I also did a 12+ hour print over the weekend, but it didn't turn out quite as well: The filament stopped extruding about two hours in, so I only have the bottom centimeter of the print. Fortunately, that's exactly the part I've been having trouble with, and it came out very nice- no tearing (spiraling ridge/trough errors), and bumps are mostly limited to within ~10deg of the south pole.
Two things I changed over the last print were the Perimeter speed from typical speed of 16-4 and bumped it up to 26-12...
Very odd- I set the speed
slower than I had it previously, so it should have been 3 for the outermost loop. The other major changes from the globe with significant bottom-side tearing were:
- 5 loops, instead of 3
- No minimum thickness (though I don't think it makes much difference with that many loops printed for a near-sphere).
- Support only on slopes above 75deg.
- 0.75mm support gap (0.25 ended up fusing too much support to the model. I may try 0.5mm again to see if it reduces the few remaining droopy bumps, or if it re-introduces the tearing (or does nothing).
- Wipe distance = 0. This neatly removed the second line of indentations.
My theory for what's going on with with the tearing error is that PLA stretches out and thins when it bridges a gap, and then deposits an over-large lump of material when it reaches the far side of a gap (I've observed this on the first post-raft layers of numerous prints). For some reason, if the lower layer on the far side of the gap is not continuous for very long (or perhaps if it is too far from the new layer to really bond properly, as is deliberately the case for rafts and support material), the thin-bridging effect starts immediately after the thicker blob gets deposited, and you end up with a series of alternating thick and thin parts of the outer loops which appear to spiral upward in the opposite direction for the extruder's path of travel (as demonstrated beautifully by Fluttershy's hair and tail from
a few posts ago).
It should be possible to compensate for this on the raft layer by slightly increasing the flow rate for the first post-raft layer, at the cost of increasing the bonding between the raft and print. I'm not sure what can be done for layers supported by support structures. Reducing the number of contact points seemed to work for me, perhaps by causing the entire outer surface of the globe to droop slightly and bond to the lower layer, rather than the support material, so there was no bridging. I'm not sure how this would work for complicated geometries- I can see some significant (1/2 layer thickness) drooping on a few crater rims near the pole in my print.
... Just as a random thought, I'm wondering what would happen to this pattern if the extruder travel direction reversed every layer. I may need to do some exciting scripting...