Fibre Density
Chemimechanical pulping produces a central fibrous matrix where high yield comes from softened lignin holding cellulose elements together rather than absolute chemical removal. Mechanical action during the refining stage separates the wood chips after mild chemical impregnation, yielding stiff and bulky fibres that define ctmp core within multi-layer board constructions. High stiffness relative to basis weight allows the internal layer to resist compression during folding carton conversion and printing nip passage.
Layer Thickness
Caliper development relies heavily on this untreated middle stratum because voluminous fibres create distance between outer linerboards without adding excessive mass. Carton stiffness increases in proportion to the cube of total thickness, making internal bulking efficiency the primary economic driver for folding boxboard design. Production parameters demand strict control over refining energy input to prevent fibre damage while maintaining the required void volume for bending performance.
Stiffness Retention
Moisture absorption degrades internal bond strength rapidly when the surrounding structure lacks sufficient sizing chemicals to protect hydrophilic hemicellulose fractions. Converted packaging subjected to high humidity environments loses structural integrity if the central web fails to distribute compressive loads evenly across the cross section. End use requirements dictate minimum ring crush values to ensure stacking stability during warehouse storage and palletized transport.
