSlicer, particularly with the features they've added over the last 18 months. I'm still waiting to hear about the promised S3D update I was supposed to get.

@fuchsr, @bobstro, thanks for your answer! So I am not the only one, that does not understand, what is happening here... I think I'll try to compare different GCodes from PS and S3D to find out, what is happening here.

@bobstro, I agree, I totally missed M221 and never touched flow inside the printer menu up to now. It simply wasn't necessary... still learning after years of successful printing 😀

RE: Relative vs absolute éxtruder positioning

I noticed a significant between both GCode variants: S3D uses absolute extruder positioning, while PS exports everything with relative positioning, at least I configured it this way years ago. Perhaps this introduces some problems with my version of Marlin or whatever...

Fun with forgotten gcode commands and "magic" profiles

Posted by: @pandel

I noticed a significant between both GCode variants: S3D uses absolute extruder positioning, while PS exports everything with relative positioning, at least I configured it this way years ago. Perhaps this introduces some problems with my version of Marlin or whatever...

You can select whether to use absolute or relative in either slicer (Printer Settings->General->Advanced->Use relative E distances in PrusaSlicer). Not sure why they default to different settings. I think S3D mentions something about "simpler to debug gcode" and PrusaSlicer something about "more accurate". It is interesting that it's set to relative in Prusa's Mk3 presets but the default is absolute. You do have to be careful if you create custom gcode as the extruder will go nuts if you set the mode incorrectly at the start of a print! That will usually be very obvious though, so not likely your current problem.

Do check the obvious things like any gcode M221 adjustments in gcode elsewhere. Things like M92 e-step adjustments in gcode will also bite you in the butt. I had one I'd long since forgotten about in a printer preset that I inadvertently selected. Hell, double-check your selected nozzle size just to be sure!

Another possibility is that your S3D presets include some M20X gcode commands in start gcode. This was fairly common in some of the "magic" profiles being shared when I was using S3D. Be sure your Printer Settings->Machine limits in PrusaSlicer match. (I realized most of the "magic" in those magic presets was done with these commands and had little to do with all the slicer settings.)

One thing to be aware of when using Relative extrusion is that on a Prusa printer profile like a Mk3 they reset the value in the before layer change gcode section.

;BEFORE_LAYER_CHANGE
G92 E0.0
;[layer_z]

this has the effect of reducing cumulative errors.  I recall seeing this mentioned awhile ago on a github post.  Naturally I cant find the bloody post now 🙂

RE: Makes me wonder about other slicers

Posted by: @neophyl

One thing to be aware of when using Relative extrusion is that on a Prusa printer profile like a Mk3 they reset the value in the before layer change gcode section.

[...] this has the effect of reducing cumulative errors.  I recall seeing this mentioned awhile ago on a github post.  Naturally I cant find the bloody post now 🙂

I think I read something very similar. It makes sense, as errors can accumulate over time with retractions and extrusions. I'm not using them as much, but I recall most of the other slicers using absolute extruder positioning by default, and only zeroing at the start of a print. Which makes me wonder... would other Slicers benefit from doing the same? 

Edit: Just checked and ideaMaker does default to Relative and has a checkbox (default off) for resetting e length on layer change.

Many thanks for all your thoughts...

I got it! I printed a dozen single layer rectangles, playing with first layer height, extrusion multiplier and even babystepping my Z offset. Now I found some values, that seem to fit very nice and I could get rid of the M221s. I had to 1) lower first layer height, raise my Z offset and reduce the extrusion multiplier from 0.98 to 0.95. What I really don't understand is why I had to raise my Z offset! That worked pretty well over the last months without any changes. The current print (calibration cube in vase mode) seems to look very nice. Now I only have to measure the walls after the print has finished to find out, if lowering the extrusion multiplier has any noticeable effect regarding wall thickness. Fingers crossed...

Solved

FWIW: I did a full recalibration of my printer, as my heatbreak got clogged and I had to disassemble the effector. Now, after tuning everything, it works fine as expected. I begin to like PrusaSlicer very much and will use it for all prints coming to understand its possibilities.