384 Graphic Communications
Pine, fi r, spruce, aspen, beech, birch, maple, and
oak are typical species harvested for papermaking. The
length of the tree fi bers varies and determines, among
other characteristics, the strength of the paper.
Papermaking is a complex manufacturing process.
It uses both chemical and mechanical means to reduce
wood fi bers to pulp, which is the material used to ultimately
produce paper in sheet form. See Figure 21-1.
Chipping
Harvested logs are cut to uniform length, debarked,
Figure 21-2, and sent to a chipper or grinder. The
chipper cuts the logs into 3/8″ to 3/4″ chips. The chips
are sized so the digester is able to separate the cellulose
fi bers. After the chips are screened for size, they are put
in a huge cooking kettle called a digester.
Making Pulp
In the chemical pulpmaking process, chemicals
in the sealed, pressurized digester break down the
lignin present in the cellulose fi bers. The cellulose
fi bers, which once resembled soda straws, become
pulp, a mass of soft, spongy matter. The pulp is blown
into a pit where the chemicals are washed away.
Raw materials
Pulp wood
Barker
Chipper
Chip pile
Grinder
Groundwood pulp
Washer
Knotter screen
Bleaching
cell
Thickener
Screens
Blow pit
Steam
Cooking liquid
Chemical pulp
Digester
Centrifugal cleaners
Washer
Drying rolls
Stock blender
Disc refiner
Headbox
Fourdrinier
Pulp
Calender stack
Paper
Tub size
Press section
Figure 21-1. Major steps in the manufacture of paper. A modern papermaking operation requires a large
investment in equipment and raw materials.