What's wrong with this print?  

Re: What's wrong with this print?

If PLA: it looks like the vanes are warping and lifting off the bed as you are printing,. Increase the bed temp to 70c and try again.

If they still lift off, a light acetone wipe down might be needed to restore adhesion.

Re: What's wrong with this print?

Thanks for taking the time to reply. I just realized the photo I attached is very misleading! I detached the half-printed model from the bed and turned it over to get a better shot. My bad.

Anyway, I resliced the model in Slic3r rather than in S3D and it is printing fine now. So it must be something in my S3D settings. I will have to compare the two and see what's different.

Re: What's wrong with this print?

In that case - FAILED --- lol.

Did you have the Use Extra Perimeters when Needed box checked? Also, the fins look to be a fair bit more than 45 degrees; maybe even more than 60 degrees. When I've tried higher angles, I am impressed with how good the Mk3 is, but it can depend on brand of filament. Some simply work better with larger overhangs. At some point, supports are necessary.

That said...
Is this the same brand and color filament as the prior builds?
Are you sure you are using the same settings?

Slowing the print speed and lowering nozzle temp are two common suggestions for making overhangs work without supports.

Other than that, I'm stumped. There seems to be something weird happening as the printer extrudes the narrow vanes... almost seems directional on the convex side, but nothing jumps out as a reason for it. Fan blowing too hard? Extruding more than 0.4mm?

Good that you got it printing.

Re: What's wrong with this print?

After the failed print yesterday, I sliced it in Slic3r using the Prusa preset for Generic PET and got a perfect print. Same filament.

Simplify3D (S3D) has different types of settings so it's difficult to be sure the settings were exactly the same, but I think it's clear that they were not. In both slicers I sliced in Vase Mode. I couldn't find a setting similar to "Extra perimeters when needed" in S3D, but in Slic3r I have it disabled.

I like Slic3r. It's simple and generates nicely printable gcodes, but it keeps crashing on me and I have to reinstall often.
I was hoping Simplify3D would be a better option, but I'm having second thoughts... I think it may be too advanced for me.

Re: What's wrong with this print?

Very nice looking Vase ...

Slic3r hasn't really crashed on me except when I feed it bad food. That old garbage in - garbage out dilemma. If I feed it well, 1 shell, no errors, it works quite well. If I feed it 25 shells, 10,000 errors, it usually burps, but keeps going. If I feed it 100 shells, a bunch of unterminated vertices, if barfs all over me.