Paper machine
A paper machine is an industrial machine which is used in the pulp and paper industry
to create paper in large quantities at high speed. Modern paper-making machines are based on the principles of the Fourdrinier Machine, which uses a moving woven mesh to create a continuous paper web by filtering out the fibres held in a paper stock and producing a continuously moving wet mat of fibre. This is dried in the machine to produce a strong paper web.
The basic process is an industrialised version of the historical process of hand paper-making, which could not satisfy the demands of developing modern society for large quantities of a printing and writing substrate. The first modern paper machine was invented by Louis-Nicolas Robert in France in 1799, and an improved version patented in Britain by Henry and Sealy Fourdrinier in 1806.
The same process is used to produce paperboard on a paperboard machine.
Process sections
Paper machines usually have at least five distinct operational sections:- [|Forming section], commonly called the wet end or wire section, is a continuous rotating wire mesh which removes water from the paper by sucking it out of suspension via vacuum.
- [|Press section], where the wet fibre web passes between large rolls loaded under high pressure to squeeze out as much water as possible.
- [|Drying section], where the pressed sheet passes partly around, in a serpentine manner, a series of steam heated drying cylinders. Drying removes the water content down to a level of about 6%, where it will remain at typical indoor atmospheric conditions. Infra-red driers may also be also used to supplement cylinder drying where required.
- [|Size Press section], where the semi-dried paper is applied with a thin layer of starch and/ or other chemicals to improve several paper properties, reduce dusting and air permeability, increase stiffness, bursting strength and short span compression
- [|Calender section], where the dried paper is smoothened under high loading and pressure. Only one nip is necessary in order to hold the sheet, which shrinks through the drying section and is held in tension between the press section and the calender. Extra nips give more smoothing, but at some expense to paper strength. The calendar section can either be a part of the machine or in a separate part of the paper mill.
- [|Reel section], where paper coming out of the machine is wound onto individual spools for further processing.
History
Before the invention of continuous paper making, paper was made in individual sheets by stirring a container of pulp slurry and either pouring it into a fabric sieve called a sheet mould or dipping and lifting the sheet mould from the vat. While still on the fabric in the sheet mould, the wet paper was pressed to remove excess water. The sheet was then lifted off to be hung over a rope or wooden rod to air dry.Fourdrinier machine
In 1799, Louis-Nicolas Robert of Essonnes, France, was granted a patent for a continuous paper making machine. At the time, Robert was working for Saint-Léger Didot, with whom he quarreled over the ownership of the invention. Didot believed that England was a better place to develop the machine but due to the turbulence of the French Revolution, he could not go there himself, so he sent his brother-in-law, John Gamble, an Englishman living in Paris. Through a chain of acquaintances, Gamble was introduced to the brothers Sealy and Henry Fourdrinier, stationers of London, who agreed to finance the project. Gamble was granted British patent 2487 on October 20, 1801. The Fourdrinier machine used a specially woven fabric mesh conveyor belt in the forming section, where a slurry of fibre is drained to create a continuous paper web. The original Fourdrinier forming section used a horizontal drainage area, referred to as the drainage table.With the help of Bryan Donkin, a skilled and ingenious mechanic, an improved version of the Robert original was installed at Frogmore Paper Mill, Apsley, Hertfordshire, in 1803, followed by another in 1804. A third machine was installed at the Fourdriniers' own mill at Two Waters. The Fourdriniers also bought a mill at St Neots intending to install two machines there, and the process and machines continued to develop.
Close to Frogmore Mill in Apsley, John Dickinson designed and built an alternate machine type; a Cylinder Mould Machine in 1809.
Thomas Gilpin is most often credited for creating the first U.S. cylinder type papermaking machine at Brandywine Creek, Delaware in 1817. This machine was a cylinder mould machine. The Fourdrinier machine wasn't introduced into the USA until 1827.
