What explains the 3d benchy hull lines?

[viscid rose]

Share your thoughts, data, resources and references on this topic.

[woeful iris]

The only real analysis that I have seen on the subject is by Prusa ( https://help.prusa3d.com/article/the-benchy-hull-line_124745 ), but there are two credible theoies (IMHO) why it happens universally:

1. Prusa say, “We believe that the main culprit is the sudden transition from sparse infill into full top layers around the 8 mm height. At this point, there is an abrupt difference in the time a layer takes to print.” Prusa say they have improved the line by splitting the model and slicing to make sure the infill does not reach all the way to the deck on the problematic layer. To me, that “fix” seems unrelated to layer print time.

2. On the other hand, some people believe that it is caused by there suddenly being more material in that layer and it can’t shrink as much as the layers below when the filament cools.

I have seen Benchies with amnabrupt gloss change at that layer which is clearly due to the change in print speed. However, I think the line is most likely caused by the deck pushing out against the hull. Maybe that is due to cooling or maybe it is because of too much infill overlap.

I think this way because the line disappeared once I told my slicer (ideaMaker ) to print outer perimeters before anything else.

I have printed about 200 Benchies using172 filaments. Since I moved to 3 perimeters with the outside one printed first, and infill overlap set to 15%, I rarely see a hull line in PLA, PETG, TPU, or Nylon. One pops up every now and then in filament that has other problems (like Jessie White PLA); it is not 100% foolproof, but well over 90%.

No one believes without pictures, so here are some random Benchies from my collection. Clockwise from top: Filaments.ca Fluorescent Yellow PETG, Zyltech Fluorescent Red PLA, Polyalchemy Elixir Crimson Red PLA, FilamentOne Glint Green PLA, FiberForce Pantone 368 C PLA.

[humble kelp]

Ok, trying to break this down in an understandable way. There are multiple possible sources of shrinkage when printing.
1 is thermal, as the plastic cools, it shrinks.
2 is elastic, that the filament wants to return to it's original shape, that 0.4 mm strand being extruded wanting to return to 1.75 mm. You can see this by purging some filament using your printer controls. Some filaments like some silks and PETGs visibly significantly pull back into a thicker shorter strand than the 0.4 mm that came out.
3 is a result of the motion of the nozzle, it is moving around a curve, pulling the strand inward. You can observe this by the fact that vertical holes in a model end up smaller than they are in the design unless you leave an extra tolerance.
These 3 effects all come together in the same location in this model when printing fast.
The lower layers have an internal void, so 2 and 3 mean it prints slightly smaller in the first place, and you are printing fast, so those lower layers are staying warm and haven't yet fully shrunk due to 1.
Then the deck layer starts. The deck layer is more solid, and so resists 2 and 3. This layer also takes longer, so the lower portion of the model has more time to cool and thermally shrink before the next layer is placed on top. So the lower layers have retracted back just a bit more underneath the nozzle when the deck layer prints than they did when printing the lower layers: effectively bumping the printing out a fraction.
These effects all add up at the same level in this model

[willow bridge]

Hey all, I've been trying to fix it for months without any luck and finally stumbled on this server. Any help would be greatly appreciated.

I've already tried printing really slow, printing outer walls first, adjusting skin overlap, etc but haven't had any luck.

I'm on an Elegoo Neptune 3 Pro and Cura 5.4.

[paper nest]

Ok, I think I will retake the experimentation on this topic, I think black filament is good for seeing this issues? I will get some and start experimentation again

[rapid nebula]

Might post this to the #1044149747621048360 because in the forum many can jump in with suggestions. This specific channel is for the print busters question