Directional Property
Fiber orientation in machine-made paperboard creates distinct time-dependent mechanical behaviors along the three principal axes of the sheet. In paper physics, orthotropic viscoelasticity describes how the creep rate and stress relaxation vary between the machine direction and the cross direction. This directional dependency arises because more fibers align with the direction of the paper machine belt.
It dictates how the material deforms over time.
Deformational Response
Stress applied in the cross direction leads to faster deformation and higher creep because fewer fibers are aligned to resist the force. Under constant loads, orthotropic viscoelasticity causes the board to stretch more along the weaker axis, which alters the balance of the container. This deformation increases when temperature or humidity fluctuates.
It triggers early failure of the paper structure.
Package Integrity
Container design must compensate for these directional differences by orienting the strongest axis of the board to resist the primary stacking forces. Since the machine direction has the highest resistance to viscoelastic creep, carton blanks are cut so that this direction runs horizontally around the box, which provides maximum resistance to bulging. This specific orientation keeps the vertical corners of the carton straight and prevents the structure from bowing outward under heavy long-term loads in the warehouse.
