US Patent 7028431 Fire-resistant door

A fire-resistant, aluminum, cementitious-material-free, insulation-free door adapted to prevent the spread of fire and heat passing therethrough, consists of: a door frame, a door hingedly mounted on the door frame, the door having a bottom wall, a top wall, and side walls, the bottom wall, top wall and side walls enclosing a hollow central core not containing substantial amounts of insulating material, the bottom wall having an outside surface, and the top wall having an outside surface; and a layer of intumescent material on the outside surface of the bottom wall. A heat-activated self-closing mechanism allows the weight of the door to close the door in the vent of fire, by releasing gas from a supporting gas spring.