Heated bed - Second attempt

I finally managed to get my hands on a piece of Dibond material (curtsey of CraigRK). A quick calculation around the number of nichrome wires I need, the length of each wire and the capabilities of the power supply at hand lead to 4 wires just shy of the overall length of the bed. The wires were stuck on to the Dibond by means of kapton tape. I also taped a thermistor to the top side of the Dibond to measure the surface temperature. With everything powered on I eagerly awaited the temperature to rise and settle before taking a reading. Maximum temperature reached was 65C at the edges with 68 at the center.... definitely not enough!

I still decided to give it a go, but although initially the ABS was sticking, this was only for a while. When the reprap started the in-fill, the print detached from the bed and was running all over the place.

Back to the drawing board.

I needed something that could generate a decent amount of heat (around 100C to 120C) and after some though I had an eureka moment..... An iron should do the trick! I had a used iron somewhere in my pile of junk which wasn't performing well when using the steam function and we had bought another one quite some time ago, but I never threw the old one away.

I quickly dismantled it and mounted the cast aluminium part that contains the element upside-down under the Dibond sheet. I even left the thermostat on it to control the heat. I raised the heat slowly, each time monitoring the temperature but when the temperature got to around 100C I could see that the top layer of aluminium in the Dibond sheet had warped quite badly. There was a difference of 3mm to 4mm from the center to the edge. I left the Dibond sheet to cool down but the warping was still evident although less then when it was hot.

Dibond sheet was binned. In hindsight the Dibond sheet was never designed to operate at the temperatures I needed and might have been  good for HDPE at temperatures of around 65C

There was also another concern with the bed reaching such high temperatures. The bolts holding the bed where attached to a repraped part. If these exceed the 90C mark, the pressure on the part from the two nuts and washers was going to deform this part and my bed wouldn't be flat and aligned any more. With some quick thinking and a look around my workshop, I came up with what I believe is an ingenious solution ;-)

I stacked a bunch of washers and penny washers alternatively on to the 4 bolts creating a heat-sink that dissipates enough heat to keep the lower part of the bolt at less than 55C.

I'm going to have to get very creative if I want to resolve the heated bed issue on a shoe string budget.