What’s the issue?
Excessive bed heat is the culprit in this case. As the plastic is extruded it behaves similarly to a rubber band. Normally this effect is held back by the previous layers in a print. As a fresh line of plastic is laid down it bonds to the last layer and is held in place until it fully cools down below the glass transition temperature (where the plastic becomes solid). With a very hotbed, the plastic is held above this temperature and is still malleable. As new layers of plastic is put down on top of this semi-solid mass of plastic the shrinking forces cause the object to shrink. This continues until the print reaches a height where the heat from the bed no longer keeps the object above this temperature and each layer becomes solid before the next layer is put down thus keeping everything in place.
Possible causes
—Heated Bed Temperature Too High
—Insufficient Cooling
Troubleshooting tips
—Heated bed temperature too high
For PLA you will want to keep the bed temperature at around 50-60 °C which is a nice temperature to keep bed adhesion while not being too hot. By default, the bed temperature is set to 75 °C which is definitely too much for PLA. There is an exception to this, however. If you’re printing objects with a very large footprint taking up most of the bed it might be necessary to use a higher bed temperature to make sure the corners do not lift.
—Insufficient Cooling
In addition to lowering your bed temperature, if you want your fans to come on early to help cool down the layers as fast as possible. You can change this in the expert settings of Cura: Expert -> Open Expert Settings… In the window that opens you will find a section dedicated to cooling. Try setting the Fan complete on at a height of 1mm so that the fans come on nice early.
If you are printing a very small part these steps might not be enough. The layers might simply not have enough time to cool properly before the next layer is put down. To help with this you can print two copies of your object at once so that the print head will alternate between the two copies giving each more time to cool.
