Pulp is diluted and mixed to form a slurry of one part solids to approximately 200 parts water and pumped to the headbox when to start the paper making process. The mixture is pumped evenly through the slice — which is a narrow slot in the bottom of the headbox — onto the polyester forming fabric. The fabric filters the water and retains the wood fibers to “form” the sheet. This is a critical part of the process because it affects the sheet’s final properties.

This forming section consists of a long moving belt of fine mesh screen supported by rolls or foils. For many decades, this screen belt was made of bronze and called the “wire.” Today it is made of synthetic polymer — primarily polyester monofilament — and is called the polyester forming fabric. Newer machines (gap formers) trap the slurry out of the slice between two forming fabrics. As the forming fabric moves along the forming section, water drainage is assisted by foils. The fiber then passes over the suction boxes to pull more water from the furnish. Still more water is removed at the couch roll just before the sheet leaves the forming section. At this point, the sheet consistency is about 20%.

Recently research has produced synthetic forming fabrics that yield improved drainage, higher production and longer life of polyester forming fabric at a lower cost per ton of paper produced.