Layer Separation
Structural failure occurs when the forces holding the individual plies or fibre networks together are overcome by external stress. Internal bond delamination describes the splitting of a board into layers, often during the printing or converting process. This defect is measured using techniques such as the Scott Bond test, which quantifies the energy required to pull the material apart.
Mechanical Cause
High tack inks or aggressive adhesive application can exert enough vertical force to initiate a rupture in the middle of the sheet. If the moisture content of the board is too high, the hydrogen bonds between fibres are weakened, which makes internal bond delamination more likely. Excessive calendering can also crush the bulk of the material and damage the interlocking structure that provides cohesive strength.
In multi-ply grades, the interface between different types of pulp is often the weakest point where separation begins.
Strength Management
Refining the pulp to a higher degree increases the number of available bonding sites and improves the resistance to splitting. Chemical additives like starch or synthetic resins are often sprayed between layers to reinforce the connection and prevent internal bond delamination. Mills must balance these strength requirements against the need for stiffness and bulk, as more bonding usually leads to a denser sheet.
Regular testing ensures that the material can withstand the high speed peeling forces found in modern packaging lines.
