Packaging Reference Library · Technical Reference

Corrugated Board Construction

Corrugated board is made of flat liners glued to a fluted medium. This sheet explains the parts of the board and the difference between single-face, single wall, double wall and triple wall.

Ref PK-0102ASTM D4727

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Corrugated fiberboard is a sandwich. The medium is paper that is heated, steamed and pressed into a wave shape, the flute. The liners (also called facings or linerboard) are flat sheets glued to the tips of the flutes with starch adhesive. Together they make a stiff, light board that resists crushing and bending. The machine that makes it is a corrugator. ASTM D4727 is the standard specification for corrugated and solid fiberboard sheet stock and covers the terms, grades and construction used to specify board.

How to identify

Look at a cut edge. Count the flat liners and the wavy mediums. One medium between two liners is single wall. Two mediums with three liners is double wall. Three mediums with four liners is triple wall. One medium with only one liner, so the flutes are exposed on one side, is single-face, usually sold in rolls as a wrap. The flute letter (A, B, C, E, F) describes the size of each wave. See the flute profile chart.

Board constructions

Construction	Layers	Example flute call-out	Typical use
Single-face	1 liner + 1 medium	B or E single-face	Roll wrap, sleeves, protective layers
Single wall (double-face)	2 liners + 1 medium	C, B, E	Most shipping boxes, mailers, retail packs
Double wall	3 liners + 2 mediums	BC, EB	Heavy or dense products, bulk boxes
Triple wall	4 liners + 3 mediums	Combinations such as AAC	Bulk bins, heavy industrial loads, export crates

How the parts contribute

Liners carry most of the bending stiffness and burst resistance. Heavier liners give a stronger board and a better print surface.
Medium holds the liners apart, which gives the board its thickness and stiffness, and its flutes carry edge crush load.
Flute size trades cushioning and stacking strength (larger flutes) against print quality, fold quality and lower cube (smaller flutes).
Grain direction: flutes run in one direction. For stacking, box flutes should run vertically.

Key points

Board strength is stated on the box maker's certificate as an edge crush (ECT) or burst rating. Construction and rating together define the board. See the ECT vs burst and box certificate sheets.
Moisture weakens corrugated board. Cold storage, humid warehouses and wet docks all reduce strength.
Liners and medium are measured by basis weight in pounds per thousand square feet.
Double wall is not always required for heavy items. A higher-grade single wall may do the job. Ask the box supplier.

Common mistakes

Calling every brown box "cardboard." Corrugated and paperboard are different products.
Specifying flute only, with no strength grade.
Running flutes horizontally on a stacked case.
Assuming double wall is twice as strong as single wall.

Governing standards: ASTM D4727 (corrugated and solid fiberboard sheet stock); box maker's certificate and carrier rules for strength ratings.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

More in Technical Reference

← All Packaging sheets