Dynamic Z Direction Tensile Stress Propagation in Recycled Fibers during High Speed Creasing
High speed creasing generates Z-direction tensile stress waves that fracture short recycled fibers unless matrix clearance absorbs elastic strain energy.
Acoustic Wave Speed
Elastic Velocity
Velocity of high-frequency ultrasonic pulses through a paperboard sheet measures the internal fiber bonding and elastic stiffness of the material. Acoustic wave speed corresponds directly to the elastic modulus divided by material density. Instruments track the transit time across a known span of board to determine this value.
Board Density
Fiber orientation and refining intensity dictate the rate of sound propagation through the fibrous web. Elevated refining increases inter-fiber bonding, which raises the elastic modulus of the sheet and speeds up the sonic pulse. Conversely, a bulky sheet with low density and high air volume slows the wave down because the acoustic path becomes discontinuous.
Calendering can alter the density without changing the elastic modulus, which affects the measurement in predictable ways. By evaluating the ratio of velocities in different directions, operators determine the mechanical anisotropy of the board, which relates directly to how the sheet behaves under tension on printing presses.
Stiffness Analysis
Process control in modern mills relies on continuous ultrasonic scanners to monitor board stiffness without destroying the web. Real-time feedback from these sensors allows operators to adjust the refining intensity or jet-to-wire ratio during production. This automated tracking keeps the board within the specified stiffness range.