Matthew Harrison - Dec 01 2018
Demystifying Thin Walls
This blog post was brought to you by yet another awesome feature that has recently been added to Slic3r Prusa Edition - the thin wall thickness recommendation. The contents of this post is something that I've had in my mind for a while but I've never had the numbers to prove it. Thanks to Slic3r, I do now.
So without further ado, let's get up close and personal with our thin walls.
A common misconception that I hear a lot is that when designing thin walled areas in a 3D model, you should make the thickness a multiple of your nozzle diameter. Some of you who know more about how slicing works will say that's wrong and it should actually be a multiple of your extrusion width - but here I'm going to explain why neither of those is actually the correct answer.
The difference between nozzle size and extrusion width
As you will know, 3D printing works by pushing plastic filament into a hotend, where it is melted and extruded through a nozzle. A standard 3D printer nozzle is 0.4mm in diameter, but once the plastic is pushed through the nozzle and down onto the layer below it, it actually expands to be usually somewhere between 0.42 and 0.48mm wide (depending on slicer settings). This is the intended behaviour, and it is for a very good reason: if extrusions are not squished to be a greater width than the nozzle orifice, this will result in poorer adhesion between each layer of the print.
In the image below you can see Slic3r's estimation of a cross sectional shape of an extrusion. All the following calculations will be based on this estimation.
Here I will make a comparison between two extremes - a high layer height print with a thin extrusion width, and a low layer height print with a larger extrusion width. On the left is a representation of a 0.3mm layer height print, where the extrusion width has been set to 0.4mm, and on the right, a 0.15mm layer height print with the extrusion width set to 0.48mm.
You can see that both the layer height and the extrusion width settings have a massive impact on how much of the plastic will contact the layer below.
Another interesting part of this is the voids that exist between extrusions, which I've highlighted in yellow above. If the slicer is only extruding enough plastic to lay the extrusions side by side as depicted, then those voids will remain unfilled. This leads us perfectly on to the next part of the blog post, which is why making your thin walls multiples of the extrusion width isn't correct either.
In an attempt to fill the voids between each extrusion, Slic3r actually overlaps them by a small amount. How much? I hear you ask.
layer_height * (1 - PI/4)
That much. So if you really want to design your thin walls to perfectly fit a certain number of perimeters, you'll need to know the layer height at which the model will be printed, and also your extrusion width setting for perimeters. This can be found under Print Settings > Advanced in Slic3r.
Then to calculate what width to design your thin wall as, use the formula below:
number_of_perimeters * extrusion_width - layer_height * (1 - PI/4) * (number_of_perimeters - 1)
This basically adds all the extrusion widths together and then subtracts the overlap between them.
Alternatively to calculating it for yourself, you can use the recently implemented "recommended object thin wall thickness" tooltip in Slic3r Prusa Edition, as seen below. This will take your existing settings and automatically perform the calculation for you!
A note about using this for extruder calibration
The overlap between extrusions is the reason that my extruder calibration guide uses only a single perimeter cube. I have seen some other calibration guides that recommend using two or more perimeters, which is fine, but just make sure that you are considering the extrusion overlap in your calculations.
In fact, I would say that if you are willing to deal with the more complex calculations, using more than one perimeter would give you a much more accurate measurement to use for an extruder calibration.
In conclusion to all of this I would like to say, it's not by any means necessary to follow this complicated process of calculation to determine the 'perfect' thin wall thickness. No matter what thickness you choose, Slic3r will do an excellent job of filling it in with perimeters or gap fill. But, if you do want it to be perfect then Slic3r just made it a whole lot easier for you.
The purpose of going into such an in depth explanation was just to prove the point that there is no sense in using your nozzle diameter to calculate a thin wall's thickness.
If you would like to read more about the extrusion calculations under the hood of Slic3r, I would highly recommend reading the Slic3r Manual page on flow math. It's where I got most of the information in this blog post from!