Tensile Strain Measurement in Recycled Paperboard Grades
Recycled paperboard strain measurement requires direct optical extensometry under ISO 1924-3 to isolate true sheet elongation from clamp slippage.
Drying Shrinkage Restraint
Mechanical Restriction
Paper machine dryer section physical holding forces control cross-direction sheet dimensional stability during the evaporation process. Cross-machine drying shrinkage restraint maintains web width while setting residual internal stresses within the fiber network. When paper web moisture drops from sixty percent to under eight percent, unconstrained cellulose fibers contract laterally and reduce total sheet yield.
Applying mechanical tension through dryer fabrics limits this natural contraction, shifting sheet behavior toward higher tensile stiffness and reduced ultimate elongation. This boundary holds across all fourdrinier and cylinder machine configurations where mechanical contact supports the web.
Tensile Consequence
Drying cylinders press the wet sheet against high-tension synthetic fabrics to suppress differential cross-machine shrinkage. Mechanical restriction along the edge zones typically proves weaker than in the center, creating a structural profile variance in hygroexpansivity across the reel. Paper converters experience this dimensional gradient as web curl or edge flutter during high-speed printing and laminating operations.
Increasing fabric tension from two kilonewtons per meter to four kilonewtons per meter reduces cross-direction shrinkage, elevating tensile energy absorption across the central sheet section. Higher restraint values alter fiber-to-fiber bond geometry, locking individual fibers into extended orientations before hydrogen bonds fully set. Consequently, register control improves during multi-pass offset printing, whereas sheet ductility drops predictably.
Operational Boundary
Over-restraining the web generates excessive micro-fractures in fiber-to-fiber bonds whenever local moisture gradients spike. Exceeding critical draw thresholds causes web breaks in the transfer section rather than controlled elongation.