Nozzle size vs Overhangs

[morgandc]

Just curious if anyone has seen a correlation between nozzle size and how well unsupported overhangs do. It seems like a smaller nozzle would weigh less and cool faster therefore making unsupported overhangs a bit better.

[IMBoring25]

Unless you manually set them, extrusion widths will probably correlate to nozzle size. If a specific layer height is desired, a steeper angle overhang should be possible with a larger extrusion width (i.e. nozzle) because more of the wider extrusion will be supported.

If you're asking about bridges, it would probably depend on the properties of the material you're using and your other settings.

[Jimustanguitar]

Haven't played with different nozzle diameters much, but I can tell you that layer height will effect overhang angles.

[mhackney]

The general design principle for FDM printing is unsupported overhangs should be less than 45deg. Think about it, it doesn't matter how wide the extrusion is, at 45deg only 1/2 of the upper layer is supported and the other 1/2 overhangs. For extrusions 1mm wide or less, there really isn't a lot of slump so this is a reasonable design rule up to about 1mm wide nozzles. Wider and you may get slump without efficient cooling so you'd expect less than 45deg overhang.

Also, printing perimeters inside->out gives you better overhangs since the outer perimeter now has a surface to bond to.

[IMBoring25]

at 45deg only 1/2 of the upper layer is supported and the other 1/2 overhangs.

Are you sure? It seems as though this should only be true in special cases. At 45 degrees, the perimeters will shift laterally by the layer height every layer height increment. That should leave the extrusion half supported only in the case where the extrusion width is exactly twice the layer height.

[mhackney]

Ok, so the more complex answer is:

The practical maximum overhang is 45deg and that is really best achieved by an aspect ration of 2:1 for width (nozzle) vs layer height. This will give you the 50% overhang.

As you increase layer height (keeping nozzle fixed) you have increasing overhang at 45deg to the 1:1 case where you end up with 100% overhang (or no support). So anything from 2:1 to 1:1 is not good simply because there is more than 50% overhang. You can try to play games with massive cooling, etc and can do a bit better.

As you decrease layer height (keeping the nozzle fixed) you can, in theory, go less than 45deg overhang but in practice, the now much thinner layers need more than 50% support in order not to slump. Again, massive cooling can maybe allow you to go a bit greater (but usually introduces other issues) so the practical limit remains at 45deg overhang.

You can often get by with 1 or 2 layers of steeper overhang like on the bottom of a print where you might use a fillet rather than a chamfer. You might be able to get by with a couple of really thin unsupported layers. But a better design is to use a chamfer.