Its finally here, heatbed is tested and ready to make your prints flat as pancake. It is a long story of testing and developing to make it really working and long lasting, you can check some background info in previous version here http://www.thingiverse.com/thing:2172 .
Active heated area is 20x20cm big and overall size is 214x214mm, its compatible with Mendel and all kinds of other RepStraps, just check if you can fit it on your bed. Its double-sided, so its more rigid and will not damage the heating element if you set wrong Z offset (thanks Joaz for this idea).
Its powered by 12V, prototype draws 9.9A when cold which goes down to 7.5A when hot. With isolation on the bottom of this heater, it reaches 100Â°C in about ~4,5 minutes, measured on the side of the heat field. These ratings may be slightly different on each PCB. It can be easily converted to use 24V or 48V.
I know guys from RepRapsSource and convinced them to fabricate these beds, so you can buy professionally fabricated version at http://reprapsource.com/en/show/6403 and trust me, I have had troubles etching first prototype which was only 1/4th of size so its worth to buy it, its made in Germany..
If you want to etch it by yourself, you need 35um copper layer and be sure to have all traces perfectly same, if some place is thinner it will generate more heat and burn the board. Also, you have to cover it before heating otherwise copper will corrode really quick.
You have basically two options. Bed has 3mm holes in corners so you can easily mount it with few screws, standoffs and some insulation will help with quicker heating. Second option is to use magnets as I did here http://prusadjs.cz/2010/04/heatbed-mk4-we-are-almost-there-simple-cheap-hot-swap/. In both ways I strongly recommend to use plywood or something similar as base, acrylic will warp.
Preparation for printing
You just need tope some kapton/pet tape on the printing side.