Deformation Speed
Mechanical deformation per unit of time during high-speed folding or creasing operations determines the structural integrity of a carton blank. The converting strain rate represents the velocity at which tensile or compressive forces are applied to the substrate during its passage through finishing equipment. In a high-speed gluer or folder, paperboard undergoes rapid bending within milliseconds, necessitating a high rate of elongation.
Standard static tensile tests cannot predict the behavior of the fibre matrix under these dynamic conditions.
Board Response
Viscoelastic properties of paper fibres dictate how the material responds to sudden mechanical loads. At a high converting strain rate, the polymeric cellulose chains do not have sufficient time to relax or slide past each other, which increases the apparent stiffness and tensile strength of the board. This behavior makes the substrate more brittle and susceptible to cracking or rupture along the score lines.
Adjusting the fiber composition or applying a fine moisture spray before the crease helps mitigate the stress.
Process Adjustment
Optimising the tooling geometry or machine throughput assists in managing this dynamic property. Operators can decrease the feed rate to reduce the impact velocity, which lowers the strain rate and allows the fibers to deform more gradually. Alternatively, widening the female creasing groove reduces the localized strain by spreading the deformation over a larger area.
This modification lowers the risk of structural failure in the outer plies of the board.
