After struggling with warping on larger prints, I decided to try adding a heated bed to my Simple. I am delighted with the results. It has completely eliminated the warping problem and allows me to print at full speed.
Here's what I used. As usual, you use information from the internet at your own risk, and if you are not comfortable with things like voltage and current ratings, get help from someone who is. Electricity at the current levels involved here are a modest fire hazard if adequate safeguards are not in place.
1) MakerFarm 4.5x6" Heated Bed with Prewired Thermistor
I could have individually bought the high temp wire and sleeving and assembled it, but the price difference was very small.
2) 3/16"x 4 x 5 Borosilicate Glass from McMaster Carr
Borosilicate glass is less likely to break when warmed and cooled. I figured the thicker stuff would be less likely to break when I pried things off the bed.
3) A solid state relay to control the bed. The Printrboard has an output that can directly drive a heated bed this size, but there are reports that the Molex connector isn't up to the task and can overheat. Additionally, a short on the bed could damage the Printrboard. The Printrboard is an expensive piece of hardware, so I chose to isolate it from the bed.
Update: I added a heat sink to the SSR. It got fairly hot during long prints, even running at 50% of it's rated current capacity. The heat sink helped a great deal.
In this configuration, the relay is the only load driven by the PrintrBoard's heat bed output. The relay has to be driven by the 12V output, and be able to switch enough DC current and voltage to run the bed with a margin of safety. Also be aware that the control outputs on the solid state relay have a polarity that must be observed. There is good information on wiring up the SSR here. Be aware that unlike typical magnetic relays, SSRs have a polarity on both the driven side and the load side.
4) A large 12V power supply I already own. The power supply that comes with the Simple is not sized to run a heated bed. Since I was using a relay it was easy to power the bed from another supply. Some people are using a modified ATX computer supply, or 12V supplies are available in a variety of capacities. MakerFarm says the bed alone can draw up to 8 amps.
5) 4 medium sized binder paper clips to hold the heated bed and glass to the plywood bed.
6) Permatex Ultra Copper high temperature silicone sealant to secure the thermistor to the heat bed.
An interesting note on the relay: I first chose a mechanical relay. It turns out that the Printrboard's firmware is set up to run a fast PID loop which is well suited to run semiconductors. It causes a mechanical relay to chatter several times per second. Changing it to a mode more suitable for a mechanical relay requires recompiling and flashing the Printrboard firmware. That's a lot of work, and carries a bit of risk, so I chose to buy the solid state relay.
The first part of the process was to secure the thermistor to the underside of the heated bed. I placed the thermistor where I wanted it, taped the wires to hold it in place, and then put a blob of Permatex Ultra Copper over it to hold it down. I was careful not to put any Permatex between the thermistor and bed.
Next, I marked the plywood bed of my printer for cutouts so that the heated bed would fit flush on it. That required cutouts for the bed wires and the thermistor. The circular cutout for the thermistor was done with a Forstner bit. The small cutouts for the wires was done with a hobby knife.
I then found that the full width clips limited my print area a bit - the head would smack one when it went home. I used a bench grinder to cut them in half.
I got good results heating the bed to 55C, with blue tape on the glass. Prints come out perfectly flat and are hard to get off the bed. A friend tells me hairspray works well on the heated glass on his Rostock Max and the parts come off the bed more easily, so I intend to try that soon.
I have been extremely pleased with this so far - it resolved my warping issue entirely, allows me to print at normal speeds, and in theory, I should be able to print ABS now. Total cost was around $65, since I had the power supply already.
Update: Prints ABS great at 90-95C. I have switched to ABS pretty much exclusively now.