General nozzle size vs. layer height/width principles

I know I have run across these in various places in the past, but i can't seem to find them again....

What are the general principles/guidelines for the maximum layer height given a certain nozzle orifice size? And also what would be the max width/height for a given nozzle?

So for example I have a J-head hot end with a .35 nozzle... what would be the largest recommended layer height and width?

Im also using Slic3r which automatically calculates the width (unless told to do otherwise), so layer height is what im most currious about.

I think i remember reading somewhere that you wouldn't want your layer height more than about 60% of your nozzle size. Is that in the right range?

It depends on the die swell, which depends on the plastic. If you extrude into mid air and measure the filament that is the maximum cross sectional area you can extrude.

The width over height ratio should be 1.5 or more for strong objects. It can be bigger as long as you don't exceed the die swell area. Also if you have a very fine nozzle tip it should not be wider than that or it won't be pressed flat.

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[www.hydraraptor.blogspot.com]

Thanks Nophead, im printing in PLA and when i extrude into thin air, the strands come out about .37 in thickness.

My nozzle is fairly fine, the flat part of the tip is 1.3mm in diameter, so it makes sense like your saying that the width can't be any wider than 1.3mm or it won't be pressed flat.

If i calculate what you suggest....

The cross sectional area of my extrusion into air would be (.37/2)²*3.14 = .1075mm²

With a width/height ratio of 1.5 and a layer height of .3.... the cross section area of my flattened extrusion would be ((.3/2)²*3.14) for both half circles on the ends + (.15*.3) for the rectangle left in the middle = .1157mm². So this would be a greater cross sectional area than my extrusion into air.... which i guess isn't good because it would make the extruder have to put out a greater volume than it easily can. Is that the right logic?

Trying a smaller layer height... say .2 The cross section area of my flattened extrusion would be .0514mm². So that gives us a much smaller cross sectional area than what we get extruded into mid air. Maybe that's easier on the extruder due to less volume to push through, but would that not be optimum either?

I guess something around .29 would be ideal for a layer height with a width/height ratio of 1.5 because it would give me a cross sectional area of .1080mm² which is fairly close to my cross sectional area into air of .1075mm²

Seems like with .35 nozzle orifice if i was always shooting for a 1.5 width/height ratio that would lock me into always using a layer height of .28

I know that's not true though because people print at all different layer heights given a certain nozzle orifice size as long as they don't exceed a certain maximum for that size orifice. So is the width/height ratio negotiable also? Is there a maximum you don't want to exceed?

daufhammer Wrote:
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> Thanks Nophead, im printing in PLA and when i
> extrude into thin air, the strands come out about
> .37 in thickness.
>
> My nozzle is fairly fine, the flat part of the tip
> is 1.3mm in diameter, so it makes sense like your
> saying that the width can't be any wider than
> 1.3mm or it won't be pressed flat.
>
> If i calculate what you suggest....
>
> The cross sectional area of my extrusion into air
> would be (.37/2)2*3.14 = .1075mm2

Yes
>
> With a width/height ratio of 1.5 and a layer
> height of .3.... the cross section area of my
> flattened extrusion would be ((.3/2)2*3.14) for
> both half circles on the ends + (.15*.3) for the
> rectangle left in the middle = .1157mm2. So this
> would be a greater cross sectional area than my
> extrusion into air.... which i guess isn't good
> because it would make the extruder have to put out
> a greater volume than it easily can. Is that the
> right logic?

The biggest area are the internal threads which can be assumed close to rectangles, so just use height * width.

The constraint isn't to do with the extruder throughput. That can be an arbitrary value up the limit where the hobbed bolt loses grip due to it not melting fast enough. The layer height constraint is that if you make a filament bigger than the die swell value you are compressing it rather than stretching it. That doesn't work because it starts to squiggle, it is like trying to push a piece of string. It also won't span gaps as it is coming out longer than the gap it is spanning. To build properly you need to be stretching it a bit so the cross section is smaller than it comes out.



>
> Trying a smaller layer height... say .2 The cross
> section area of my flattened extrusion would be
> .0514mm2. So that gives us a much smaller cross
> sectional area than what we get extruded into mid
> air. Maybe that's easier on the extruder due to
> less volume to push through, but would that not be
> optimum either?

It isn't to do with volume through the extruder, as that depends on how fast you build as well. It is how much you are stretching it. The more you stretch it thinner the more it wants to cut corners and eventually it will just snap.

>
> I guess something around .29 would be ideal for a
> layer height with a width/height ratio of 1.5
> because it would give me a cross sectional area of
> .1080mm2 which is fairly close to my cross
> sectional area into air of .1075mm2
>
> Seems like with .35 nozzle orifice if i was always
> shooting for a 1.5 width/height ratio that would
> lock me into always using a layer height of .28

That is just the maximum. You use as low a layer height as you want, but you have to keep the width within limits.
>
> I know that's not true though because people print
> at all different layer heights given a certain
> nozzle orifice size as long as they don't exceed a
> certain maximum for that size orifice. So is the
> width/height ratio negotiable also? Is there a
> maximum you don't want to exceed?

There is a minimum width / height ratio because if it is much less than 1.5 you aren't squashing the round filament into ovals, so you have cylinders touching tangentially forming only weak bonds.

For a given nozzle and plastic there is:
a maximum layer height (because the plastic has to be stretched lengthwise, not compressed).
a minimum width (because there is a limit to how much you can stretch it before it snaps).
a maximum width equal to the size of the flat tip of the nozzle.

You can change height and width within these constraints.

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[www.hydraraptor.blogspot.com]