We are coming down to the final stages of the paper making process. We’re finally getting into the actual process where the paper is made into sheets
A paper machine can be thought of in a most basic sense as a very large winding continuous conveyor belt where a slurry of pulp, various chemicals and other additives enter at one end and emerge as a dried finished sheet of paper at the other end. Of course in practice the actual processes and control systems involved are much much more complicated than that but the goal of a continuous flow of paper off the end of the machine is the production manager’s goal! We will divide the paper machine into two main parts just to make it easier. The wet end and the dry end. Fairly self-explanatory.
The front or first part of the machine is the wet end, where the diluted pulp, chemicals and other assorted additives start their journey to becoming a finished roll of paper. There are some basic component sections common to most modern paper machines be they newsprint, LWA (lightweight coated), SC (Supercalendared), etc. These different machines and different types of paper they produce, utilize a whole myriad of chemicals and additives that frankly are too numerous to explain or even list. Plus, it could get rather dull. New chemicals and additives are constantly being tested on paper machines all over the world every day in order to meet customers ever changing needs. The industry is constantly evolving to meet customer’s quality needs while keeping output costs as low as possible.
So let’s begin. Rather than trying to describe all different sorts of paper machines, Newsprint, SC, etc. let’s assume our paper machine produces a high brightness grade of SC paper. The type typically used in magazines and glossy add inserts in newspapers, etc. The bleached pulp that has been produced in the pulp plant and stored in a silo, is pumped out of the silo and diluted down to a low consistency. From there, it is pumped into a mixing chamber of some sort where many of the additives previously mentioned are combined with the pulp. These additives could be a wide assortment of things such as kraft pulp, clay, various optical brighteners, dye, retention aid and on and on. This “stock” is then pumped to a mix tank where it sits ready to be pumped to the wet end of the machine.
Located at the very start of the paper machine is the headbox. It’s a tank that spans the full width of the machine. It’s purpose is to evenly distribute the stock to the end of the machine. The first stage of the wet end of the machine that receives the stock from the headbox is called the “former”. Basically, this is a pair of continuously rotating loops of fabrics referred to as wires where a very low consistency (wet) slurry of stock is evenly sprayed onto the wires. As the wires convey the stock from the headbox towards the next section of the machine, the press section, the stock drains and begins to settle on the bottom wire to form a wet paper web. It actually looks like paper at this point but is still too wet to be conveyed onwards without the support of fabrics.
Once the paper web leaves the former it enters the press section again between two fabrics which wind between large press rolls which squeeze more liquid from the paper web in an even uniform manner. When it exits the press section of our machine the paper sheet still needs to be dried further so it now moves to the dryer section which is the beginning of the “dry end.” Stay tuned for the final stage of the paper making process!