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JamesK

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realthor
I always believed that the flat on the nozzle is to cope with the imperfections of early 3d printers, it flats out the plastic that comes out and ensures better bonding with next layers, as they don't remain rounded.

Well, sort of yes and no. No to the first part, that's a common misconception, yes to the second part. It comes down to what shape are we trying to extrude and how do we get that. A round cross section extrudate would be bad, because there is minimal contact area and poor bonding

So to get better bonding we aim to extrude filament that is wider than it is high, something like

The tip of the nozzle provides a constrained space into which we can extrude the filament to get a variety of extrusion widths

The smallest width that can be extruded is limited by the bore of the nozzle - if you try and extrude less than that the system is under constrained, you can't guarantee where the filament will go and it will likely have intermittent contact the lower surface. In my simplistic view of the world, the largest width that you can extrude is determined by the diameter of the flat. If you try and extrude wider than the diameter of the flat the system is again under constrained. I'm not sure what that would look like, but given the viscosity of the plastics we use, I'm guessing it would look like this

So, my understanding (which I'm happy to have explained as to why I'm wrong) is that the nozzle bore sets the minimum extrusion width and the maximum rate of flow, while the maximum width is set by the diameter of the flat. A nozzle with a small flat looks like it should have a rather small range of usable extrusion widths, which seems like a disadvantage to me, as variable extrusion width is something that slicers can make use of to improve print quality. On the other hand, I don't yet see a disadvantage to having a large flat on the nozzle, so I'm interested in Srek's observations that the narrower tips have advantages in bridging and overhangs. I'm not disputing Srek's observations at all, just trying to understand them.

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Chri
the width of the outcoming filament should not be wider than the nozzle output size.
so for sure the filament output should flat (2nd picture) and not be round, but at least is should not be wider than the nozzle size.
Chri

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JamesK

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Chri
the width of the outcoming filament should not be wider than the nozzle output size.
so for sure the filament output should flat (2nd picture) and not be round, but at least is should not be wider than the nozzle size.
Chri

That's an interesting suggestion Chri, what's the reasoning behind it? Clearly it 'can' be wider than the nozzle size as that is how 99% of the prints I make are done. The nozzle bore sets a minimum width for the extrudate (since if you try and extrude less than the nozzle bore it's undefined where the extrudate would be), so if it should also not be wider than the bore you're in fact saying that the only possible extrusion width is the size of the bore. Clearly extruding wider than the outer dimension of the flat face of the nozzle is a bad idea, but I don't immediately see what's wrong with the range of widths between bore size and flat size.

[edit: you can take the above argument too far of course. If you make a nozzle with a very wide flat and try and extrude at multiples of the bore width print quality goes down as the shape of the extrusion becomes less and less well defined. I've done some tests at 2x bore size and they show signs of poorly controlled flow. Up to about 1.5x bore seems very workable in my experience]

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Srek
I'm talking about this one
[b.bonkers.de]

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Srek
I spent one year of my spare time developing and testing the Merlin hotend. I tend to become a bit defensive if someone claims a print is of good quality despite the hotend beeing used, especially when that person never used it himself. The Merlin has some shortcomings, a lack of print quality is not one of them.