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Extrusion Width Calculations (and multiplier tuning)

Posted: Wed Aug 14, 2019 3:02 pm
by pggibson
This is kind of an observation/request for confirmation regarding extrusion width and how it relates to tuning your extrusion multiplier, and some things to be aware of if you are moving to Simplify3D from another slicer. Hopefully someone else can confirm my findings. My basic argument goes like this though if you don’t want to read the whole thing:

"If you are using 0.4mm single wall thickness measurements to dial in your extrusion multiplier, the thickness you should actually be hoping to measure is 0.443mm"

Let me explain:
Before I got Simplify3D, I had always used the method outlined on Matt’s Hub to help calibrate my e-steps and extrusion multiplier. (basically setting the extrusion width, printing a single wall cube and tweaking the extrusion multiplier until the measured wall thickness matched up to the set extrusion width) I noticed however that when I used this method with Simplify3D, I always ended up with subsequent prints looking and acting slightly under extruded.

I read an article somewhere about the difference in how Simplify3D and other slicers calculate the extrusion width and tool paths to ensure that adjacent extrusions fuse well (I can’t find the article at the moment, unfortunately).

In a nutshell, what that article outlined and what I found in my research matched up: Simplify3D and Slic3r Prusa Edition differ in how they approach this problem. I did some tests slicing a 20mm cube with no infill, no top layers, 4 perimeters, a 0.4mm extrusion width set manually, and 0.2mm layer height. I generated GCode from Simplify3D and Slic3r Prusa Edition with these settings and then examined the GCode.

If I understand it correctly, this is what is going on:

If you set your extrusion width to 0.4mm, Slic3r attempts to create an extrusion that is actually 0.4mm as measured from the curved edge of the extrusion to the other curved edge when looking at it in cross section. Simplify3D on the other hand calculates as if it were extruding a line that is perfectly rectangular in cross section (there is a good reason for this!).

The result is that the Simplify3D extrusion will be slightly wider than the Slic3r since there is a rounding on the sides of the extrusion. In practice, it ends up being more like 0.443mm wide. This little extra "squish" on the Simplify approach makes it bond nicely to the extrusions adjacent to it – it ends up with about 0.043mm of overlap. In Slic3r, it compensates by moving the actual tool paths closer together. The overlap these closer toolpaths create is also about 0.043mm.

In both slicers, the outer perimeter toolpaths are exactly 19.6mm part for a 20mm cube which indicates that both slicers are treating the extrusion width as a true 0.4mm wide.

I made a diagram outlining the differences.

Why does it matter? For me it finally solved the “underextrusion mystery” when I had been so careful with my single wall thickness tests. It also explains why some people get better results with multiple shells on those tests because the error is minimized due to the intrinsic overlap between the shells. Honestly, I think the Simplify3D approach is better because it makes it easier to design for (predictable tool paths) and I bet a dollar to a donut it makes it faster to slice.

In conclusion (if I’m right) and you are calibrating your multiplier with single wall extrusion, set your extrusion width to 0.4mm and shoot for 0.443 when you measure it!

Re: Extrusion Width Calculations (and multiplier tuning)

Posted: Wed Aug 14, 2019 5:41 pm
by airscapes
Very interesting and fits with what i have seen found over the past year. I switched to a 2 wall square which I use to verify starting height and to some extent Extrusion width (should be .2 or .3 larger than the model. Then print a different model that is just a 30x30 square 2 layer high of my chosen layer height (so I have a few of them).
This is the extrusion square.. using a loop there should be no space between the line and the height should be correct for the model height. Used with the 2 wall, I have been able to get much stronger parts. Yes, the thickness of the 2 wall is normally .2-.4 fatter than the model as you suggest and my Extrusion multiplier is .2 - .3 higher than with the old one wall alone. Now if the .4 under sized hole thing could be resolved I would be thrilled.. still using 4.01 :-)