My standard pic to show the effect of a dummy cooling tower (ruler is in mm and cm).
In addition to what gr5 said, printing in thin layers, slow and as cool as possible also helps. Also, do not use 100% infill but rather 50% or less. But I could never totally get rid of the effect. At some point the hot nozzle just keeps sitting on top of the tiny model, preventing it from cooling down and solidifying. That is a limitation of this printing method.
It's not getting enough time to cool. I usually fix this by printing 3 or 5 of the object at the same time. Right click on the part and select multiply. If your fans are on the side then position the parts in a row left to right. If your cooling fans are in the front or back then position the parts on a line front to back.
Alternatively you could print a small tower on either side of this print (that is at least as tall).
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