History of paper a brief:
Paper was first discovered in ancient China, and the first material used for writing was a "tree" called papyrus. These papyrus materials are not defined as real paper. It was first documented in China during the Eastern Han period
(25-220 CE), usually told to a court official. Spread to the Islamic world during the 8th century. The algorithm of the 11th century was brought to medieval Europe. Later in the 19th century came the Western developments in the field of paper with the invention of wood-based papers.
Paper making:
Paper making is the manufacture of paper and cardboard, which are used widely for printing, writing, and packaging, among many other purposes. Today almost all paper is made using industrial machinery and Automation.
The stock passes through one or two head boxes onto the paper machine. The paper web is formed from the head box onto the wire and dewatered through the wire primarily by the action of gravity and suction/vacuum. Further dewatering by mechanical means takes place in the press section where water is taken out of the sheet by pressing between felts. The final drying takes place in the drying section of the paper machine where the sheet runs against steam heated cylinders to get its final dryness of 91-93%. The collected water is reused for diluting the thick stock coming from the stock preparation.
Semi Chemical Fluting is a paper with just one ply and therefore the paper machine has one head box and one wire. Kraft-liner is normally a two-ply product and therefore requires a paper machine with two head boxes and usually two wires. The base brown ply contains the internal machine broke pulp in addition to wood fibres from the usually integrated pulp production and could also contain recycled paper pulp. The top ply is normally wood pulp from the integrated pulp production that is more refined and cleaner to give the top surface the right characteristics and printability. For white surface grades bleached fibres are of course used for the top ply.
Test liner mostly consists of two plies of paper. Depending on the type of Test liner the fibre composition of mixes of types of recycled paper can be different in each layer. In general a better grade of mix is used for the upper layer for reasons of appearance and strength. In order to increase its strength Test liner receives a surface treatment in the size press. This involves the application of a starch solution to one or both sides of the sheet. The top ply of Test liner is given an even, mostly brown color by coloring the mass or by means of the size press treatment. This coloring is never given to White Top Test liners. The addition of special additives makes it possible to produce Test liners with special properties such as extra water-repellent, low-germ and anti-corrosion grades.
Recycled Fluting can be a one-ply or two-ply product. Usually a size press treatment with a starch solution is applied in-line on the paper machine in order to obtain sufficient strength and stiffness properties. The most common surface treatment of recycled fibre based corrugated board materials is done by a size press. Essentially a size press comprises two revolving rubber covered rolls, pressed together, through which the paper web passes. In the nip formed by the rolls there is a starch solution. The paper absorbs some of this solution, is pressed between two rolls and goes into the "post dryer” section of the
paper machine in order to evaporate from the paper excess water absorbed from the starch solution in the size press.
After the paper machine there is a slitter winder where the big jumbo reel from the paper machine is rewound and cut down to customer reel formats according to customer orders. Finally the reels are weighed, marked, labelled and prepared for shipment to the customer, the corrugated board industry.
Important parts of paper machine:
1. Head box. 2. Forming section.
3. Press section. 4. Pre-Dryer section.
5. Size press. 6. Post dryer section.
7. Calendar. 8. Pope reel.
9. Rewinder.
1. Head box:
As mentioned, head box is one of the most important sub processes of paper machine. The function of head box is to spread the stock uniformly on the wire. That means, head box is designed to
cause a steady state movement of the pulp to wire. Head box is generally categorized as:
Further, pressurized head box is categorized into,
Most of the paper machines use pressurized head boxes in which the pressure is maintained through pumps (such as fan pump).
The Fourdrinier table of a paper machine consists of foils, table rolls, breast roll, couch roll, suction boxes,
wire rolls, and other parts. The fabric on which the sheet of fibre is formed is a finely woven synthetic fabric of strands made of especially fine drawn and filaments, woven into a screen. Various types of weave are used to obtain maximum fabric life and to reduce wire marking on the wet sheet.
The foils are located under the forming wire. In addition to supporting the wire, the negative pressure created in the expanding nip on the underside of the fabric, causing drainage of the water through the wire. Older paper machines sometimes use table rolls for this purpose. However, most modern paper machines now use foils. After the foil section on the forming table, the moving fabric passes over a series of vacuum boxes and then over a couch roll. Often, a dandy roll is located on top of the forming fabric over the suction boxes. This is a light open structured roll covered with wire cloth, resting lightly upon the surface of the sheet. Its function is to flatten the top surface of the sheet and improve the finish. A pattern on the dandy roll may leave translucent patterns on the wet paper, in the form of names, insignia or designs. These are called watermarks.
The paper web formed on the wire part or the forming section passes onto the press section. Pressing of the web on the paper machine follows entering of wet web through the nip of two rolls running under pressure. Under the effect of pressure between the two rolls
further water removal of paper is obtained and its compactness and strength is increased. The pressing operation is important for the capacity and economy of a paper machine and has great influence on paper quality. It determines the dryness of the sheet entering the dryer section. It has a major impact on the structure and runnability of the paper and thus the operating efficiency of the machine. The dewatering capacity of a press section and the properties of paper depend on the design and the number of nips. The risk of web breaks is determined by the general design of press section and the arrangement of open draws if any.
