Internal Resistance
Resistance of paperboard to internal sliding forces determines the structural integrity of the sheet during creasing and folding operations. Fiber shear occurs when parallel forces act in opposite directions within the plane of the material, causing the individual fibres and bond sites to slide past each other. This reaction is fundamental to the localized delamination needed to make neat, crack-free folds.
The shear strength of the sheet must be balanced to allow this internal separation while preventing total board split.
Deformation Mechanism
Moisture and refining are the primary variables that determine how the fibrous network yields under shear forces. Heavy refining increases the bonding area between fibres, raising the shear resistance and making the board stiffer. If the fibre network is too dry, it becomes brittle, and the shear force will result in a clean break rather than the desired ply separation.
Spraying or conditioning the board helps lower the resistance to shear during converting.
Converting Impact
Low shear resistance leads to premature failures such as flaking or peeling during die-cutting. Press settings must be adjusted to match the shear limit of the specific board grade. The sheet must hold together until the fold is made.
