Interlaminar Failure
Layer separation inside folding boxboard occurs when internal bonding forces fail under mechanical stress during high speed packaging conversion. Mechanical strength along the z axis depends heavily on refining levels achieved during stock preparation on the paper machine. Insufficient fibre entanglement across middle plies creates a weak plane susceptible to rupture when carton blanks undergo sharp creasing and subsequent gluing.
Finished cartons experience structural compromise if internal bonding values drop below specific threshold limits established for heavy creasing operations.
Ply Adhesion
Refining energy applied separately to individual stock furnishes determines internal bond strength through effective fibre bonding per unit area. Modern multi cylinder paper machines combine recycled secondary fibres with bleached chemical pulps while maintaining strict control over couch pit consistency. Starch addition at the size press penetrates upper surfaces but rarely reaches inner structural plies where weak points originate.
Converters measure this internal resistance using standardized internal bond testing apparatus which records energy absorption values during abrupt impact separation.
Structural Integrity
Die cutting operations exert severe shear forces along crease lines where folding boxboard delamination typically manifests under inadequate moisture conditions. Rapid bending moments force inner plies apart if converting machinery operates outside optimal temperature and humidity parameters for the specific carton grade. Finished folding boxboard packaging relies entirely on balanced ply construction to withstand the rigors of automated filling lines without interior structural failure.
