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Differences in PLA vs. layer evenness

Posted by orcinus 
Re: Differences in PLA vs. layer evenness
October 08, 2012 10:54AM
Yes, that was my original guess as well.
See first post:

I've been thinking what the cause of that correlation might be and so far, my working theory is that the brittle filaments get mangled much more by the hobbed bolt, as they're less elastic (and less plastic). Instead of the teeth digging into the filament and deforming it, chunks of filament break off, clogging the bolt teeth and slipping of the filament during retraction and post-retract extrusion.

I've tried playing with it, but didn't get any consistent results. I've ended up settling with a relatively short retract (0.5) and high retract speed (75). Lowering the retract speed was making things slightly (but not dramatically) worse.

The print with diagonal banding was actually with retract turned off.

I've tried printing two copies of an object (small herringbone gear) to see if i can deduce something from it.
The fill actually changed from one copy to the other - with the extrusion rate tuned in, one would be getting not enough extrusion (gaps between fill lines) and the other would be getting too much (nozzle dragging through solid fill). Confirming it's definitely an issue with flow, not Z.
Re: Differences in PLA vs. layer evenness
October 08, 2012 11:20AM
what slicer are you using, and what firmware?
Re: Differences in PLA vs. layer evenness
October 08, 2012 04:37PM
Tested this with 0.7.2b, 0.8.4 and 0.9.x.
Gave up on 0.9.x pretty fast because it creates artefacts and uneven layers on its own.

The last two or three shots in this thread (the single wall calibration objects and the herringbone gear) were sliced with 0.7.2b.

Edited 1 time(s). Last edit at 10/08/2012 04:37PM by orcinus.
Re: Differences in PLA vs. layer evenness
October 08, 2012 04:38PM
Well that's bizarre. My reply kept getting rejected as spam until i logged out and back in.
Re: Differences in PLA vs. layer evenness
October 08, 2012 06:18PM
If it 'crunches' would the filament drive, which is pushed towards it with a spring, move differently? It is a very small movement, how precisely/cheaply can we measure that movement?
Re: Differences in PLA vs. layer evenness
October 08, 2012 06:52PM
Could it be that the bad filament absorbed more water from the air? Once you break the seal on PLA it "goes bad" pretty quickly in humid climates. You could try baking it in an oven for 4 hours or so and see if it helps. I was able to revitalize some PLA I had that way, but it may have been a different issue than you have.

When it absorbs water you can sometimes hear it pop and fizzle as the water turns to steam if it is really bad, or you may see it come out "cloudy" as the water expands within the heated filament itself. quality of print may be affected well before these visible/audible signs become apparent.

Edited 1 time(s). Last edit at 10/08/2012 06:54PM by John Meacham.
Re: Differences in PLA vs. layer evenness
October 09, 2012 07:02AM
Possible. No hissing and popping nor cloudiness, and i keep the filament stock in a sealed bag.
But i sometimes leave a piece of it (cut to length in preparation for a print) in the open, already threaded into the extruder, overnight or even a few nights.

I'll see if "baking" might help.

@Jasper1984 - there's no need for a spring-action filament drive, as the idler on the other side is already spring-driven against the drive.
Re: Differences in PLA vs. layer evenness
October 09, 2012 08:14AM
i think what we have here is that the air flow is not consistently constant in terms of

1) specific volume of air directed at a very specific area = change in temperature = change in mechanical character of filament
2) the exact specifics of the air humidity/ temperature. which affects its ability to soak up latent heat around it.

to me, when air is fan impelled to direct at something, it is a turbulent wave, it is not a jet. it could be useful if a kind of air scoop is done pre/post fan, to massage the moving air into a more uniform "subject".

what do you guys think?

orcinus Wrote:
> In other news, i found out what has the greatest
> effect on "random" banding depicted here.
> Cooling! Or, rather, fans! And no, i'm not talking
> about not having fans or about a lack of cooling.
> It's the cooling/fans that causes the banding.
> [dl.dropbox.com]
> g
> Left = fans off (which is why the top few layers
> on the side farthest from the camera sagged)
> Right = fans on
> No idea what's going on here. I've got one stupid
> hypothesis - which is that the airflow is too
> strong and is actually blowing the weaker-adhering
> layers away a fraction. And i've got a less
> stupid, but pretty fuzzy hypothesis - which is
> that some filaments react weirdly
> swell/contract-wise when heated and cooled
> quickly.
Re: Differences in PLA vs. layer evenness
January 19, 2013 03:00PM
Hey Orcinus,

Perhaps you've figured this one out already, or have moved onto another hobby entirely, who knows.

Regarding the photo of your single walled print: The horizontal banding went away when you turned the fan off. The high air flow from the fan is cooling the extruded filament enough to cause it to contract slightly, in the short period before the next layer is added. The perimeter cools and the top half shrinks and curls inwards. If you view the extruded filament in cross section then 12 oclock becomes 11 oclock. The next layer is laid down, but due to the intricacies of fluid dynamics - path of least resistance presumably - it is still deposited on 'top' of the previous layer at it's 12 oclock. This next layer is over cooled and curls too, causing your single walled print to 'arch' inwards. This effect is cumulative - each successive layer adds to the the arch. Eventually a new layer finds it easier to lay down on top of the arch, rather than add to it - at 2 oclock say. And the process begins all over again. The result is repeating bands.

So try the same print with different fan speeds. See what you get. I run my fan at 45/255 PWM. I'd run it even slower if friction would permit.

Edited 1 time(s). Last edit at 01/19/2013 03:02PM by Woodsmoke.
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