Similar designs
Records show Charles Kinsey of Paterson, NJ had already patented a continuous process papermaking machine in 1807. Kinsey's machine was built locally by Daniel Sawn and by 1809 the Kinsey machine was successfully making paper at the Essex Mill in Paterson. Financial stress and potential opportunities created by the Embargo of 1807 eventually persuaded Kinsey and his backers to change the mill's focus from paper to cotton and Kinsey's early papermaking successes were soon overlooked and forgotten.Gilpin's 1817 patent was similar to Kinsey's, as was the John Ames patent of 1822. The Ames patent was challenged by his competitors, asserting that Kinsey was the original inventor and Ames had been stealing other people's ideas, their evidence being the employment of Daniel Sawn to work on his machine.
Related inventions
The method of continuous production demonstrated by the paper machine influenced the development of continuous rolling of iron and later steel and other continuous production processes.Pulp types and their preparations
The plant fibres used for pulp are composed mostly of cellulose and hemi-cellulose, which have a tendency to form molecular linkages between fibres in the presence of water. After the water evaporates the fibres remain bonded. It is not necessary to add additional binders for most paper grades, although both wet and dry strength additives may be added.Rags of cotton and linen were the major source of pulp for paper before wood pulp. Today almost all pulp is of wood fibre. Cotton fibre is used in speciality grades, usually in printing paper for such things as resumes and currency.
Sources of rags often appear as waste from other manufacturing such as denim fragments or glove cuts. Fibres from clothing come from the cotton boll. The fibres can range from 3 to 7 cm in length as they exist in the cotton field. Bleach and other chemicals remove the colour from the fabric in a process of cooking, usually with steam. The cloth fragments mechanically abrade into fibres, and the fibres get shortened to a length appropriate for manufacturing paper with a cutting process. Rags and water dump into a trough forming a closed loop. A cylinder with cutting edges, or knives, and a knife bed is part of the loop. The spinning cylinder pushes the contents of the trough around repeatedly. As it lowers slowly over a period of hours, it breaks the rags up into fibres, and cuts the fibres to the desired length. The cutting process terminates when the mix has passed the cylinder enough times at the programmed final clearance of the knives and bed.
Another source of cotton fibre comes from the cotton ginning process. The seeds remain, surrounded by short fibres known as linters for their short length and resemblance to lint. Linters are too short for successful use in fabric. Linters removed from the cotton seeds are available as first and second cuts. The first cuts are longer.
The two major classifications of pulp are chemical and mechanical. Chemical pulps formerly used a sulphite process, but the kraft process is now predominant. Kraft pulp has superior strength to sulphite and mechanical pulps and kraft process spent pulping chemicals are easier to recover and regenerate. Both chemical pulps and mechanical pulps may be bleached to a high brightness.
Chemical pulping dissolves the lignin that bonds fibres to one another, and binds the outer fibrils that compose individual fibres to the fibre core. Lignin, like most other substances that can separate fibres from one another, acts as a debonding agent, lowering strength. Strength also depends on maintaining long cellulose molecule chains. The kraft process, due to the alkali and sulphur compounds used, tends to minimize attack on the cellulose and the non-crystalline hemicellulose, which promotes bonding, while dissolving the lignin. Acidic pulping processes shorten the cellulose chains.
Kraft pulp makes superior linerboard and excellent printing and writing papers.
Groundwood, the main ingredient used in newsprint and a principal component of magazine papers, is literally ground wood produced by a grinder. Therefore, it contains a lot of lignin, which lowers its strength. The grinding produces very short fibres that drain slowly.
Thermomechanical pulp is a variation of groundwood where fibres are separated mechanically while at high enough temperatures to soften the lignin.
Between chemical and mechanical pulps there are semi-chemical pulps that use a mild chemical treatment followed by refining. Semi-chemical pulp is often used for corrugating medium.
Bales of recycled paper for unbleached packaging grades may be simply pulped, screened and cleaned. Recycling to make white papers is usually done in a deinking plant, which employs screening, cleaning, washing, bleaching and flotation. Deinked pulp is used in printing and writing papers and in tissue, napkins and paper towels. It is often blended with virgin pulp.
At integrated pulp and paper mills, pulp is usually stored in high density towers before being pumped to stock preparation. Non integrated mills use either dry pulp or wet lap pulp, usually received in bales. The pulp bales are slushed in a pulper.