As you can see in a structure picture of a wooden fire rated door, there is an interior layer named crossband adhering to the fire resistant core. Normally, it is plywood or MDF.
And today, we are going to take a look at plywood, a piece of board that seems easy to understand but actually not.
Plywood is an engineered wood product made up of layers of thin sheets of wood veneer that are glued together. It is seen as one member of the family of manufactured boards.
As real wood, it is also a natural insulator, and therefore provide good protection of heat loss. Also with its good fire resistance property, it is a great choice for fire rated wooden doors.
Give a close check at a piece of plywood, you may find what a creative and interesting structure it has.
Firstly, smaller and lower-quality plywood can only separate its layers at right angles to each other. Higher-quality plywood products will be divided into 5 layers in steps of 45 degrees (0, 45, 90, 135, and 180 degrees) to provide strength. This helps to strengthen the whole door leaf structure of the wood fire rated door.
Secondly, plywood always has an odd number of layers, such as 3, 5, 7. So here comes the question: Why are they all single-layered? The purpose of using a single layer of plywood is to make the plywood have a middle core layer, which can make the thin plates on both sides be restrained by the core layer, so that the middle layer is also contained by the outer layer. Therefore, it is always glued together one piece horizontally according to the grain, and one piece is overlapped and glued together so that the thin layers are restrained from each other and are not easy to warp or break.