Types of press:
Several general type of wet presses are used on paper machines. Press arrangements are combination of various types. All have the primary function of water removal and secondary functions such as transfer of the web from one felt to another, smoothing the web surface or otherwise affecting paper properties.
a) Plain Press
b) Double felted press
c) Suction press
d) Hot press roll
e) Extended Nip Presses
Designing of press:
Pressing on a paper machine typically takes place in three or sometimes four press nips. The wet web is transferred to the press form the forming section by a “suction pick up” at the couch roll. In this typical three-nip press, the first nip is made up of a suction roll and granite roll with the second nip being another suction roll on the same granite roll. This is followed by a free draw, followed by a third press made up of a granite roll and suction roll.
4. Pre-Dryer section:
Drying in paper making serves two functions. First, it removes the remaining water in the web that cannot be removed by vacuum or pressing. Second, it causes fibres to bond together by hydrogen bonding.
Drying Stages:
Drying is achieved by raising the web temperature in the web to a level at which the vapor pressure of water in the paper exceeds the partial pressure of water vapour in the ambient. This pressure difference is the driving force for the evaporation of water from the
web.
The drying process consists of a) heat transfer to the web and b) mass transfer of vapor from the web. Drying of paper has three stages.
The first stage is a warm up stage. The second stage is a “constant-rate stage”. Here the drying rate is constant because sufficient water remains in the web that heat and mass transfer within
the web are not rate controlling steps.
The third stage is the “falling-rate stage”. Here there is not sufficient water to completely fill the web. Water near the paper surface in contact with the hot roll evaporates, causing complex
mass transfer of vapor and liquid diffusion within the web. Water near the hot surface evaporates and diffuses outward. Some liquid flows in towards the hot surface. The final point of evaporation takes place from both surfaces.
On the paper machine, the size press is used to apply surface size to dried paper. Starch is the most frequently used binder in surface sizing. Besides raising surface strength, starch also imparts stiffness, lowers water sensitivity, reduces dimensional changes and raises air leak density of the sheet.
After size press paper web is passed through a post dryer section. It is group 4-8 cylinders. In this section paper
web is drying more. After size press paper web is passed through a post dryer section. It is group 4-8 cylinders. In this section paper web is drying more.
7. Calender:
After the dryer, the paper web is passed through a calender stack. This is a vertical stack of solid iron rolls which impart successively higher compression cycles to the paper as the paper passes downward through the stack. The function of the calender stack is to reduce the thickness and to impart a smooth surface to the paper web for good printability.
A typical pope reel is shown in Figure 1. It has a set of arms that pivot around the reel drum called primary arms. Another set of arms that have pivots slightly above floor level are called secondary arms. Both sets of arms control the nip load between the winding parent roll and the reel drum. An empty spool is loaded and waiting in the primary arms. When the parent roll in the secondary arms grows to a desired size, a spool starter accelerates the spool to a surface speed slightly faster than the reel drum. The primary arms rotate to a turn-up position where the spool is lowered onto the reel drum. At this time, a turn-up device such as a goose neck, water jet turn-up device or a tape turn-up break or cut the web and cause it to begin winding on the empty spool. The secondary arms pivot the full parent roll away from the reel drum and release it to a stopping station where brakes are used to stop its rotation. The primary arms simultaneously lower the new spool onto the rails. The secondary arms pivot back towards the drum and contact the spool of the new parent roll. The primary arm clamps then open which allows the primary arms to rotate back to the raised position in preparation to receive another empty spool and repeat this sequence.
9. Rewinder:
A parent reel is a large paper machine wide roll. Its diameter is typically from 2 to 4 meters and its width from 2 to 10 meters; it can weigh over 100 tons. It is not reasonable to transport reels of this size. They are also too large for printing houses. Paper winders cut and wind the large reels coming from the paper machine to customer rolls. From one parent reel, several sets of customer rolls can be made. There are basically two types of paper winders nowadays: two-drum and multi-station. In the two-drum winder, rolls are lying on two drums side by side. The weight of the rolls is directed onto contact points between the rolls and drums. This contact point is called a nip. In multi-station winders, each roll is wound independently in separate stations.
1. The Unwinding Stage:
The unwinding stage consists of an unwind stand onto which the parent roll of paper is loaded and secured. The stand holds the roll about its central axis and allows it to rotate. The roll is held either by inserting two chucks each with mechanical griping elements on both ends of the roll which expand to grab it, or by clamping the two exposed ends of a shaft which runs along the whole length of the roll’s core on bearings on the back stand.
When initially unwinding heavy rolls, large inertial forces are experienced which may tear the paper or deform pressure sensitive materials. This is avoided by using driven unwinds which can
be braked when necessary to maintain the accurate tension of the material during the process.
2. The Slitting Stage:
The slitting stage is where the actual separation of the web into multiple smaller slits occurs. From the winding stage, the unwound continuous web of paper is passed through a series of spreading rollers and through the slitter head. The slitter head is made up of a series of either rotating knives or razors and it does the cutting of the web. The desired slit widths are set manually or automatically depending on the type of machine being used by varying the locations of the knives on the slitter head. Before the slitter head are rollers to straighten the sheet to ensure good quality cuts.
3. The Rewinding Stage:
From the slitter head, the slits are rewound onto smaller cores on the rewind stand to the desired footage or diameter. Various factors such as the type of material being wound and the desired hardness of the final roll determine the type of winder to be used under the given conditions. The rollers which are after the slitter head keep the slit sections separate as they enter the winding and prevent the sheet from weaving back and forth between adjacent rolls in the roll set.
Comments
Post a Comment