Prints are now sticking too well to the bed. What to do? 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.
Options to check (in order of probability of occurenece and success):


Z-calibration


Maybe you should perform calibration again. It's possible that nozzle is now too close which causes the filament is spread on the heatbed, which causes that the bottom surface is "too flat". It means all separate filament tracks are spilled and they create "glass flat" surface. You did mention that the model sticks too much even to kapton. It suggests z-calibration.


Temperature calibration


Check if the temperature is not too high. Reducing it a bit can reduce sticking. If filament is too soft then material sticks usually better (causes the same as in #1)


Heatbed cleaning


If your HB is scatched (even not visible scratches) then it's possible that previous printouts left little grains in such scratches. Cleaning HB could then help. Are you using glass? if yes - change glass side to check this option. Eventually replace glass with new one.


Heatbed surface


If your heatbed is scratched then filament can penetrate such scratches and increase sticking.


Filament


Did you change filament vendor? If not then maybe your filament had changed during a time/humidity/sun/cigarete smoke. This is very doubtful option but who knows.

What methods you can use to detach model 


Paper knife


I also have the same problem when I use paper glue stick. Some vendors produce such sticky glue that I affraid to break a glass (which I use of course). When this happens I use a paper knife. Glass is scratchproof enough. Be careful - if your model suddenly detaches releasing knife... just be careful. Unmount HB or unmount glass first of course.


Hammer


I know people use hammer to detach sticked model. I would suggest this method only for big and relatively simple elements. Especially for solids (fully filled with the material, without any grid/honeycomb inner structure). One short hit in the same surface as the HB. Be careful of course and unmount HB or unmount glass first.


Fridge


As HB material and filament have most likely different thermal shrinkage factor it might help. And guess what - unmount HB/glass first ;)