This isn’t going to be accurate, it’s ignoring a key aspect of the heat that will be generated, friction. When designing materials for prosthetics we have to be aware of how much friction occurs between the material and skin. If the amount of friction is too great, the material can create enough heat to damage tissue.
The formula for the skin friction coefficient is cf=τw12ρeue2, where ρe and ue are the density and longitudinal velocity at the boundary layer’s edge.
Depends on the plastic, you can safely heat most polypropylene and polyethylene based plastics. If it’s putting off noxious fumes then it probably has urethane, styrene, or vinyl in it.
The worst plastic to overheat that I’ve worked with is kydex. Even though it’s most common application is as a thermoplastic, if you over heat it the stuff off gasses hydrogen chloride.