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.
Fiber Length Shortening
Refining Effect
Mechanical reduction of the longitudinal dimension of papermaking fibers occurs during stock preparation as a consequence of intense shear and cutting action in the refiner. This fiber length shortening reduces the average aspect ratio of the wood pulp. Refining is optimized to fibrillate the fiber wall rather than cut it, because excess shortening impairs the strength properties of the sheet.
Kraft Pulping
Refiner plate geometry and rotational speed control the intensity of the treatment applied to the slurry. When pulp passes through narrow bar clearances, the mechanical impact shears the fibers transversely. Softwood fibers, which are naturally longer and more flexible, are particularly susceptible to this damage during high-consistency treatment.
Hardwood fibers are less affected due to their shorter initial length, though they still experience some trimming when subjected to extreme refiner loads. This mechanical action must be balanced to ensure that the fiber network maintains sufficient tensile strength without suffering from excessive drainage resistance on the wire.
Web Strength
Tear resistance in finished linerboard suffers when the average length of the constituent fibers decreases too much. Shorter fibers have fewer contact points and smaller bonding areas across which to distribute stress. The resulting paper web becomes brittle and prone to breaking during high-speed printing runs.