Machine Direction Fiber Orientation Mechanics in Packaging Board
Jet-to-wire velocity ratios govern machine-direction fiber orientation, setting board anisotropy ratios that dictate box compression and score crack resistance.
Cross Machine Shrinkage
Transverse Contraction
Evaporative drying along paper machine dryer sections induces dimensional contraction as hydrogen bonds draw adjacent cellulose fibres together within the consolidation web. During web manufacturing, cross machine shrinkage measures the percentage reduction in sheet width occurring from the wet press section to the reel. TAPPI T 439 describes testing protocols for evaluating dimensional stability in paper networks.
Mechanical restraint applied by dryer fabrics limits longitudinal shrinkage while permitting substantial transverse movement at sheet edges. Edge zones contract up to three times more than the web center due to unconstrained shrinkage forces operating near deckle edges. This differential contraction alters cross-deckle basis weight and caliper profiles across the finished roll width.
Web Distortion
High moisture variation across converting reels triggers localized web distortion when paperboard absorbs atmospheric humidity. Excessive cross machine shrinkage induces latent internal stress that causes sheet curl or diagonal twist during sheet-fed offset printing. Printers encounter registration errors on multi-color press runs when dimensional movement alters sheet dimensions between printing units.
Adjusting pocket ventilation systems and dryer felt tensions flattens the shrinkage profile across the machine deckle.
Strain Limit
Evaluations stop applying when paper webs experience full re-wetting and mechanical elongation during aqueous coating operations. Saturated paperboard networks lose original bonding geometry, replacing drying shrinkage history with wet expansion dynamics.