If you are having problems with the heated bed and aluminum plate I have a solution. I built a Mega Prusa with an aluminum bed and silicone heater attached to the bottom. The heater is 110 AC voltage and I use a cheap solid state relay wired into the ramps and plugged into a power strip. It works flawlessly. My bed is about 18 X 18 inches and I think the silicone heater is about 17 X 17 inches. You can see the pic of the printer attached. Let me know if you have questions about the set up.
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