Cardboard is a versatile packing material used to package everything from TVs and washing machines to pizza. Within the U.S., 95 percent of all products are shipped in corrugated boxes. In one year, the U.S. produced more than 25 million tons of corrugated cardboard and millions of tons of this versatile material is used each year. Cardboard is a stiff, strong, light-weight material made of up of one to three layers. Carl Dahl, a Swedish chemist, invented the process for pulping wood into a strong paper, that he called "kraft paper". The manufacture of cardboard, using kraft paper, began in 1906 through development of machinery, and has become an efficient process.
Processing Pine Chips
Large cardboard companies own thousands of acres of pine trees which are harvested annually and replaced with seedlings. Tree trunks are put through the "kraft process", also known as the "sulfate process", where chemicals break down wood chips in a large pressure tank, known as a "batch digester".
Producing Kraft Paper
The fibrous pulp is cleaned, refined and, as a slurry, is pumped into the paper-making machine, called a "fourdrinier machine". It is up to 600 feet long and contains a wire mesh, within which large sheets of paper are formed. The sheets are fed into huge, steam-heated rollers and wide, felt blankets that remove all water. Then they are formed into large rolls of four different sizes. Tough kraft paper resists tearing, splitting or bursting.
Sorting and Grading
The most common roll sizes are 67 inches and 87 inches wide and the larger size rolls can weigh up to 6,000 pounds each. The paper is sorted by grades that are combined to make cardboard containers of different strengths. Various grades are used for the corrugated medium, which is glued to single or double flat sheets called "liners". The more layers the corrugated box has, the stronger it becomes.
Corrugating the Cardboard
Rolls of kraft paper are loaded into one end of the corrugator machine. Rolls advance through pre-heating rollers where high pressure steam is forced into the paper, reaching temperatures of 350º F to 365º F. The corrugating rollers are covered with "flutes", that are horizontal parallel ridges, like the teeth of wide gears. As the hot paper passes through the corrugating rolls, the flutes trap and bend it, forming a fluted middle portion in each sheet of cardboard. Corrugating machines have interchangeable, corrugating rollers with various flute sizes.
Gluing the Pieces
A layer of liner is glued to both sides of the corrugated sheet and each piece of cardboard passes over steam-heated plates, drying the glue. A finished piece of corrugated cardboard looks like a sandwich with the corrugated portion compressed between two liner sheets.
Making Box Blanks, Printing and Waxing
A "slitter-scorer" trims the cardboard and cuts it into large sheets called "box blanks". They are stacked and routed through a flexograph machine. This machine prints the cardboard sheets which are next stacked and banded for shipping. Wax is applied to vegetable and fruit boxes as orders are assembled.