Corrugated cardboard is made by gluing a wavy, fluted sheet of paper between flat outer sheets called liners. The process combines heat, pressure, and starch-based adhesive on a machine called a corrugator, which can run at speeds up to 400 meters per minute. What looks like a simple brown box is actually an engineered sandwich of paper layers, each chosen for a specific purpose.
The Raw Materials
Corrugated board uses two types of paper. The flat outer layers are called linerboard, and the wavy middle layer is called the corrugating medium (or just “medium”). Both are made primarily from wood pulp, though recycled fiber from old corrugated containers is a major ingredient. Kraft linerboard, the strongest variety, blends virgin wood pulp with recycled pulp across multiple layers, with the top layer using longer, stronger fibers for a smoother surface. Test linerboard, a more economical option, is made entirely from recycled pulp.
The corrugating medium is also typically made from recycled pulp, with chemical strengthening agents added to improve its physical properties. Corrugated boxes are one of the most recycled products in the world. The EPA reported a recycling rate of 96.5 percent for corrugated boxes in 2018, which means the fiber in your next shipping box has likely lived several previous lives.
How the Corrugator Works
The corrugator is the heart of the process: a machine that can stretch the length of a football field. It takes in flat rolls of paper at one end and produces finished sheets of corrugated board at the other. Here’s what happens along the way.
First, the medium is conditioned with steam sprays that moisten and heat it. Hot, damp paper becomes pliable and easier to shape without cracking. The softened medium then passes between a pair of steam-heated corrugating rolls, which are large metal cylinders with interlocking ridges and grooves, like massive gears. As the paper feeds through, the rolls press it into a continuous wave pattern. This is where the signature rippled shape is formed.
Immediately after, a thin layer of starch-based adhesive is applied to the tips of the freshly formed waves. The first flat liner is then pressed against those glued tips, creating what’s called single face board: one flat sheet bonded to one fluted sheet. This happens at the “single facer” station of the corrugator.
The single face board then moves down a long heated section called the double backer, where the second liner is glued to the exposed side of the flutes. Heat from steam-powered plates activates the adhesive and drives out moisture, locking everything together. The result is a continuous ribbon of corrugated board that’s cut into sheets at the end of the line.
The Starch Adhesive
The glue holding corrugated board together is surprisingly simple. It’s a water-based paste made primarily from starch (typically corn or wheat), with small amounts of sodium hydroxide and borax. The sodium hydroxide lowers the temperature at which the starch gelatinizes, and the borax acts as a crosslinking agent that improves the bond’s strength once it sets. No synthetic plastics or complex polymers are needed, which is one reason corrugated cardboard is so easy to recycle.
Flute Types and What They Mean
The wavy layer isn’t one-size-fits-all. Different products need different levels of cushioning, stiffness, and printability, so manufacturers use several standard flute profiles that vary in wave height and frequency:
- A flute: The thickest option at 4.7 to 5.0 mm, with about 33 flutes per foot. Provides excellent cushioning for fragile items.
- B flute: Thinner at 2.5 to 3.0 mm with roughly 47 flutes per foot. The tighter wave pattern resists crushing and gives a flatter surface for printing.
- C flute: The most common all-purpose profile at 3.5 to 4.0 mm with 39 flutes per foot. It balances cushioning and stacking strength.
- E flute: A micro-flute at 1.0 to 1.8 mm with about 90 flutes per foot. Used for retail packaging and pizza boxes where a smooth printing surface matters.
- F flute: The thinnest standard profile at 0.8 to 1.2 mm with over 120 flutes per foot. Common for small consumer packaging like cosmetics and electronics accessories.
More flutes per foot means a smoother outer surface and better print quality. Fewer, taller flutes mean more air space inside the board, which translates to better cushioning and insulation.
Single Wall, Double Wall, and Beyond
The number of fluted layers determines the board’s overall strength. Single face board is the simplest: one liner glued to one fluted sheet, with the other side of the flutes left exposed. It’s flexible and often used as wrapping material or interior padding.
Single wall (also called double face) is what most people picture when they think of cardboard. One fluted layer is sandwiched between two flat liners. This is the standard construction for shipping boxes.
Double wall board adds a second fluted layer separated by an intermediate liner, for a total of five paper layers. This construction handles heavier loads and provides more protection. Manufacturers can even combine different flute sizes in double wall board, pairing a large B or C flute for cushioning with a smaller E flute for surface smoothness. Triple wall board exists for industrial applications like shipping pallets and bulk containers, though it’s far less common.
Converting Sheets Into Boxes
The corrugator produces large flat sheets, but those sheets still need to become actual boxes. This happens at a converting plant, sometimes in the same facility, sometimes at a separate location. The key piece of equipment here is the flexo folder-gluer, or FFG, which handles multiple steps in a single pass.
First, the flat sheet is printed using flexographic printing, a process where raised rubber or polymer plates transfer ink directly onto the board. This is how shipping boxes get logos, handling instructions, and barcodes. Next, the sheet is scored (creased along fold lines) and slotted or die-cut to create the box’s flaps and shape. Finally, the machine folds the blank along its score lines and glues one joint to form a flat, knocked-down box that’s ready for shipping to the end customer, who pops it open and fills it.
For boxes with irregular shapes, windows, or complex designs, a separate die-cutting step uses a custom steel-rule die (essentially a giant cookie cutter) to punch out the shape before folding.
How Strength Is Measured
Not all corrugated board is created equal, and manufacturers use two primary tests to rate a box’s durability. The Edge Crush Test (ECT) measures how much compressive force the board can withstand before buckling, reported in kilonewtons per meter. This test predicts how well boxes will stack on a pallet without collapsing, making it the more relevant test for warehouse and shipping applications.
The Mullen Burst Test measures how much pressure, in pounds per square inch, the board can handle before a hole punches through it. A sample is clamped between two plates, and a rubber diaphragm pushes against it until it bursts. This test matters more for situations where boxes might be punctured by sharp or heavy contents. Most corrugated boxes sold today are rated using ECT values, which better reflect real-world stacking conditions and allow manufacturers to use lighter-weight paper while still meeting strength requirements.