The paper machine is always the heart of any paper mill. On the paper machine, the fibres which have been mixed with large quantities of water are laid out in a very thin layer on an endless-band meshed screen, with a weave similar to a mosquito net. The water is then successively removed, so that the “stock” mixture which is 99% water and 1% fibre at the start, is about 93% fibre and only 7% water in the final sheet. The paper machine performs this fascinating transformation.
To do this, the fibres are mixed with chemicals and fillers and much water to form the stock with 1% fibre. This is then ejected from the headbox in a fine thin rectangular jet at the enormous rate of 2,000 litres per second and a speed of 100 km per hour. The jet lands in the gap between the two meshed screen or forming fabrics. Water is first drained by gravity and then pulled out with vacuum suction boxes, raising the sheet to 20% fibre.
Next, more water is harshly but carefully pressed out between spongelike felt surfaces bringing the fibre content up to 50%. Finally, the sheet is threaded over large steel steam-heated cylinders at 135°C to evaporate the remaining water, so we end up with approximately 93% fibre and 7% water in the final sheet.
To achieve the perfect paper quality, two key steps are vital. The first is continuous online quality scanning, and the second is what’s called calendering, a smoothing process to remove bumps and creases. Quality scanning is done by the QCS, which is a high-tech monitor travelling back and forth across the sheet while measuring all the key paper properties. These include the weight in grams per square meter, the brightness or whiteness, the thickness and the amount of mineral fillers. Advanced data analytics then make sure that all these properties are within the specifications in all directions on the sheet. This is critical for getting a consistent and uniform printing surface and high-quality feel to the paper.
Calendering is a smoothing process done at the end of the paper machine in which a hard heated metal surface presses the paper against the softer backing roll to make the sheet smooth and glossy and also to keep the sheet thickness consistent. The process is somewhat like ironing of a shirt on an ironing board, but on a vastly larger scale.
Finally, the paper is wound on the jumbo reel which is a giant roll of paper weighing up to 40 tons, with a length of about 100 km. This is then slit into smaller rolls for delivery to customers.
The pulpwood enters the mill and is washed, debarked, cut and chipped before moving on to the pulping step.From logs to chips
The wood chips are steamed, refined, relaxed and bleached before entering the paper machine.From chips to pulp
The paper pulp enters the paper machine and exits as a finished sheet of paper after just 15 seconds.From pulp to paper