I'm simulating a heat staking operation on a very small plastic part, so am simulating the heat staking tool as a solid body with a hot surface.

I need to simulate not only the heat flowing into the plastic part (that locally melts during the heat staking), but then continue the simulation with the hot tool removed, and then evaluate how the heat dissipates into the plastic part.  (We have a biological sample mounted in the plastic part, which will be damaged if the plastic gets too hot.)

I'm thinking that I could either to a time based study with a time based thermal resistance, where the thermal resistance between the tool and the plastic part goes to infinity once the heat staking is done, but that doesn't make the contact surface available for natural convection cooling.

Any suggestions on how to simulate this?