Fluting
The wavy inner layer of corrugated board. What fluting paper is, how it's made on a corrugator, and why the wave geometry — not the fibre density — provides structural strength.
Fluting — also called corrugating medium or simply medium — is the wavy paper layer at the core of corrugated board. In a standard single-wall corrugated sheet, it sits between the two flat liner sheets, bonded at the peaks of each wave. It is not the outer faces you see and print on; it is the invisible structural layer that actually resists compression.
The name comes from the fluted wave shape. The paper itself is called corrugating medium or fluting medium; the action of forming it is called fluting or corrugating.
How fluting is made
Corrugating medium starts life as a flat paper reel. The papermaking specification is distinct from liner: medium is typically lighter (80–135 g/m² in most markets; some heavy-duty applications run higher), and the fibre furnish skews toward shorter recycled OCC fibre or semichemical pulp, depending on the market.
On the corrugating machine (the corrugator), the medium reel feeds into a pair of heated fluted rolls — like a giant gear mesh — that corrugate the flat sheet into the wavy profile. The roll temperature runs 160–200 °C; heat and moisture (from steam injection) soften the fibre network, allowing permanent forming without cracking the sheet. The geometry of the fluted rolls determines the flute profile: A, B, C, E, F, or N. See flute profiles for the full dimensional breakdown.
After forming, the medium is immediately glued (starch adhesive) to the single-face liner on the upper face, then the double-backer liner on the lower face, to produce finished corrugated board. The board must maintain flatness (no warping) while the starch sets under heat.
Why the wave geometry is the structural mechanism
The important conceptual point — often misunderstood — is that the corrugated wave provides strength primarily through geometry, not through raw material density.
Consider a flat sheet of 100 g/m² paper. If you compress it from the top, it will buckle almost immediately. Now form that same sheet into an arch or wave. The arch resists far more load before buckling because the curved geometry redirects compressive forces into axial tension along the curve. A corrugated column of medium works by the same principle: each flute crest is a small arch, resisting top-to-bottom compression.
This is why fluting research shows that flute geometry precision — consistent height, consistent pitch, no flattened or crushed peaks — matters enormously to the compression performance of the finished board. A heavier medium with poorly formed flutes will underperform a lighter medium with precise geometry.
The primary test for corrugating medium quality is CMT (concora medium test), which tests a strip of medium after re-corrugating it in the lab, measuring the force required to crush it. See CMT.
Fibre and grades
Most fluting medium globally is made from recycled OCC (old corrugated containers). Short recycled fibres bond adequately for the medium's load case — it needs flat crush resistance (CMT) and ring crush (RCT) rather than the high burst and tear values required of virgin kraft liner.
The Indian fluting paper above — 100–250 g/m² recycled OCC — is representative of a large segment of the Asian market, where recycled medium is standard. In North America and Europe, semichemical medium (using a neutral sulphite semichemical process, NSSC) was historically prevalent because it retained more lignin and produced stiffer, higher-CMT medium per unit weight. Recycled OCC has largely displaced NSSC medium in recent decades as secondary fibre quality has improved with better OCC sorting.
Virgin kraft medium exists but is uncommon — the cost premium over OCC medium is difficult to justify given that the inner layer of a box has no presentation surface to protect.
The corrugated stack
In a standard single-wall (3-ply) corrugated sheet, the structure from outside to inside is:
- Outer liner (kraft liner or testliner)
- Fluting (corrugating medium)
- Inner liner (kraft liner or testliner)
Double-wall adds a second medium-and-liner pair. Triple-wall adds a third. The total board grammage of a single-wall box ranges from roughly 320 to 550 g/m² depending on component specifications.
The fluting's contribution to edge crush test (ECT) — the metric that predicts box compression strength — is significant. Medium with a higher ring crush value (RCT, ISO 12192) produces boards with higher ECT at equal liner specification. See ECT.
WPI editorial note: When qualifying a corrugated box supplier, ask for CMT values of the medium being used, not just the board ECT. A supplier can hit ECT specifications by over-specifying the liner while running sub-optimal medium — then cut liner weight later and expect you not to notice.