The 12'' Polished Silver 40-80mm All Aluminium Fire Rated Letterplate is lined with special intumescent material which expands to 50 times its size, blocking the hole in the cavity and preventing smoke coming through the cavity.The Fire Resistant Anti Vandal 300mm all aluminium 12 inch letterplate comes with both an external and internal flap to protect your customers homes, and is available in 40-80mm depths for composite and wooden doors.