Large flat ABS print curling during cooldown
Short Answer: Make the environment temperature is a bit warmer to offset the cooling rate.

Long Answer

All materials shrink as they cool, but some materials shrink at a much higher rate than others. This is primarily driven by temperature coefficients and thermal conductivity of the individual materials. There are calculators available online that you can get more involved with discovering how this works mathematically.

However, in layman's terms, the temperature between your build plate and the first layers of plastic are so drastically different, that at a certain point the plastic becomes under tension as it shrinks faster/more than the build plate. Over time, since the surface tension between the part and the build plate is no longer in the way, the corner can continue to shrink.

Here are some options:


Ensure your build plate temperatures are correct acceptable. I've had success with 112C in NW United States. Increasing the temperature will be an attempt to bridge the gap between BP temps and plastic temps during the final cool-down.
Search for ways to "anchor" the part to the build plate. There are lots of resources on the internet on how to do this, so I'll spare you the list.
Increase your environment temperature. I highly suggest this because it can help reduce other errors you might encounter throughout the printing process, such as de-lamination, warping, brittleness, etc. I'm not recommending placing a space heater next to your printer (really, don't do that!), but perhaps even a space heater in the room can help regulate the environment temperature a bit better.
Consider other environment variables. Consider things like "Is my printer next to a window?", "Do I live in a particularly cold environment in general?", or even "Am I baking cookies in the next room?". A lot of these seemingly meaningless variables play an important role in ensuring quality prints.


(I know there's more, so I'll add onto this answer a bit more as I remember them)