Why does papermaking begin with cellulose fibers suspended in so much water if the final result is supposed to be a dry sheet of paper? Initially, this process can seem wasteful, since water is added to make stock, only to be removed again during forming, pressing and drying. But water isn’t just something we’re trying to get rid of at a later point in the process. In the early steps, water acts as the vehicle for allowing cellulose fibers to separate, move around, mix and spread over the length of the forming section. A wet mass of pulp cannot self-distribute across the width of a paper machine.
To prepare the pulp for the next step, water is added to the pulp and mixed to reach a consistency that will work for the subsequent processes. This includes refining, fillers, sizing and any other stock treatments necessary prior to getting the suspension to the headbox. Diluting the pulp gives the fibers more room to move around and prevents them from staying bundled up together in large clumps. This is very evident when looking at the role water plays at the headbox and forming section. The headbox sends out a diluted stock that gets distributed across the wire. As the stock moves along, water starts to drain off and the fibers stay behind on the forming fabric, eventually building up into a wet web. This shows that sheet formation consists of two things happening simultaneously: the distribution of fibers and the loss of water. Having a suspension that is not well diluted or distributed would complicate the process of obtaining good formation.
One way to think about papermaking is by picturing a water removal chart instead of a fiber processing chart. Imagine starting with prepared stock and walking through the process, from the headbox, through the forming section, press section, and finally to the dryer section. At the forming section, water drains away in large quantities while the network of fibers starts to form. The press section then mechanically removes more water from the web. Drying is used last to remove the rest of the water down to the amount needed for the finished paper. This explains why the dryer section is not capable of doing all the dewatering on its own. Evaporating all water with heat would be far more energy-intensive than removing as much water as possible through drainage and pressing beforehand.
For the student learning about papermaking, it’s also helpful to understand that each step is performing a specific function in the process. Formation builds the fiber network, pressing compacts the wet web and squeezes out water, while drying brings the moisture content down. Drying isn’t the mechanism by which the sheet was formed. Water also impacts the way you might interpret various defects or properties of the paper. Formation issues could be created while the fibers were still circulating in the suspension. The later stages of moisture removal could impact the dimensional stability and handling characteristics of the paper.
Finally, you could assess the properties of your paper such as basis weight, thickness, opacity, smoothness, or strength. None of those properties should be attributed solely to drying just because the dryer section is near the end of the process. The web had already passed through multiple stages dependent on water content. Pay attention to what happens to both fiber and water when reviewing a pulp and paper process flowchart, rather than focusing only on the fiber. Water first enables the creation of a controllable suspension, then it drains off in successive steps. Viewing papermaking in this way, as a controlled transition from a dilute fiber suspension to a stable sheet, makes it much easier to accept why papermaking uses so much water.