Paper manufacture

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PAPER MANUFACTURE VANLALNGHAKA UG116143

Transcript of Paper manufacture

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PAPER MANUFACTURE

VANLALNGHAKAUG116143

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CONTENTS

INTRODUCTION THE PAPER MACHINE THE HEADBOX THE FOURDRINIER WET END

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1.INTRODUCTION

Paper consists of a web of pulp fibers (normally from wood), usually formed from an aqueous slurry on a wire or screen, and held together by hydrogen bonding.

The basic steps are same for either hand- or machine-made paper .

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INTRODUCTION

The steps are as follows: 1. Forming - applying the pulp slurry to a screen. 2. Draining - allowing water to drain by means of

a force such as gravity or a pressure difference developed by a water column.

3. Pressing - further dewatering by squeezing water from the sheet.

4. Drying - air drying or drying of the sheet over a hot surface.

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2.THE PAPER MACHINE

The paper machine is a device for continuously forming, dewatering, pressing, and drying a web of paper fiber. Until recently, the most common type of wet end was the fourdrinier, where a dilute suspension of fibers (typically 0.3-0.6% consistency) is applied to an endless wire screen or plastic fabric.

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THE PAPER MACHINE

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THE PAPER MACHINE

Water is removed by gravity, or the pressure difference developed by table rolls, foils or suction equipment, and the drilled couch.

The web at this point is 18-23% consistency. More water is squeezed out in the press section

to a consistency of 35-55%. Finally the sheet is dried with steam heating in

the dryer section.

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THE PAPER MACHINE

Machines that use two wires to form and drain water from the dilute, pulp slurry are called twin wire formers. These have become popular since the late 1960s for printing and lightweight papers.

During the 1970s, many multi-ply formers were developed for heavyweight board.

These formers use up to seven wires consecutively and are modifications of another type of paper machine, used for heavy-weight board, the cylinder machine, which has been used since the early 1800s.

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THE PAPER MACHINE

For every 1 lb of fiber 200 lb of water are used. For this 1 lb of fiber about 195 lb of water (98%) are

removed at the wet end with the web leaving at a consistency of 20%.

Another 2 to 3 lb of water are removed in the press section (1 to 1.5%) with the web leaving the press section at 35-50% consistency. The remaining 1 to 1.5 lb (0.5 to 0.75%) of water are removed by the dryer section.

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3.THE HEADBOX

Headbox It is a pressurized device that delivers a uniform pulp

slurry on the wire, through the slice, at approximately the same velocity as that of the wire.

The main purpose of the headbox is to convert a low consistency fibre suspension flow (stock) in a round pipe into a uniform flat jet several meters wide that will fall on the wire to start the dewatering process.

Original headboxes were open, unpressurized, and used a hydrostatic head for the necessary pressure.

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THE HEADBOX

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THE HEADBOX

Almost all headboxes on paper machines operating below speeds of 2500 ft/min have two to five perforated rectifier rolls (holey rolls) inside the headbox that create microturbulence to keep the fibers in suspension, giving even formation.

The rolls are 0.2-0.8 meters (8-32 in.) in diameter with holes 1.5-2.5 cm (0.6-1.0 in.) in diameter that occupy

40-52 % of the roll area and rotate at 5-20 rpm. A secondary headbox may be used part of the way down the

table to give a top coat of high quality fiber relative to the rest of the sheet.

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THE HEADBOX

Originally, the pressure to accelerate the stock to the speed of the paper machine was supplied by a hydrostatic head of liquid.

But high speed paper machines require too much pressure for this to be practical, so paper machines now used closed, pressurized headboxes to supply the necessary pressure.

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THE HEADBOX

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THE HEADBOX

Slice It is a rectangular slit in the headbox where the pulp

slurry is applied to the wire. It consists of a lower, fixed apron and an upper,

adjustable lip controlling the slice height. The slice height controls how much stock is applied to

the wire (and therefore the basis weight). The slice height is variable across the width of the paper

machine to insure uniformity of the paper across the width of the paper machine.

