Heat Transfer of a Plate in Air

[rjsfocus]

I am setting up a system where a rectangular plate is heated by a set of heater cartridges, producing about 4000 watts. This plate is in free convection air. It is used to melt plastic at regular invervals. I am trying to model the system so we can estimate temperatures of the plate to control the system better, by using Simulink. I am having trouble linking several equations together though. I can figure out the temperature and time it takes to reach the temperature, in a vacuum at least, through Q=CM(Tf-Ti), and q=Q/t. The heaters are heated like a convection oven, electricity is either supplied or stopped, based on the temperature of the plate.
What I am looking to do is find the temperature of the plate during a cycle, such as when temperature is roughly steady state, but is then decreased when energy is transferred to the plastic, and to air as well if I can. Does anyone have any idea how to calculate the temperature drop when it hits the plastic, and then how long it takes for the temperature to increase? I feel it is similar to say a fridge, it runs at a constant temperature until an item is put into it. The temperature drops and then rebounds.
Thanks for any help that can be provided.
Rich

 

[SteamKing]

If you put an object in a refrigerator and the object is hotter than the interior of the fridge, then heat will transfer from the object to the interior of the fridge and raise the temperature. Once the increased temperature goes past the thermostat set point, then the refrigeration unit starts to remove the excess heat from the interior until the temperature drops below the thermostat set point. If the object is colder, there is no excess heat to remove.

 

[rjsfocus]

Is there a way to calculate both temperatures? I plan on modeling in Simulink but I really don't know what equations to start with.