I also remember that the first batch of IRLB8743 I bought had a much higher ON impedance than it should be according to the specs - so they got much too hot. The second batch was ok and the MOSFETs kept cool. You can measure the voltage drop over the MOSFET while heating between the "-" of the heatbed and the GND from the power supply. For 10A current it should not exceed 50mV, which would mean about only 0.5W heat dissipation...
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