Layer height vs nozzle diameter

[Harlock974 0]

Hello

 

When searching the net to find how do layer height and nozzle diameter relate, we find that layer height should be between 25 % and 80 % of nozzle diameter (see here).

For example, if I use a 0.8 mm nozzle, I should print between 0.2 and 0.64 mm height.

 

But like many things repeated indefinitely on the net, no one explains why. Technically, what will be the issue if I print a 0.12 mm layer height with a 0.8 mm nozzle ?

[ahoeben 1,951]

3 hours ago, Harlock974 said:

Technically, what will be the issue if I print a 0.12 mm layer height with a 0.8 mm nozzle ?

The extrusions will have relatively low number number of steppermotor steps, so there is less precision in how much material gets extruded on a move.

 

There is no hard limit of what is "ok" and what is not, but with a lower layer height, you get less precision in how much material gets extruded, so (theoretically) you get very small under- and over extrusions along the print due to rounding. This is even more a problem for "relative" extrusion mode, since the rounding of steps happens at each move instead of over a full print.

Edited March 25 by ahoeben

[Slashee_the_Cow 271]

BTW: I'm moving this to the "Improve your 3D prints" board because it doesn't have anything in particular to do Cura, just a question about printing in general.

 

You can (to a degree, but you shouldn't and you probably won't want to) mitigate the problem slightly in one of three ways, since the problem is not being able to control the flow rate finely enough to straight out print, or keep any pressure in the nozzle chamber, if you increase the flow rate then the margin for error on the extruder's end becomes higher because you can be extruding enough to keep some pressure in the nozzle chamber.

Here's three ways you shouldn't and don't want to do it:

1. Have a line width so wide you could ride a bicycle along it. Higher width = more material = more flow, but in this case you'll exceed the "60-150%" guideline for nozzle ratio to line width well before you hit a high enough flow rate (and why can't you print wider than that? You can't just squirt molten plastic out of a nozzle and tell it to spread out 2mm in either direction and not go anywhere else.
2. Print faster. Higher speed = more flow. But again, you'll reach the limit of how fast your material should be printed, or possibly how fast your printer can print, before you can get enough flow to get away with printing layers too short. And higher speed = less accuracy, especially if you run it fast enough to get it ringing.
3. Increase the "flow" setting in the print settings to a ridiculous amount. This will just make it straight out overextrude the whole thing, which will cause you to lose all semblance of detail, or even surfaces, and possible give you a problem when you come around 0.12mm higher and the filament has decided that increasing in height a bit is a great way to expand as it cools down so your nozzle crashes it.

For those wondering (and those not): I am aware I spend far too much time thinking of ridiculous circumstances. It is not limited to this subject.

[Harlock974 0]

Thanks for the detailed answers.
What I understand is that if I try to print a 0.12 mm height, there is not enough vertical room to allow the flow coming from a 0.8 mm nozzle to draw a clean line, and material will overflow the side of a theoretical 0.8 mm width line. Is it correct ?  

[Slashee_the_Cow 271]

31 minutes ago, Harlock974 said:

Thanks for the detailed answers.
What I understand is that if I try to print a 0.12 mm height, there is not enough vertical room to allow the flow coming from a 0.8 mm nozzle to draw a clean line, and material will overflow the side of a theoretical 0.8 mm width line. Is it correct ?  

Not exactly. It might just run straight through and not fill the nozzle chamber, so you might get small underextruded or overextruded bits if the extruder doesn't keep up perfectly, which it probably won't because of how the motors in it have an amount they move per step so it can't do a tiny adjustment up or down.

 

Normally when printing the flow rate is enough that you'll fill the nozzle chamber, which then basically acts as a "buffer" of sorts to make it come out at the intended rate, but printing a very short layer on a 0.8mm nozzle isn't enough flow to fill the chamber, so there's no buffer to keep it even.

 

Another thing is that there's a lot more room in the nozzle than will be taken up by a 0.8mm line at a 0.12mm layer height, so it won't necessarily come out accurately in position.