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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