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J:\Pfib\texth\dok\11-2.doc KEP 2001-11-20 1

FORMING 1, GENERALLY

1. Forming process ..........................................................................................................2

1.1 Dewatering .............................................................................................................2

1.2 Wet web strength.....................................................................................................4

2. Forming and paper properties...................................................................................6

2.1 Formation ................................................................................................................6

2.2 Fibre orientation ....................................................................................................10

2.3 Distribution of the fine material in the Z direction ............................................... 15

3. Factors influencing the forming process .................................................................16

3.1 Forming of fibre flocs ........................................................................................... 16

3.2 Fibre orientation in the sheet .................................................................................273.3 Distribution of the fine material in the Z direction ............................................... 28

CEPATEC AB

Knut-Erik Persson

GalleryGlossaryMain reg

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1. Forming process

1.1 Dewatering

During the forming process, thestock has to be dewatered in a

way which an even fibre

network is be created. A web is

formed.

What influences the dewatering and what makes the fibres in the

network keep together?

If there is a large

amount of fine material

in the stock or if the

fibres are swollen and

soft, the stock drains

less and it takes a longertime to dewater it.

If a higher concentration in the head

box is chosen, the amount of water

leaving the stock decreases and theforming takes place more quickly.

However, the increasedconcen-

tration makes it more difficult

to form the sheet. Therefore, this is

normally not an acceptable way to

speed up the forming process.

Fig. 1. Forming section in a

liner machine. (11-001.tif)

Fig. 3. Microscope

photo. Beaten

chemical fibres.

(Sunds Def.)

(11-003.tif)

Fig. 2. Microscope

photo. TMP pulp.

(STFI)(11-002.tif)

Fig. 4.

(11-004.tif)

Fig. 5.

(11-005.tif)

Fig. 4 and 5. Pictures illustrating

how the stock volume decreases

when the fibre concentration

increases.

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The stock temperature is another factor,

influencing the drainage.

The higher the temperature is, the lower

the water viscosity will be and the faster

the stock will drain.

Adding a retention chemical isanother way to increase the

drainage rate.

Fig. 6. Dewatering on a Four-

drinier machine.(11-006.tif)

Fig. 7. Diagram showing the

connection between temperature

and viscosity of the stock.

(11-007.tif)

Fig. 8.

(11-008.tif)

Fig. 9.

(11-009.tif)

Fig. 8 and 9. Equipment forpre-

paration and dosage of a retentionchemical.

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The dewatering does notdepend

on the stock conditions, only.

The design of the wire section is

importantas well. Dewatering in two

directions is always faster than in one.Consequently, a two-sided dewatering

is often used in new machines.

1.2 Wet web strength

In the finished paper the fibres

bind to each other with

hydrogen bonds. Tomake

these forces work, thefibre

surfaces must be in direct

contact with each other.

However, the wet web has a

certain strength, too. The

reason is the so called surface

tension.

Fig. 10. Two-sided dewatering on

a hybrid machine.(11-010.tif)

Fig. 11. Illustration. Enlarged

surface section showing two fibre

surfaces binding to each other with

hydrogen bonds.(11-011.tif)

Fig. 12. Open transfer of the webfrom the wire to the press section

in an old paper machine.

(11-012.tif)

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Wet fibres are surrounded by

a thin water layer.

When two such fibres get into

contact with each other, the waterlayers will overlap in the contact

point.

Forces, trying to keep the

water layers together, arise

and the fibres will keep

together, too.

Fig. 13. Illustration. Two fibres

surrounded by a water layer.

(11-013.tif)

Fig. 14. Illustration. Two fibres

in close contact. The water layers

are overlapping in the contactpoint.(11-014.tif)

Fig. 15. Illustration. Forces

arising between two wet fibres.

(11-015.tif)


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2. Forming and paper properties

2.1 Formation

The forming processmeans a lot for all paper properties. Inthis part of the presentation, some paper properties strongly

related to the forming are presented.

The local distribution of the

fibres in a paper is called

paper formation.

A simple, but not always

correct, way to judge the

formation is to view the

paper in transmitted light. If

the formation is bad, the

paper seems to be patchy

and is said to have a wild

look-through.

