Heat bed issues

Hey guys,

I'm currently running Makerfarm's Prusa 12in i3v. I have been using this printer for about two years with no major problems excluding regular maintenance. I recently switched the stock 300mm x 300mm PCB heater plate with a 3mm aluminum 200mm x 300mm plate. I figure I sacrifice a little build space for a sturdier build platform in which I can later install an inductive sensor for auto level features. Anyway, the current set up uses a Rumba board with Marlin. The heater bed output is connected to a relay which switches 12v of power directly from the power supply to the heater bed. Previously, I had no problem reaching temperatures of 120celsius with the stock 300mm x 300mm PCB board and two plates of 1/8in glass. Now, however, this smaller heat plate seems to cap out around 83celsius. This seems odd that a smaller heat plate composed of a very conductive material should struggle to keep up with its larger cousin which heats two panes of glass with no issues. The only change in this system is the heater plate itself. I'm using 14 gauge wire to connect the heater bed to the relay and relay to power supply.

The thermistor reads the heater temperature, not the glass temperature. Glass is an insulator. Since it doesn't conduct the heat away from the heater, the heater gets hotter faster (but the glass doesn't) and gives the illusion of heating quicker. Set the heater to 60C and when you read 60C on the computer, try putting you finger on the print surface. It's probably just a little warm. The thermal resistance of the glass means it is going to take a while to get the glass surface up to the set temperature. The aluminum conducts heat away from the heater transfers it to the air. When the rate of heat transfer out of the aluminum = heat transfer in, the temperature stabilizes. You need more power.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I was hoping that wasn't the problem, but so be it. I've been investigating these silicone heater pads. After watching one of Tom Sanladerer's videos about using silicone heaters, I think this will be my best bet. I figure wiring it up to mains 110v AC with a solid state relay to the RUMBA board I'm using and keep my fingers crossed.