The dryness and finql sheet density are intimately related for a given furnish. It is not possible to change this relationship by pressing, except by a very small amount. The mechanical furnishes have a very steep slope because of the stiff fibre walls compared to the chemical pulp. The roll press and the shoe press data lie along the same dryness-density line for a given pressing temperature. Within a very small tolerance, there is no inherent difference between the two press types when it comes to achieving density and dryness.
Density is achieved by a given impulse during the water removal process.
The water removal mechanism is one of mechanical liquid displacement, a sheet volume reduction process. For a given density there generally is higher dryness when the sheet is pressed longer. Rewetting is bad, not just for dryness, but also for density. Rewetting causes the sheet to lose density because the water re-enters the fibre network and allows it to expand. Using a blotter in place of a press felt eliminated rewetting in the above data.