the only solution would be a heated bed.
I have tried several approaches for the proper selection of the print surface. I’m actually printing on the BuildTak foils or on a polyimide foil (e.g. Kapton), depending on the materials I use (using both side of the glass plate). Using the same material the adhesion on the printer plate can be different. I tried some times ago to print an area of 5 *12 cm² and got a lift-off. On the polyimide foil all edges got up, on the Build-Tak only one edge came up, but I was not able to remove this part without damage because in the center of the part it was sticking too much.
So my conclusion is: you can print small parts up to approx. 5cm*5cm without major problems like edge lift off. For larger areas the internal stress will become too high, so the warping/lift off will occur. This is just a rule of thumb: for softer materials the area can be larger, for more stiffer materials you are more critical.
The effect of the heat bed is to maintain a large amount of the material near the heat bed junction at a temperature above the glassification point to allow the relaxation of this internal stress.
So for me one of the next tasks will be to implement a heated bed. Marlark has offered some times ago a custom size heat bed http://forum.velleman.eu/viewtopic.php?f=64&t=14714&hilit=heat+bed, but it seems it is sold out. So I don’t know how to continue on this topic yet. For me a heatbed must replace the glass plate and shall allow the usage of both sides since I will maintain my approach of using two different print surfaces. I would be happy to find an easy solution.
And if you find a trick or any idea to overcome the described weakness please let me know.