Designing parts with proper wall thickness

[KiddingMe]

Hello, sorry for the stupid question.

When designing a part, if I know I want the wall to be only two shells thick (just for example), what would be the ideal wall thickness to produce good solid walls where the two shells are fused together well?

I'm using the stock V3b. My guess would be .8mm but when I tried that they still weren't fused together. Could be a extrusion width issue but would like to hear what width you would use.

[Jules]

Yeah, two threads don't always bond well. I found that making your wall width at least three threads thick (1.2-1.5 mm) worked a lot better for adhesion, with infill of at least 20%. The infill overlaps a little bit, and runs in an opposite direction, stabilizing it much more than the two thin-walls side by side. And it doesn't add much to the width.

You can increase the extrusion width by a certain percent on the first layer to help with bonding, but the rest of the wall might not always attach. Or if you need a really thin wall, just use a single thread thickness.

[KiddingMe]

Dang, I was really hoping there was a way to fuse two together better. I'll try the tripple-thread-width with infil as you suggested.

I want more strength then single shel walls give you but don't want to increase print time more then necessary.

[Jules]

Well, maybe the other guys have an idea......there might be something I haven't tried.

[Tim]

I'm using the stock V3b. My guess would be .8mm but when I tried that they still weren't fused together.

First, what slicer are you using? They can have slightly different behavior, especially for marginal cases.

Generally, if you set your extrusion width to, say, 0.4 (for a 0.35 nozzle), then a wall of 0.8 should be two filaments wide, as you suppose. Sometimes you need to make it a bit wider than that due to round-off errors. If the space is too narrow, then at least in the Simplify3D slicer, it will fail to generate any wall at all. But if you widen it to the point that it is just wide enough to accommodate two filament widths, and when you print it, they are not bonding together, then you can try over-extruding; that is, increase the "extrusion multiplier". If the extruder multiplier is set at 0.9, try increasing it to 1.0 or 1.1. You can also help the bonding by printing at a higher temperature, to make it more "melty"; try increasing it by 10 degrees C and see if that helps. Note that changing the extruder multiplier is actually increasing the wall width compared to what the slicer thinks/says it is, so keep that in mind given your definition of "proper wall thickness".

(edit) I would add that you should run a thin-wall calibration piece to check if your problem is simply that your extruder multiplier is set too low to begin with.