How Paper is Made

A schematic of the papermaking process is shown below. In this case, a chemical pulping process is depicted. However, if the digester, blow pit, and washer were replaced with a mechanical refiner, the drawing would also describe a mechanical pulp and paper mill. (Move your mouse across the drawings below to learn more about the processes shown.)

Note that once the wood has been converted to pulp, the pulp is beaten (more on this in a moment), refined in a Jordan refiner, and then sent to a large machine where a slurry of fiber is metered onto a moving wire. In this machine, called a Fourdrinier, a slurry that is 99% water by weight flows from the head box onto the wire, after which suction boxes pull water rapidly away. The result is a fiber mat that is then pressed, dried, coated and/or sized, re-pressed, and cut into desired sizes of paper.

A Fourdrinier unit is pictured here. A slurry of fiber is being metered onto the moving wire at the far right in this picture, with movement of the wire toward the left. The drier units can be seen at the far end of the production line. (Photo courtesy: American Forest and Paper Association)

The beating and refining processes are very important in the papermaking process. An examination of beating, in particular, provides several insights as to some of the technology involved.

This is a photo of softwood that has been chemically pulped. Note that the fibers are straight, smooth, and largely undamaged.

For the most part, however, smooth surfaces and rounded, undamaged fiber are not what is needed in making a quality sheet of paper. Fibers must be flattened to increase the contact area (and thus the bond potential) between them.
Flattened fibers can be readily seen in this highly magnified photo of the surface of paper. (Photo by John Crist and Ron Teclaw)
Moreover, by unraveling microfibrils from the cell walls, surface area (and thus hydrogen bonding potential) can be greatly increased as illustrated by this photo of mechanically produced fiber. (Photo: John Crist and Ron Teclaw)
The way that fibers are flattened, and subjected to a mechanical rubbing action that unravels microfibrils, is through the use of a beater. A simple beater (a Hollander beater) is shown here. A slurry of fiber goes around and around in the tub, each time passing between a fixed bedplate and the ribs of a rotating beater wheel. The opening between the bedplate and wheel is set to about the width of a single wood fiber, meaning that fibers are pounded and deformed with each pass. (Image adapted from TAPPI)

The longer fibers remain in the beater, the more beat-up they become, and the more the surface area of the fiber is increased.