Thanks @gr5! Here is the slice view. The source of this print is https://www.thingiverse.com/thing:27233 (I'm just playing around with fun things at this point!) I'll try lowering the temperature, and also observe whether the fan is at 100% (is there an easy way to do the latter?)
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gr5 2,172
Short answer - cooler. More fan, lower temps. But first double check that cura sliced it the way you wanted in cura slice view. It might be that it's actually sliced like this for some reason (bad cad, strange cura setting).
Notice how the base of the part also curves inward and then recovers and curves back out.
This is caused because liquid PLA is like a mucus - like snot - when liquid. It sticks to itself nicely when still liquid (unlike many other materials such as PVA). So as it comes out of the nozzle it's already (in milliseconds) cooled a bit and shrunk a bit and is like a liquid rubber band and as it is placed on the bottom most layer - glass - it sticks where it is supposed to but the next layer up is pulling inwards so for a few layers it pulls inward - but as you get farther from that hot heated bed it recovers.
Lowering the temp of the heated bed will improve that lower area but then your part might not stick as well so for a small part like this you could probably just lower the heated bed from 60C (typical default for PLA) to 50C. That will help the lower area and possibly the upper area.
For the upper area you should be able to lower your nozzle temp another 10C. Whatever you are printing now try 5C to 10C cooler. Also it is CRITICAL that the fan be at 100% by the time it gets to the arrowed region in your photo. That could be related to the problem or the entire problem.
Or it might be sliced this way - I would love to see it in slice view.
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