An adjustable head restraint assembly for a vehicle seat is provided with a post for supporting a head restraint for movement along the post. This embodiment comprises a housing, a primary heating zone located in the housing and comprising one or more heating chambers. The air knife also cools down the boundary layer of the substrate so that, as the thermoplastic coating is applied, this cooled-down and gas-evacuated boundary layer provides a measure of insulation between the coating and any gasesvolatiles which have not been evacuated from the substrate.