Fig. 16. Microscope photo. Fine

paper. (STFI) (11-016.tif)

Fig. 17. Visual judgement of the

paper formation.(11-017.tif)

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Of course, instruments in the mills

can measure the formation more

exactly.

A better formation makes

the paper more even and

improves its printability.

The formation influences

the paper strength,but

how much depends on howa certain degree of formation

is achieved.

A good formation will notonly be of importance for

the properties of the

finished paper. It willalso

enhance the production of

the paper.

Fig. 18. Sensor measuring the paper

formation.(11-018.tif)

Fig. 19. Picture from a printing office.

(Norra Skne) (11-019.tif)

Fig. 20. Reeling up of a kraft liner.

(11-020.tif)

Fig. 21. Paper machine for liner

production. (11-021.tif)


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With good formation, it will

become easier to dewater and

press the web.

However, above all, the formation

means the most during the drying

process.

When the fibres dry, they shrinkcrosswise and become thinner. In

the cross point the fibres are fixed

together. A fibre shrinking

crosswise compresses another

fibre lengthwise. As a

consequence, the whole sheet

shrinks.

If the formation is bad, the paper

dries and shrinks unevenly. There

will be tensions in the paper and it

may get a cockled finish.

Fig. 22.

Fourdrinier

section.

(11-022.tif)

Fig. 23.

Press nip.

(11-023.tif)

Fig. 24. Drying cylinders in a multi

cylinder dryer.(11-024.tif)

Fig. 25. Microscope picture

showing how the overlying fibre is

compressed lengthwise when the

underlying fibre shrinks crosswise.

(STFI) (11-025.tif)

Fig. 26. An example of a paper with acockled finish. (11-026.tif)


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A paper that should have a

high gloss has to be

calendered. At this

operation the formation is

most important.

If the paper sheet has a bad

formation, the thicker parts

of the sheet will be harder

pressed than the thinner

ones. As a result, these

points will get a higher

gloss. The paper will become

top calendered.

If the paper has a very

uneven formation the thick

parts may still be moist

when the paper leaves the

drying section.

Moist parts are easier compressed

in the calender. The freesur-

faces in the paper sheet, which

can reflect the light are reduced on

those points. The spots become

more transparent.

Fig. 27. Calender. (Twin roll with a

soft nip; soft calender.) (11-027.tif)

Fig. 28. Drying section in a fine

paper machine.(11-028.tif)

Fig. 29. Paper that has been

calendered and made transparent at

certain points. (11-029.tif)


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In another lighting the transparent

parts appear as dark spots.

This is called blackening.

2.2 Fibre orientation

During the forming,the fibres are

not only to be distributed,but also

directed or orientated.This is

another factor influencing the paper

properties.

In many papergradesit is de-

sirable to have the properties the

same aspossible in all directions.

Examples of grades with such

properties are fine paper and

sack paper.

Fig. 30. Transparent parts appearing

as dark spots. (11-030.tif)

Fig. 31. Forming of sack paper.

(11-031.tif)

Fig. 32. Different

types of writing

paper. (11-032.tif)

Fig. 33. Sacks.

(11-033.tif)

Fig. 32 and 33. Examples of grades

where the properties have to be as like

as possible in all directions.


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In fine and sack papers, the fibres

are to be equally orientated in all

directions.

However, sometimes it is

desirable to have the fibres

directed, as much as possible, in

the machine direction.

Newsprint and tissue are

examples of such grades.

Fig. 34. Illustration. Sheet with the

fibres equally orientated in all

directions.(11-034.tif)

Fig. 35. Illustration. Sheet where the

fibres are more orientated in the

machine direction than in the cross

direction.(11-035.tif)

Fig. 36.

Newspapers.

(11-036.tif)

Fig. 37. Tissue

paper.

(11-037.tif)

Fig. 36 and 37. Examples of grades

where the strength has to be higher in

the machine direction than in the crossdirection.

M

M

C

C

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The more the fibres are orientated in the

machine direction, the higher the

strength in that direction will be. A high

strength in the machine direction makes

the paper more resistant to the tensilestress in aprinting press.

With machine direction

orientation the web becomes

stronger and it will be easier

to produce the paper.

