How Cardboard Works

Marshall Brain

Corrugated cardboard is one of those things that you never really think about, but chances are you see it every day. Look around you. As I am sitting here looking around my office, there is a cardboard box of printer paper, a cardboard box from a UPS package received yesterday, and a cardboard box containing one of the kids' toys. The garage and closets are filled with boxes. Empty cardboard boxes are so common at any retail store that there is usually a special compactor out back to handle all of them. Cardboard is everywhere.

What special properties make cardboard so ubiquitous? And where does all of this cardboard come from? Let's look at the miracle of corrugated cardboard!

Tens of millions of tons of cardboard are used in the United States every year -- something like 200 pounds of corrugated cardboard for every man, woman and child in the country.

The reason why cardboard is so popular is because it has several unique properties:

  1. A sheet of corrugated cardboard is made out of three sheets of ordinary Kraft paper glued together with glue made of cornstarch. Paper and glue are relatively inexpensive, so cardboard is inexpensive.
  2. Cardboard is completely recyclable and biodegradable.
  3. Cardboard is about half air, making it very light for its size.
  4. Compared to a sheet of paper, cardboard is incredibly rigid.
  5. Cardboard has some give, so it provides a little padding for the things inside the box.
  6. Cardboard is strong and resists punctures, but it is also easy to cut with a knife.
  7. When a box gets crushed it tends to stay together rather than shattering like wood would.
  8. Cardboard boxes fold flat for shipping without losing strength, making transportation much easier and less expensive. It is also very easy to put the boxes together when they are needed.

About the only disadvantage of cardboard is that it does not like water. When cardboard gets wet, it tends to get soggy and disintegrate. That is not too big a deal because most boxed goods are kept in dry trucks, warehouses, closets and garages.

When you find a solution that has that this many advantages and no significant disadvantages, it tends to dominate its category. That is why you see cardboard boxes everywhere you go.

The source of the strength

How can it be that 3 sheets of paper and a little glue can create something so strong, even though the resulting piece of cardboard really is not much thicker than the original 3 sheets? The answer to this question has to do with I-beams and trusses.

Think about how strong a steel I-beam is at a construction site. An I-beam has a top-plate, a bottom plate and a stringer in between. Despite the simplicity of an I-beam, this structure has a great deal of strength, especially if you are applying weight to the top of the I-beam.

Now look at a piece of cardboard. What you have could be thought of as a series of paper I-beams lying next to each other. These I-beams are not as strong as steel, but they are much stronger than flat sheets of paper. The I-beams give the cardboard tremendous strength along the beams. This is the direction in which the cardboard is strongest.

You could also think of corrugated cardboard as a truss structure . That provides strength in the other direction. See How Bridges Work for details on I-beams and trusses.

The strength of the resulting cardboard is determined by several factors, including:

  • The strength and thickness of the two liner sheets (the flat sheets on either side of a sheet of cardboard).
  • The strength and thickness of the corrugated sheet in the middle (called the medium).
  • The thickness of the corrugation (the distance between the two liner sheets)
  • The number of corrugations per foot in the medium.

There are several different ways to measure the strength of a box that is made from a sheet of cardboard. A box design is tested and the box receives a Box Certificate indicating the amount of abuse a box can take. A typical box certificate like this gives the gross weight limit for a box and the edge crush weight. Puncture force and bending force are other measures for a flat cardboard sheet.

Making Cardboard

This video demonstrates the process of making cardboard:

To make cardboard, you start with trees -- typically de-barked pine trees that have been recently harvested. These trees go though a chipper and the chips end up in a big pressurized tank. In this tank, the chips will undergo the Kraft process. The chips mix with chemicals like sodium hydroxide (aka lye), which breaks down the glue (lignin) holding the wood fibers together. Once the process is complete, this unbleached wood pulp is formed into huge rolls of Kraft paper. The rolls can weigh several tons each, and they head off to the cardboard factory.

At the factory, the process of making flat sheets of cardboard is straightforward. The cardboard is made of two flat sheets of paper (the liners) with a corrugated sheet (the medium) glued in between. The glue is made of corn starch mixed with water. It is easy to visualize this machine -- three rolls of paper feed in three sheets. One sheet goes through a set of corrugating rollers. Glue gets applied, and the two liner sheets get glued to the medium. Once the glue sets, you have a rigid sheet of cardboard.

The next step typically slices or die-cuts the cardboard into the shape of a box blank, and this blank gets folded and glued appropriately. These finished boxes are left flat for shipping and are assembled when they are needed.

More info: - Corrugated Cardboard