The method according to claim 20 used to make bunstock wherein: 192 to 500 PBW said isocyanate to is introduced to the screw of the extruder; and 10 to 201 PBW said other ingredients with less than 2% by weight of non-encapsulated blowing agents are introduced to the screw of the extruder of which 10 to 85 PBW are fire retardants, 1 to 5 parts is a dispersing agent and 1 to 5 parts is a surfactant