Fig. 38. Paper web in a print-

ing press. (11-038.tif)

Fig. 39. (11-039.tif)

Fig. 40.(11-040.tif)

Fig. 39 - 41. Magazine paper machine: forming, pressing, drying.

Fig. 41.(11-041.tif)


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How the fibres are orientated in the sheet

does not only influencethe strength

properties of the web and the finished paper.

During the drying process, the fibre

orientation effects the web also in anotherway.

The fibres always shrink more

crosswise than lengthwise

during the drying. If most fibres

are orientated in the machine

direction, the paper web will

shrink mostly in the cross

direction.

Thus, if the fibre orientation is

different in machine direction, a

different shrinking is achieved inthat part.

Fig. 42. Paper web in the

drying section. (11-042.tif)

Fig. 44.

(11-044.tif)

Fig. 43.

(11-043.tif)

Fig. 43 and 44. Paper web during the

drying. The main shrinking direction is

marked.

Fig. 45.

(11-045.tif)

Fig. 46.

(11-046.tif)

Fig. 45 and 46. Paper web during

drying. The main shrinking directionis marked.


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The fibre orientation is often

different in the edges. This is one

reason, why the edge rolls may

sometimes create problems inthe

paper use.

How the fibres are orientated in the

paper can be estimatedby measuringthe tensile strength in various

directions. However, this is a

detailed and most time consuming

procedure.

With the help of new, modern

instruments the fibre orientation canbe defined safely and quickly.

Sometimes, the fibre orientation isdifferent on the two sides of the

paper, that is one side is more

lengthwise orientated than the other.

Fig. 47. The edges of a paper web

often have a different fibre

orientation compared to the other

part of the web. (11-047.tif)

Fig. 48. Measure of the tensile

strength. (11-048.tif)

Fig. 49. Apparatus for

determination of the main fibre

direction.(11-049.tif)

Fig. 50. Illustration. Paper with

different fibre orientation on thetwo sides.(11-050.tif)


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If the humidity in the atmosphere

surrounding the paper is changed,

thepaper will shrink or widen

differently on its two sides.

The result will be a curly paper and

the phenomenon is called curl.

2.3 Distribution of the fine material in the Z direction

It is not only the paper formation and

the fibre direction in the sheet that

have a direct influence on the paper

properties. The stock contains filler

and fine material, too. How that

material is distributed in thethickness direction of the paper, the

Z direction, is of great importance.

Fig. 51. Sheet pile in a printing

office. (11-051.tif)

Fig. 52. Illustration. Curl.

(11-052.tif)

Fig. 53. Microscope picture. Cross-

section of a paper sheet. Note the

even filler distribution. (STFI)

(11-053.tif)


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A usual problem, in one-sided

dewatering, is that most of the fine

material will be localised closest to the

top side of the paper. The paper is

unequal-sided or two-sided.

Such a paper may give curl.

The surface strength and the printingproperties are also strongly influenced

by an unequal-sided paper.

3. Factors influencing the forming process

3.1 Forming of fibre flocs

We have seen how some important paper properties are influenced by

the forming process. Now, we have to take a step backwards and study

what influences the process as such.

The quality of thesupplied stock and

the conditions

during the

forming are both

of great im-

portance in the

formingprocess.

Fig. 54. Microscope picture.

Cross-section of a paper sheet.

The filler share is here higher

closest to the papers top side.

(STFI) (11-054.tif)

Fig. 55. Offset printing.

(Norra Skne) (11-055.tif)

Fig. 56.

Stock preparation

department.

(11-056.tif)

Fig. 57. Paper

machine.

(11-057.tif)


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3.1.1 Stock fibre properties

The fibres in a stock easily

tangle andbind mechanicallytoeach other,they create flocs.

Such fibre flocs can be quite

stable, so rather great forces are

needed to split them up.

During the forming, fibre

flocs are always created.

If these flocs are not broken down,they

will remain in the finished paper. The

paper will get a bad formation.

Fig. 58. Photo.

Fibre flocs in a

stock. (11-058.tif)

Fig. 59.

Photo. Stock

with broken

fibre flocs.

