Warping/lifting on a single and multi part printing

I have noticed that the more of the identical parts I try to print at the same time the more warping/corner lifting I experience. I can print two of the same parts without any issues but if try to print 4 of these parts the corners start to lift. My theory is that parts simply have too much time to cool down before the next layer starts.
I have experienced this with different materials and both using and not using heated bed.
What are your experiences on trying to print multiple parts on a single bed?

Edited 1 time(s). Last edit at 10/01/2015 02:18PM by Edvardas.

I just finished a 5 hour print of 6 Halloween pumpkin 'things' for the 'boss' using PLA and a heated bed with tempered glass and hair spray. No problem. That was after printing 8 goblins for aforementioned 'boss'. No problem and same application of hairspray lasted for both of them plus several prints last night.
If you're one of the masochists using ABS, I can't help you. Which is it?

Maybe Nylon ?

Nylon warps bad enough while printing just a single part Would not try to do multiple of those.
I am talking about ABS and PETG. ABS of course is worse.
I think I could get them to stick to the printbed without lifting but removing them would require too much cursing, blood and damaged stuff.

I can remember photos of printbeds that are full of reprap printer parts. So I guess it is possible. But I wonder what was required to remove those parts.

Printing multiple parts requires a levelled bed. Before I had my Prusa i3 levelled, I only had one corner where the first layer was sticking.

Also a brim to keep the edges down and a temp difference between print and remove. I print PLA at 45°C on painter tape. When I wait until the bed is cool, a fingersnip removes the parts.
-Olaf

Edited 1 time(s). Last edit at 10/02/2015 11:52AM by o_lampe.

I use this stuff, its cheap and worked really well as advertised, I tried all 3 material they listed and all worked out perfect, 8 euro a pack goes a long way, I used just little under half the pack and I got 40+ prints out of it so far, with the biggest part being 185mm x 185mm x 185mm in size, not exactly a square box but it was that big, start to finish not a single lift

can't find link I bought it from, but same shop relisted it

printing paste