The lining of flannel inside a hollow board typically refers to the placement of a layer of flannel material on the interior or surface of the hollow board (also known as hollow board material). This is done to enhance functions such as insulation, sound absorption, protection, and decoration. The flannel lining is usually made from various types of flannel materials, such as flannel fabric, fleece, or flannel textiles. Different thicknesses and densities of flannel can be selected to meet specific requirements.