(11-059.tif)

Fig. 60. Forming section in a board

machine. (11-060.tif)

Fig. 61. Example of papers

with better and worse

formation.(11-061.tif)

WORSE

BETTER

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Thus, fibre flocs cause bad

formation.

What determineshow much

fibreflocs there will be in thestock?

A stationary fibre will occupy aspace equal to its own volume.

The fibres in water follow the

water movements. If there are

whirls, or turbulence, in the water

the fibres will rotate.

The largest possible volume the

fibre can sweep over, correspondsto a sphere with a diameter as

large as the fibre length.

Fig. 62. Forming in a Fourdrinier

machine. (11-062.tif)

Fig. 63. Illustration. Fibre.

(11-063.tif)

Fig. 64. Illustration. Fibre rotating

in water. The sweep volume is

marked.(11-064.tif)


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The fibre in a softwood pulp is

about three times as long as

the fibre in a hardwood pulp.

The sweep volume increases with the

cube of the fibre length. So if the soft

wood fibre is three times as long as the

hardwood fibre, it will sweep over avolume of 333, thus 27 times larger

than that for the hardwood fibre.

However, the hardwood fibre is lighter

than the softwood fibre. In orderto

get the same weight there mustbe

about four short hardwood fibres to

each long softwood fibre.

In spite of the number of hardwood

fibres being four times higher, the shortfibres sweep over a smaller volume

than the long fibres do.

Fig. 65. Microscope

picture. Longsoftwood,

chemical fibres.

(STFI)(11-065.tif)

Fig. 66. Micro-

scope picture.

Short, hardwood

chemicalfibres.

(STFI)(11-066.tif)

Fig. 67. Illustration.

Comparison between the sweep

volume of a short hardwood

fibre and a long softwood fibre

rotating in water. (11-067.tif)

Fig. 68. Illustration. Four short

hardwood fibres have the same

weight as one single softwood

fibre.(11-068.tif)


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The ideal way to form a sheet would be

to have enough space to let the fibre

move freely in the stock until theyare

deposited in the created fibrenet work.

However, forming a paper under suchconditions is not realistic from practical

or economical reasons.

Consequently, the fibres, not being able

to move freely, will tangle and form

flocs.

The risk for such floc formation

increases with the fibre length.

Therefore, forming a sheet from long

fibres requires a lower stock

concentrationthan forming it fromshort fibres.

Fig. 69. Illustration. Few fibres

in water. Here, the fibres can

move freely. (11-069.tif)

Fig. 70. Illustration. Many fibres

in water. Here, the fibres can not

move freely. Fibre flocs are

formed. (11-070.tif)

Fig. 71.

(11-071.tif)

Fig. 72.

(11-072.tif)

Fig. 71 and 72. Illustrations.

Many short and few long fibres in

water.


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Fibres easily form flocs. However, what really happens when a floc is

formed?

If there is not enough space forthe fibres to move freely, the

fibres penetrate into each others

rotation zones. Then, the risk that

the fibres tangleincreases.

The turbulence force in the stock

make the fibres move. As long as

the moving force is greater than the

force hooking the fibres, no flocs

will be created.

The fibres are elastic and therefore,

they bend when they move.

However, if the turbulence decays,the fibres stop moving. An immobile

fibre will take back its natural form.

Fig. 74.

(11-074.tif)

Fig. 73.(11-073.tif)

Fig. 73 and 74. Illustrations showing

how the movement space decreases

when the fibre concentration

increases.

Fig. 75. Illustration. Turbulence

whirls in the stock.(11-075.tif)

Fig. 76. Illustration showing

fibres straightening out when the

turbulence whirls have dis-

appeared. (11-076.tif)


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The fibres, however, prevent each

other from straightening out

completely. As a result, they

bind mechanically to each other,

a floc is then created.

Refined chemical fibres aresofter and more elastic, than

unrefined ones. These

properties make them more

disposed to entangle and lock

each other. Thelonger

the fibres are, the greater

the risk for floc formation.

The fibres in mechanical

pulps are short and stiff.

Such fibres arenot so

easily entangled and

and interlocked.

Consequently, mechanical,

fibres have less tendencyto form flocs.

