Mechanical Separation
Structural failures occur when the internal bonds between the layers of a multi-ply board or the fibers within a single sheet are broken. Often appearing during the printing or converting process, fiber delamination results in the peeling apart of the material into two or more distinct layers. This indicates that the Z-directional tensile strength was insufficient for the applied load.
Stress Response
High-tack inks or rapid changes in tension on a printing press pull on the surface of the board. If the internal bond strength is lower than the force of the ink tack, fiber delamination will occur. The moisture content of the board and the degree of refining of the pulp heavily influence this resistance.
Quality Consequence
Separated layers cause the board to lose its structural integrity and fail to protect the contents of the package. In high-speed folding operations, the weakened material may buckle or tear, leading to machine jams and wasted stock. The visual appearance of the printed surface is also ruined when the top layer peels away.
Manufacturers test for this by measuring the Scott Bond value. Proper starch application and pressing on the paper machine help to prevent these failures. It is most common in boards with recycled content where fiber lengths are shorter.
Maintaining correct humidity during storage reduces the risk of layer separation.
