Bottom corners curl up during printing (warping?)

[jeex]

Hello,

I use the Ultimaker2 which has a heated glass plate at 60 degrees celsius (140 Fahrenheit) as standard setting. As you can see in the image the bottom corners curl up slightly despite the brim i use. Even the brim is pulled up by the object. The filament is PLA.

556b24282e838.JPG

As the object is on the plate for many hours, i think it shrinks like loundry that is overheated. So first question: how can i tell gcode to lower the temp of the glassplate after say 5mm. The object then is heavy enough by itself not to move around. And i can put some "glue" under the object to fixate it.

Or is there another reason why this happens?

Thanks.