Structural Definition
Multi-ply packaging material combines flat paperboard facings with arched fluted medium bonded by starch adhesive into a rigid structural sandwich. Commercial corrugated board derives bending stiffness, stacking capability, and flat crush resistance from this engineering geometry rather than raw sheet mass alone. The structure serves transport packaging and protective point-of-sale displays across grammages spanning 250 to over 1500 grams per square metre.
Base components utilize unbleached kraftliner, recycled testliner, and semi-chemical or recycled fluting papers. The definition excludes solid containerboard laminates that lack wave-form fluting voids.
Flute Geometry
Fluting rolls press conditioned paper webs between gear-cut teeth under steam pressure to mold sinusoidal waves of specific heights and pitches. Common structural profiles include A, C, B, E, and F flutes, with tooth heights descending from five millimetres down to under one millimetre in micro-flute variations. Single-wall board uses two liners around one fluted medium, whereas double-wall and triple-wall boards alternate multiple medium layers to multiply vertical compression strength.
Adhesion relies on gelatinized corn or wheat starch suspension, which cures rapidly across flute tips at temperatures exceeding 140 degrees Celsius. Inadequate heat transfer leaves uncured starch that weakens structural shear transfer across plies.
Converting Tolerance
Rotary die cutters and flexographic folder-gluers demand precise caliper retention, as excessive crush during printing compresses flute peaks and permanently reduces box compression performance. Target edge crush test ratings govern box stacking calculations under international shipping protocols. Caliper variation along the corrugator run must stay within five percent of target thickness to avoid registration drift during slotting.
Moisture levels between seven and nine percent prevent both board warp and scoreline fracturing during folding. High relative humidity diminishes compressive strength rapidly by plasticizing cellulose fibre networks inside the fluted arch.
