In the mold, the correct temperature transfer between different materials is of crucial importance. The heat transfer inside the mold depends on a material depended-value, the heat transfer coefficient. The higher it is, the better is the transfer of the thermal energy from one material into the next one.
As the hot runner system should transfer as little heat as possible to the remaining mold, an air gap helps to receive a small heat transfer coefficient, to get an isolated system.
It is necessary to know, that the heat transfer coefficient is no a constant value. It depends on material specific properties, like specific heat capacity, density and thermal conductivity, as well as geometry and roughness of the surface and for moving fluids on the flow behavior.