I recently read Nophead’s most recent article, and it was very interesting.
History
I’ve been working on Volumetric 5D, mainly to fill a need I see, and several people have been either helping me or happened to be thinking along similar lines. We all noticed that when we extruded at what, volumetrically determined, would be ~85% fill of plastic it would yield about the best looking objects.
A couple of people then tried to measure the unwieldy extruded filament and it looked like it was losing volume. I hoped it was wrong.
Nophead said we were wrong
Early on hophead was saying we were wrong. Dave Durant posted a profile-creating program on thingiverse, and in the description Dave mentioned the 15% loss theory. Nophead commented that he felt that that wasn’t the case.
At that point I still wasn’t firm on a theory of what’s going on, but Enrique had already added changes to Skeinforge based on the math that I had been working on with an 85% “packing density.”
Feeling better, bad timing
So now that Skeinforge 40 is out with the new math in it Nophead does some experimenting to disprove that the density loss theory. I welcome the disproval, really, but couldn’t it have been sooner?
Anyway, on IRC Dave and I had just been discussing how the math goes funny with layer heights around .2mm and lower. We were both seeing too much plastic with ABS. We had both come to the conclusion that the “corners” were not filling in — that surface tension or something was keeping the ABS from filling in every void there was.
It wasn’t until Nophead’s blog post that I realized that most or all of the “packing density” we were seeing is because of this. <facepalm>
What’s going on again?
Well, basically the filament is laid down with rounded edges, like icing being piped onto a cake. Then the extruder comes back along one side of that extruded filament for the next pass and is trying to fill in all of the gaps between the two passes.
For PLA — a very pliable and rubbery plastic at melting temperatures — this mostly works. The PLA easily deforms and allows the new filament to fill in the spaces.
ABS hardens so fast and has such viscosity that — as Nophead put it — to fill in all the gaps would be like injection molding. I think it’s slightly worse because we’re dealing with very tiny places to fill and we would require tremendous pressure to fill them. Way more than we can expect from our current range on extruder designs, at least.
I do feel better about this theory. I do wonder if there is a certain minimum radius we can expect the filament to fill in. Knowing that would help us more accurately determine how much volume to output to get as reasonably close to 100% fill as we can.
Headaches
ABS fumes have started to make my head hurt, so I switched back to PLA. However, I already had a Mk6 all geared up for thin filament so I wanted to make that work. I ordered some 1.75mm PLA from MakerGear and started printing right away.
And immediately started to get jams.
I suspected that the PLA was liquifying in the tube because it was overheating. I also blamed the rapid-reversal ooze-prevention I was using, but turning it off didn’t really help. I tried removing some tube to stop thermal transfer inward:
That didn’t work:
So I ordered a MakerGear hot end and stepper setup, and am getting similar results.
But, thinking about this new theory about filling the gaps in: I wonder if it’s simply too much back-pressure in the nozzle from using it as an injection molder?
I hope I can test this tomorrow…
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