Fig. 77. Illustration showing how

the fibres lock each other when

they straighten out. (11-077.tif)

Fig. 78. Chemicalfibres. (STFI)

(11-078.tif)

Fig. 79.

Conical

refiner.

(11-079.tif)

Fig. 80. Mechanical

fibres. (TMP).

(STFI)(11-080.tif)

Fig 81.Chip refiner.

(11-081.tif)


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The choice of fibres

depends on which paper

that is to be produced. Thus,

thefurnish for each

paper grade is fixed.

It is during the formingprocess in which

the flocformationcould beprevented.

Before thewebis formed, the stock is

always highly diluted. This dilution is

done in the short circulation.The

longer the fibres are, the more thestock

has tobe diluted.

Fig. 82. Magazine

paper (periodicals).

(11-082.tif)

Fig. 83. Kraft

paper (bags).

(11-083.tif)

Fig. 84. Kraft paper

(sacks). (11-084.tif)

Fig. 85. Flow diagram. The

short circulation.(11-085.tif)

Fig. 86. Dilution of the stock

in the short circulation.

(11-086.tif)


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Diluting the stock is the ideal way

to prevent fibres from forming

flocs.

A paper produced of a highly

diluted stock gets a goodformation.

The better the formation is, the

stronger the paper will be.

However, the stock can not be

diluted too much. The lower the

fiber concentration is, the larger

the stock flow becomes. Soon, an

upper limit will be reached.

Thus, there is maximal limit beyond which the stock can not be

diluted. The next step is to limit the size of those flocswhich in spite

of all are formed.

Fig. 87.

(11-087.tif)

Fig. 88.

(11-088.tif)

Fig. 87 and 88. Photos. Stock before

and after diluting.

Fig. 89. Liner is one example of a

strong paper formed from a highly

diluted stock.(11-089.tif)

Fig. 90. Diluting before the fan

pump. (11-090.tif)


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3.1.2 Stock turbulence

The way to prevent the fibres from forming large flocs is to have enough

stock turbulence.

If the turbulence is strong, the

shearing forces tearing up

the flocs becomesgreater than the

forces keeping them together.

Whetherthe fibre floc isdecreased

only, or dispersed totally, depends

on thecharacter of the turbulence.

When the turbulence is estimated, the intensity and thesizeofthe

whirls must be taken into consideration. How to define the intensity

and thesize of the whirls?

The intensity is thevelocitydifference

between two adjoining whirls.

The sizeis the area influenced by everysingle whirl.

Fig. 91.

(11-091.tif)

Fig 91 and 92. Photos showing stocksbefore and after generating turbulence.

Fig. 92.

(11-092.tif)

Fig. 93. Illustration. The

intensity of whirls.(11-093.tif)

Fig. 94. Illustration. The size

of thewirls, thescale.

(11-094.tif)


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When the diameter of a simple whirl

is approaching the fibre length,

micro turbulence is created.

If the turbulence is morecoarse, the flocs are only

partly broken down.

The smaller the turbulence

whirls are, the greater the

probability to release single

fibres will be.

Fig. 95. Illustration of micro

turbulence.(11-095.tif)

Fig. 96.

(11-096.tif)

Fig. 97.

(11-097.tif)

Fig. 96 and 97. Illustrations showing a

fibre floc being brokendown

by a coarseturbulence.

Fig. 98.

(11-098.tif)

Fig. 99.

(11-099.tif)

Fig. 98 and 99. Illustrations showing a fibre

floc broken downby micro turbulence.


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However, what will happen to the

fibre flocs does not depend on the

turbulence, only. The fibre

properties are also of importance.

If the fibres are long and elastic the

number of points locking each fibre

increases and the forces keeping the

fibres together become greater.

Then, the force needed to separate

the fibres is increased.

Thus, long fibres do not only form flocs easily. The fibre length, too,makes the flocs difficult to break downagain.

The stock turbulence can never totally prevent the fibres from forming

flocs, but it limits the size of the flocs. The smaller the floc size, the

better the paper formation will be.

However, the strength of the paper never becomes as high when the

flocs are broken downagain, as when flocs have never been created.

3.2 Fibre orientation in the sheet

During the formingprocess, the fibres

tend to orientate in the flow direction

of the stock.

