Fourdrinier machine
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Although there were inventions that predated it, the Fourdrinier machine is the basis for most modern papermaking, and it has been used in some variation since its design in 1806. Named after its inventor, the Fourdrinier accomplishes all the steps needed to transform a source of wood pulp into a final paper product.
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Pulp preparations
Harvested tree trunks are cut into logs of four to eight foot lengths, then sent to a very large horizontal debarking drum, which rotates and strips the logs bare. The freshly debarked logs are then fed into a chipper, which reduces the logs to handheld-sized chips. The chips are then passed along to a digester where they are cooked for a number of hours, a process that softens the wood to a large degree. The digester can be one of two types: sulphite or sulphate. In a sulphite digester, the principle chemical constituent is calcium acid sulphite and the method is referred to as the acid process. The sulphate, or Kraft, process is the younger of the two, and uses an alkaline system that reduces cooking time.
After the cooking is complete and the lignin content has been removed, the softened chips are fed at high pressure into refiners where the chips are forced between rotating steel plates. The refiner plates shatter the chips into a soup of brown fibres. Chlorine is used to bleach brown fibres to a brighter white colour, and calcium hypochlorite (sulphite process) or chlorine dioxide (sulphate process) are also used for whitening. Caustic soda (sodium hydroxide NaOH) is used to wash the pulp of any impurities, and the steps are repeated in order to obtain the desired brightness.
Sections of the machine
Wet end
There are four main sections to the Fourdrinier; the first is typically known as the wet end. Pulp may be delivered to the Fourdrinier machine in a liquid slush (pulp - water mixure) directly from the pulping process. Alternatively, pulp may be supplied in dried sheets which are then broken down in water to produce a similar slush, before being fed to the wet end. In the wet end, pulp is combined with sizing, fillers, colours, and possibly waste paper called broke, and passed on to the refiner where the fibres are subjected to brushing and rubbing.This action causes the fibrils of the smaller fibres to partially detach and bloom outward. More washing is done and the pulp enters the headbox, a unit that loads the pulp onto a moving wire conveyor. A gate called the slice determines the thickness of the pulp on the wire, which revolves around the Fourdrinier table, vibrating and aligning the fibres. Suction boxes below the wire gently remove water from the pulp with a slight vacuum.
Press section
The second segment of the Fourdrinier machine is the press section and it removes the most water via a system of nips formed by rolls pressing against each other. Some of the water is carried away by a press felt and the other part is either removed by suction through a vacuum chamber in the press roll or by flowing out of the converging nip into a tray. The press section also provides a smoothing action to flatten out the "sheet".
Dryer section
True to its name, the dryer section of the Fourdrinier machine dries the pulp by way of a series of steam-heated rollers that stretch the web somewhat, removing the moisture. Additional sizing agents are added to the web to alter its characteristics, and these may include resins, glue, or starch. Sizing improves the paper's water resistance, decreases its ability to fuzz, reduces abrasiveness, and improves its printing properties and surface bond strength.
Calender section
The calender stack is a series of rollers that the web is run between in order to further smooth it out, which also gives it a more uniform thickness. The pressure applied to the web by the rollers determines the finish of the paper, and there are three types of finish that the paper can have. The first is machine finish, and can range from a rough antique look to a smooth high quality finish. The second is called a super-calendered finish and is a higher degree for fine-screened halftone printing.
The third type of finish is called a plater finish, and whereas the first two types of finish are accomplished by the calender stack itself, a plater finish is obtained by placing cut sheets of paper between zinc or copper plates that are stacked together, then put under pressure and perhaps heating. A special finish such as a linen finish would be achieved by placing a piece of linen between the plate and the sheet of paper, or else an embossed steel roll might be used. The web is then wound onto a roll after calendering, with a moisture content of about 6%, and stored for final cutting and / or shipping.
Alternate papermaking techniques
While the Fourdrinier machine uses a wire conveyor to create the web, another type of machine makes use of a cylinder mold that rotates while partially immersed in a vat of dilute pulp. The pulp is picked up by the wire and covers the mold as it rises out of the vat. A couch roller is pressed against the mold to smooth out the pulp, and picks the wet sheet off of the mold, passing it along for further treatment by equipment similar to the remaining elements of the Fourdrinier machine. A handmade paper is produced in low volume and requires the papermaker to scoop pulp out of a vat with a screened frame. The pulp is pressed gently and allowed to dry. Handmade papers are becoming fashionable once again, especially as special-order wedding invitations, and can even be bought in office supply stores.