Boxes: A Universal Packaging Method

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After screening, the chips go to the digesters (huge pressure cookers) where they are mixed with chemicals and cooked to loosen the lignin adhesive that binds wood fibers together. This reduces the wood to a pulpy mass. Once the fibers have been loosened, they are separated from the chemical cooking liquors. The chemicals contained in the "black liquor" are processed and recovered to be used again in another cooking cycle.

After leaving the digester and being separated from the black liquor, the wood fiber (now called pulp) is stored in high-density tanks until it is needed by the paper mill. Before going to a paper machine, the pulp must go through various steps of screening, washing, refining, and cleaning so the fibers will weave together in a tight mat during their long journey through the paper machine.

In the making of linerboard, a solution of 99.5% water and 0.5% pulp (wood fiber) is sprayed onto a rapidly moving screen wire on the paper machine. This process is done in two steps, which, in effect, creates a double-layered sheet. The water is removed when drained through the screen, leaving a soft mat of pulp that forms the paper sheet.

The paper travels through a series of rollers in the press section where much of the additional water is squeezed out. The linerboard then enters the dryer section, where steam-heated drums evaporate the remaining moisture from the sheet.

The final movement between polished cylinders on a "calender-stack," smooths the surface and takes the paper to the reel, where it is wound into giant rolls. When the reel reaches a designated size (about 10 feet in diameter), it is lifted off the paper machine by a crane and taken to a rewinder. Here the giant reel of linerboard is cut into smaller rolls as ordered by the customer, then stenciled, labeled and banded for shipment.

Most of the chemicals used in making kraft paper end up in the spent cooking liquor and are recovered. Through the processes of evaporation, combustion (burning) and chemical reaction (mixing) at various stages in the recovery cycle, 95% to 98% of the cooking chemicals are reclaimed and recycled. The ability to recycle chemicals makes paper much more economical to produce.