Paper isn’t formed by making a thick layer of pulp and pressing and drying it flat. Before there’s a sheet, the fibers are dispersed in a large volume of water, resulting in a flowing fiber suspension known as stock. Here, the fibers are sufficiently separate that they can move and redistribute rather than being clustered together in a solid mass. This makes a big difference because part of the development of the final sheet is occurring while the fibers are suspended in the water.
The stock must be diluted to a certain consistency before it enters the forming section, so the fibers can spread across the width of the machine. The stock flows out of the headbox and is laid down on a forming fabric, commonly called the wire. The goal is not to lay fiber down on a surface; rather, we want to ensure that the flow is such that the fibers can arrange themselves into a fairly uniform structure as the water drains away through the fabric. If too much fiber is deposited in some areas and too little in others, the final paper will exhibit non-uniform formation.
One of the most critical events in the formation of the sheet, therefore, is drainage. Water flows through the wire, leaving the cellulose fibers behind to contact one another. The flowing suspension transitions to a fragile wet web. Although the wet web contains a great deal of water and has very little strength, the general structure of the fiber distribution has been established. While pressing and drying will significantly alter the moisture content and help to consolidate the fiber structure, they will not be able to rectify a sheet whose formation was poor to start with.
An easy way to observe formation without any special equipment is to take two or three paper samples and hold them up against a light source. Rather than evaluating which sheet looks “better,” describe what you see. Is one sheet relatively uniform, while the other exhibits regions of cloudiness or areas that are lighter or darker or have clusters of fibers? Turn the sheets around and examine them again. Recording your observations allows you to describe the formation before you attempt to identify the cause.
While it might be tempting to ascribe any irregularity in the paper to bad fiber distribution, in reality there are many variables in play. Fiber furnish, refining, stock consistency, flow conditions, drainage characteristics, and retention can all affect formation. Even the nature of the fibers themselves is relevant. Long fibers, short fibers, fillers, and other additives won’t all exhibit the same behavior in suspension. In the context of a basic exploration, the best strategy is to keep in mind a number of potential process variables, rather than leaping to a single explanation.
Another way to get a clearer picture of the process is to make a simple process flow. List out stock preparation, headbox, forming section, drainage, wet web, press section, and dryer section, and identify the points where water is removed and where the fiber network is formed. The forming section will be where the suspension becomes something resembling a sheet. The pressing section will remove more water mechanically, and the dryer section will remove additional water by heating.
Once you know the basics of this sequence, examining a finished sheet becomes much easier. Paper formation isn’t an afterthought applied at the end; it occurs while the fibers are being distributed and water is being removed. When you hold up a sheet and look at it against the light, notice the variations in the fiber network, and ask yourself at what point the sheet became rigid enough that subsequent pressing and drying could not significantly change it.