From basic storage boxes to multi-colored card stock, cardboard is available in an array of sizes and forms. A term for heavier paper-based products, cardboard can range in manufacturing method as well as aesthetic, and as a result can be found in vastly different applications. Because cardboard doesn’t refer to a specific material but rather a category of materials, it is helpful to consider it in terms of three separate groups: paperboard, corrugated fiberboard, and card stock.
Paperboard is typically 0.010 inches in thickness or less, and is essentially a thicker form of standard paper. The manufacturing process begins with pulping, the separation of wood (hardwood and sapwood) into individual fibers, as accomplished by mechanical methods or chemical treatment.
Mechanical pulping typically involves grinding the wood down using silicon carbide or aluminum oxide to break down the wood and separate fibers. Chemical pulping introduces a chemical component to the wood at high heat, which breaks down the fibers that bind cellulose together. There are approximately thirteen different kinds of mechanical and chemical pulping used in the U.S.
To make paperboard, bleached or unbleached kraft processes and semichecmical processes are the two types of pulping typically applied. Kraft processes achieve pulping by using a mixture of sodium hydroxide and sodium sulfate to separate the fibers that link cellulose. If the process is bleached, additional chemicals, such as surfactants and defoamers, are added to improve the efficiency and quality of the process. Other chemicals used during bleaching can literally bleach the dark pigment of the pulp, making it more desirable for certain applications.
Semichemical processes pre-treat wood with chemicals, such as sodium carbonate or sodium sulfate, then refine the wood using a mechanical process. The process is less intense than typical chemical processing because it doesn’t completely break down the fiber that binds cellulose, and can take place at lower temperatures and under less extreme conditions.
Once pulping has reduced wood to wood fibers, the resulting dilute pulp is spread out along a moving belt. Water is removed from the mixture by natural evaporation and a vacuum, and the fibers are then pressed for consolidation and to remove any excess moisture. After pressing, the pulp is stream-heated using rollers, and additional resin or starch is added as needed. A series of rollers called a calendar stack is then used to smooth and finish the final paperboard.
Corrugated fiberboard is what one typically refers to when using the term “cardboard,” and is often used to make various types of corrugated boxes. Corrugated fiberboard is comprised of several layers of paperboard, typically two outer layers and an inner corrugated layer. However, the internal corrugated layer is typically made of a different kind of pulp, resulting in a thinner kind of paperboard that isn’t suitable to be used in most paperboard applications but is perfect for corrugating, as it can easily assume a rippled form.
Corrugated fiberboard is manufactured using corrugators, machines that enable the material to be processed without warping and can run at high-speeds. The corrugated layer, called the medium, assumes a rippled or fluted pattern as it is heated, wetted, and formed by wheels. An adhesive, typically starch-based, is then used to join the medium to one of two outer paperboard layers.
The two outer layers of paperboard, called linerboards, are humidified so that joining the layers is easier during formation. Once the final corrugated fiberboard has been created, they component undergoes drying and pressing by hot plates.
The thinnest type of cardboard, card stock is till thicker than most traditional writing paper but still has the ability to bend. As a result of its flexibility, it is often used in post-cards, for catalog covers, and in some soft-cover books. Many kinds of business cards are also manufactured from card stock, because it is strong enough to resist the basic wear and tear that would destroy traditional paper. Card stock thickness is typically discussed in terms of pound weight, which is determined by the weight of 500, 20 inch by 26 inch sheets of a given type of card stock. The basic manufacturing process for cardstock is the same as for paperboard.