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4.THE FOURDRINIER WET END

Fourdrinier The fourdrinier, or flat wire machine, is a paper

machine with a horizontal, moving, fine mesh, woven wire cloth or plastic fabric upon which the pulp slurry is deposited, forming the web.

The front side or tending side is the side from which the paper machine is controlled, whereas the other side is the backside or drive side.

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THE FOURDRINIER WET END

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THE FOURDRINIER WET END

Clothing Paper machine clothing consists of forming fabrics (wires),

press felts, and dryer felts. Wire, forming fabric It is a continuous loop or belt of finely woven screen made

from wire or plastic; the mesh size varies from 40 to 100 mesh (openings per inch).

Before 1960, wires were made from metals such as bronze but now they are mostly made from polyester which is corrosion resistant and lasts much longer, although it stretches more and cannot handle highly abrasive furnishes.

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THE FOURDRINIER WET END

The forming media has three functions: 1. to transport the fiber. 2. to permit draining the sheet. 3. to transmit power.

The power input is usually through the couch and wire turning roll.

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THE FOURDRINIER WET END

Wire showers Wire showers (Fig. 9-9) are high pressure showers on

the underside of the wire used to remove fillers and other material which may plug the wire.

Usually, they move back and forth slowly to clean all parts of the wire and prevent wire wearing in one spot.

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THE FOURDRINIER WET END

Web The continuous mat of fibers that is in the process of forming or

which has already formed the final paper is known as the web. Forming board The forming board (Fig. 9-12) consists of a large leading blade to

reach in close to the slice followed by several smaller blades, usually with gaps between them.

Breast roll The breast roll is located under the headbox and serves to return the fabric to the forming area to receive the

stock once again. It must be rigid enough to resist deflection.

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THE FOURDRINIER WET END

Deckle board They are used to prevent the stock from flowing off the

two sides of the forming fabric when a thick layer of stock is delivered to the fabric by a large slice opening.

Trim squirts, edge squirts The width of the sheet delivered to the press section is

controlled by trim squirts. The narrow bands of stock left on the fabric are knocked off by edge squirts.

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THE FOURDRINIER WET END

Table roll Table rolls are freely revolving rolls under the

fourdrinier wire that support the weight of the wire and wet web.

Water is removed from the bottom of the web by a partial vacuum at the meniscus that forms at the nip where the roll leaves the wire.

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THE FOURDRINIER WET END

Foils A foil introduced by Wrist and Burkhard (1956), is a

stationary blade 5-10 cm wide with a divergent surface so that an angle forms between the stationary fabric and the foil surface. The angle is usually between 0.5 to 3°.

Lovacs A lovac (Hansen, 1991) develops suction using water-filled

drop legs to provide siphoning action. These are sometimes called wet boxes.

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THE FOURDRINIER WET END

Dandy roll The dandy roll (Fig. 9-27) is a hollow, light, wire

covered roll that rides on top of the fourdrinier wire just ahead of the suction boxes.

This roll has four purposes: 1. to impart a water mark to the sheet. 2. to improve the top surface for printing. 3. to improve formation by mechanical shear. 4. to increase the drainage capacity of the flat wire.

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THE FOURDRINIER WET END

Couch roll The couch roll is the guide or turning roll for the

fourdrinier wire, where the paper web leaves the wire and the wire returns to the breast roll.

It has two purposes: to transmit power to the fabric and to increase the dryness of the sheet.

Lump breaker roll The lump breaker roll (Fig. 9-28) is a solid roll with a

suitable soft cover that is mounted over the couch roll to assist in drying the sheet by pressure.

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THE FOURDRINIER WET END

Pickup (transfer) felt The pickup felt is a traveling felt (blanket)

designed to pick the wet paper web off the wire and transfer it to press section. This means that

the web is supported at all times by either the wire or felt.

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THE END

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