Paper and board production - abstract
Web forming
Press section
Press section effect on paper properties
Drying
Paper and board production - abstract |
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Web forming |
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Press section |
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Press section effect on paper properties |
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Drying |
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Paper and board production - abstract |
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Hydrogen bonds |
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Rawmaterials of paper and board |
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Papermaking concepts on different paper grades |
The most common paper production process is based on wet web forming in which the raw material components of paper mixed with water form a stock suspension. Water functions as a conveyor for the raw material composition enabling its homogeneous distribution.
In the papermaking process the raw material components appropriately selected
and treated are mixed together to form a stock suspension. Then the suspension
is distributed to a thin web which is reinforced
by removing water from it by drainage 
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By means of pressing and vapourizing , the
rest of the water in web is removed and dry paper is produced.
Paper production is a continuous process, whose resulting end product is ready for finishing processes.
Important difference between board and paper is that board is usually multi-layer web and its basis weight is higher.
The paper holds together principally by means of hydrogen bonds between fibers, which are created in the drying process of paper at a dry content of about 70 to 80 %. Fiber and other raw material distributions produced in web forming have a decisive influence on the strength of creating hydrogen bonds.
The most important component of paper and board raw materials are fibers, generally wood fibers. Since the properties of wood fibers in different pulp types vary to a great extent, then also the use of mechanical pulps, chemi-mechanical pulps, chemical pulps and recycled pulps is very much subject to change on different paper and paperboard grades. Other raw materials are fillers, adhesives, chemical additives and different coating agents.
The share of raw materials and filler as well as their treatment is selected as economically as possible paying attention to the requirements of each paper grade. Also the paper production methods and particularly the web forming methods and the machine concepts vary grade by grade.
Papers:
Printing paper grades with large production volumes
are manufactured in the largest possible production units using the most economic possible raw materials. The main raw material is mechanical pulp; for newsprint also recycled fibre pulp is used. A typical basis weight range of paper is from 35 to 55 g/m2 , the production speed from 1200 to 1600 m/min, and the web width exceeds eight meters.
As regards the production level and width, the fine paper machines also belong to the same size class as the machines for woodcontaining grades. Large fine paper machines most often produce
A typical production speed range is from 1000 to 1300 m/min and the basis weight range from 50 to 90 g/m2 .
In the production of fine and special paper grades chemical pulps are mainly used, which are clearly more expensive than mechanical pulps. The production lots of special papers are normally small; a lot of flexibility is required from the production line and the machines are generally slower and narrower.
The basis weight range of special papers is extremely wide varying between 40 and 250 g/m2 . The running speeds range from 400 to 1100 m/min and the web width from three to eight meters.
Boards:
For most board grades different furnish types are utilized not only as compositions but also for forming in layers. The most important packing board grades are packaging boards:
and container boards
The number of special board grades is very high, but the production volumes compared to the previously mentioned grades are rather small.
Production of board machines are usually very high because of the high basis weight. For packaging box board the basis weight range is between 170 and 450 g/m2, velocity 300...600 m/min and width of machine from 3 to 6 m. For liner board machine the basis weight range is between 100 to 330 g/m2 , velocity from 500 to 900 m/min and width of machine up to 9 m.
Packaging boards are normally formed in three or four plies. Due to high surface quality requirements the surface ply is produced from chemical pulps and in most cases the top surface is double coated. For the centre ply, less expensive mechanical pulp is used, or recycled fibre on White Lined Chipboard. In this way it is possible to economically produce the high stiffness level required from boxboard. The stiffness demand also requires a bulk (thickness) saving machine concept from the packaging board machine.
The back ply in boxboards is often a composition of pulp and mechanical stock and sometimes the back surface is single-coated. Linerboards are normally produced totally from chemical pulp. Back ply and optional centre ply of testliner is produced from recycled fibre pulp. The furnish for fluting is normally recycled fiber pulp.
Web forming |
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Headbox |
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Wire section |
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Web forming section effect on paper properties |
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The web forming section on a paper machine comprises
. It seems that it is particularly the forming section configuration in the paper machine that is extremely readily affected by the paper grade requirements.
Headbox is located between short circulation and wire section
. In the headbox, stock suspension is distributed
evenly across the width of the wire in a controlled way .
The tasks of the headbox and the approach piping are the following:
The tasks of the forming section in web forming are the following:
it through the
forming fabric
The following important structural properties of paper are determined in the web forming section as combined effects of its parts:
(MD/CD-ratio),
orientation profiles in cross machine direction and layered fibre orientation
The function of the press section is to remove
a maximum amount of water from the web and to compress it.
The goal is to achieve a sufficiently high wet strength with the press in order
to ensure that the web is transferred to the drying section without any breaks.
On the other hand, compressing the web enables the formation of strong interfiber
bonds during web drying.
The dry content of the web approaching from the wire is in the 17 to 20% range. The dry content level after the press is from 35 to 50 %, depending on the paper grade and press design. A 1 % increase in dry content at the press will diminish the dryer-section steam consumption by 3-4 %. At a specific web moisture level, pressing is the most economical way of drying the web.
Wet pressing takes place either between a press felt and a smooth roll or between
two press felts .
Pressing reduces the web (fiber mat) volume, leaving no more space for water,
which, as a result, is channeled to the felt.
At first, pressing is performed carefully so as not to crush the web formed by the wire. Excessively strong or fast pressing would flush away fines from the web and, in the worst cases, crush the fiber network. In practice, pressing performed in stages is carried out with several nips built up by rolls. When the web proceeds through the press nips, the nip forces can be gradually increased.
Although the objective of pressing is to achieve as high a dry content as possible, the press power present in the last nip cannot be raised as high as the present equipment technology will allow. With excessively high press power applied, the paper sheet thickness would diminish too much, resulting in too low a level of bulk. Secondly, this would shorten the felt service life and cause potential roll or felt marks in the paper. At high press powers, the press would be sensitive to vibration.
Wet pressing has a strong effect on paper properties. The press geometry, rolls and their covers, felts and linear pressure combinations must be selected to conform to the running speed and the paper grade to be produced. Wet pressing affects the following quality properties:
Drying |
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Drying methods |
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Drying effect on paper and board quality |
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The purpose of the dryer section is to remove water from the web through evaporation
. Evaporation must occur efficiently, economically,
evenly and without impairing paper quality.
There are three different methods normally applied to paper and board drying:
Common to all the above methods is that the web is provided with external energy that evaporates water away from the web. Then evaporated water is led with air away from the web. The drying methods differ in principle from each other in the energy supply methods, further resulting in the use of quite different equipment solutions
Drying affects paper and board quality:
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