This is a new one. I have a Robo3D and after several hours finally got the E3D head installed by following the assembly instructions on the E3D documentation website. Everything was assembled, and I changed the Marlin software for a maximum temperature of 300, and changed the value for the thermsistor to 5. It took quite a while to heat up, which I just attributed to the fan running continuously as the instructions say to basically wire it into the mains. Unfortunately, by the time the extruder was reading about 260C, the entire thing melted down. There was no short in the thermsistor. It was inserted appropriately into the heat block. So basically, the heatsink failed with the fan running. The head melted itself. The thermsistor was (presumably) bad. It also melted part of my X-carriage, but I can ask Robo3D to print one of those and send it to me. So, needless to say, I'm not too happy. There is no way that the thing got that hot with the head (or more accurately, the thermsistor) functioning appropriately.