The longer and stiffer the fibres are, themore they tendto orientate.

Fig. 100. Illustration. Long fibres

lock each other in many points. The

floc strength becomes great.

(11-100.tif)

Fig. 101. Illustration. Alignment

of the fibres in a flowing stock.

(11-101.tif)

M


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If a local cross flow is generated during

the forming, the fibres will orientate in

the same direction. Thus, the fibre

orientation will become different

compared to the rest of the paper web.

The fact that the fibres tendto orientate in the flow directiondepends on

laws of physics. However, how much the fibres orientate and whichdominating direction they will get in the finished papermay be

influenced during the formingprocess.

3.3 Distribution of the fine material in the Z direction

Stock properties

The more fine material there is in a head box stock, the greaterthe risk becomes to get an unequal-sided sheet.

Single-sided dewatering

When the stock is

dewatered, the fibres form

a connected network. The

fibre network is finer than

the wire cloth, and thethicker it is, the better it

will catch the fine material

of the stock.Conseqently,

the content of fine material will

be higher on the top side of

the paper than on the wire

side.

Fig. 102. Illustration. The

orientation of the fibre in a

flowing stock. Note the cross

flow.(11-102.tif)

Fig. 103.

(11-103.tif)

Fig. 104.

(11-104.tif)

Fig. 103 and 104. Illustration.

Single-sided dewatering.

M


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The largequantities of

water, flowingthrough

the initially formed

fibre layers, wash off

some of the finematerial, too. This is

another reason for the

different amount of

fine material in the Z direction.

A single dewatering causes the content of fine material in paper

to be lower closest to the wire side.

Two-sided dewatering

When dewatering between

two wires, the same thing

will happen.

However, in this case the finematerial becomes more sym-

metrically distributed.It will be

lowest closest to the surfaces and

at the highest in the middle of the

paper. How much depends on how

the dewatering is done. There are

different ways to counteract the

effect and on modern formers the

fine material is quite evenlydistributed.

Fig. 105. Dewatering over foils.(11-105.tif)

Fig. 106. Illustration.

Two-sided dewatering.

(11-106.tif)

Fig. 107. Twin wire

former. (11-107.tif)

Fig. 108. Microscope photo.(STFI) (11-108.tif)


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When dewatering in two

directions, the two fibre

networks are only half as thick

as when the dewatering takes

place in one direction. This,together with a higher pressure

in the dewatering zone, makes

the stock drain very quickly.

However, the pressure pulses

needed to get this quick

dewatering at a good formation,

increases the risk for breaking

up the already formed fibre

nets. If this happens theretention will become lower.

Retention chemicals (Retention aids)

To help bind the fine material to the

fibres, retention chemicals are often used.

The wire retention becomes higher.

Besides, when the fine material forms

flocs which bind to the coarser fibres, the

fine material is more evenly distributed in

the thickness direction of the sheet. The

paper becomes less tight. The porosity is

higher.

During the forming process,the more

even distribution of the fine material is

enhancing the stock drainage.

Fig. 109.

(11-109.tif)

Fig. 110.

(11-110.tif)

Fig. 109 and 110. Illustrations.

One- and two-sided dewatering.

Fig. 111. Illustration showing

how the fibre surfaces catch up

the fine material at the use of

retention chemicals.

(11-111.tif)

Fig. 112. Dewatering over

foils.(11-112.tif)


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Thus, the retention agent improves the

retention and makes the stock to drain

more quickly. However, it is necessary

to be very careful when selecting the

retention agent.

What could happen is that the fine

material binds together and creates

flocs, which can destroy the sheet

formation .

Forming a paper means that the stock is to be dewatered and that the

fibres are to be directed and distributed in the formed network.

However, the quality demands on each paper grade is specific and the

supplied furnish has its special character.

Of course, the demands on the section forming the net work becomes

specific as well. The design of theforming section is to be treated in

the following chapters.

Fig. 113.

(11-113.tif)

Fig. 114.

(11-114.tif)

Fig. 113 and 114 illustrate how

the fine material flocs togetherand fills the vacant space

between the fibres.

http://../pdfbild/11-114.pdfhttp://../pdfbild/11-113.pdf

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