I'm a beginner. I've a Printrbot Play with a heated bed add-on. I'm using it exclusively with PLA.
It worked great initially, but then I took the nozzle out and put it back in, and the Z calibration was lost (and I didn't know the calibration was a thing). As a result, I started having issues with the piece warping up and not sticking to the bed in the corners and around the edge, when the piece was large and flat.
I've installed the heated bed. I used Kapton tape. All these made it perhaps a little better.
Finally I started tweaking the Z axis calibration (the fine adjustment for the initial distance between nozzle and bed) and that made it perfect. No warping anymore.
But now I have the opposite problem. When printing pieces with large flat surfaces that are siting flush with the bed, it's next to impossible to tear it off at the end.
I went back to using plain blue tape instead of Kapton, but it's still sticking too well to the bottom of the piece. I tried to pry it off by pushing a knife under the piece, but that has put a few scratches on the bed. Obviously I don't want to continue doing that.
What can I do? How to prevent warping, while also making sure the piece does not stick too hard to whatever is underneath?
EDIT: The accepted answer was very good and I don't want to mess with it by adding my own "answer". So here it is:
I ended up adding a glass plate on top of the bed, with a heat conductive layer between bed and plate (it's a funny looking, rubbery, chewing-gum-y material that conducts heat). Now I just apply glue stick on the glass and print. Works great. Large pieces come off on their own sometimes if I let them cool down to 30 C or lower.
I had some issues with the Z calibration sensor due to the plate, eventually got solved, but that would take too long to explain here.
I have only experienced PLA "super sticking" if the print bed is allowed to cool too much. I would suggest running a mild preheat to your bed when you find a print has stuck and the bed is cool. (What you set the temp to for the preheat would depend on the filament in question but start low and work up and you may find a sweet spot/release point for your particular PLA)
I have never encountered PLA "super sticking" printed with rafts on (regardless of printbed temperature).
Rafts will give you a sacrifical grid of PLA between your actual finished piece and the bed itself. That way you can "pry" the printed object from the bed and have more leaway for increased leverage without damage to your actual piece(use a plastic razor blade common in cell/tablet screen replacement kits and you generally won't scratch your printbed/printbed coating/tape). The lattice/net of the raft would also, generally, have less, in contact with the printbed surface area than a finished piece and thus have less holding power.