Forming_Fundamentals

7 –1

7. Forming, Fundamentals

Forming section

A Fourdrinier Wet End
The paper web is built on the forming wire at the paper machine. Different equipment is
necessary to separate water and solids.
On the figure below is shown a Fourdrinier Forming Section. It is used to produce mag-
azine papers and newsprint.
Production speed: 500 – 1000 m/min; Basis Weight: 36 – 52 g/m².

Papierzentrum Gernsbach

7 –2

7. Forming, Fundamentals

Basic functions of the Forming Section
1. Sheet Forming
- Distribute the stock over the forming fabric (wire)
- The sheet is formed by dewatering. Removal of as much water as possible on the

forming fabric
- Water removal by filtration

 Dewatering of the stock suspension only in one direction can cause
- Two – Sidedness!

2. Dewatering:
- Through dewatering an additional filter (fibre mat) is created on the forming wire
- Compact the mat to obtain fibre to fibre contact
- Consolidate the web
3. Transport of the stock suspension and the developing fibrous layer

Sheet Forming Process on the forming fabric:
 Out of the stock suspension a fibrous layer (mat) is created.
 The fibrous layer consists out of fibres, chemical additives and water!

Splitting of solids and water by dewatering

Filtration Thickening

Requirements:

 Homogenous distribution of

 solids and  equal fibre orientation

in Cross- and Machine Direction (CD, MD) as well as in the calliper.

Papierzentrum Gernsbach

7 –3

7. Forming, Fundamentals

Principle of the sheet forming process
Fines and fillers are passing the mesh of the forming fabric.
The majority of the long fibres are not passing the mesh openings. This is why at the
beginning of the sheet forming process, an initial layer of long fibres is created first.

 Two – Sidedness!
 The initial long fibre layer acts now like an additional filter. On top of this filter,

fines and fillers are settling.
 The more fines, fillers and long fibres are settling on top of the initial layer, the

more compact the fibre mat will become.
 The more water is removed, the more the volume of the stock suspension is

reduced and the fibre mat becomes more compact.
 Hydrogen bonds: they are not formed yet, because the distance in between of

the fibres is still too large!
Dewatering
Driving force for dewatering through filtration is the pressure difference between supply
side and drain side, caused by:
 Water column,
 Gravity force,
 Vacuum,
 Mechanical forces (pressing),
 Centrifugal forces.

The sheet forming process ends, if the free drainable part of the water is removed!

Initial sheet forming, activity on the wire can be described as follows:

Since the mesh opening has a certain
diameter,

 Water
 Fillers
 Fines
 and some of the short fibres will

drain through the mesh
 mainly long fibres will be kept on the
forming fabric

Papierzentrum Gernsbach

7 –4

7. Forming, Fundamentals

Describe the situation in the figure above:
 Water,  Fillers,  Fines,  and some of the short fibres will drain through
the mesh
 mainly long fibres will be kept on the forming fabric

Size of fibres and fillers, Source Fapet

Papierzentrum Gernsbach

7 –5

7. Forming, Fundamentals

The forming section can be split into four different areas:
1. Impingement zone (Suspension on the wire)

 gentle dewatering action!

 First fibres are fixed onto the wire  initial forming process

 The first fibre layer onto the wire is acting as a filter for the ongoing dewatering
process

 No flowback of the white water, to avoid that the formed fibrous layer gets de-
stroyed.

2: Sheet forming zone
 Ensure a sufficient turbulence in the stock suspension on top of / in between of the

developing fibre mat (s)
 If water is removed too quick, the fibre mat will become too dense!

Use of dewatering units which are able to create turbulences, e.g. hydrofoils

3: Dewatering with a low pressure difference Δp
 Using e.g. wet suction boxes.

4: Dewatering with a high pressure difference Δp

 By increasing the consistency of the fibre mat, the fibre mat / fibrous filter becomes
more and more dense.

 Use of dewatering units with an increased vacuum (pressure difference)

 dry suction boxes (medium level of vacuum)

 Couch roll (relatively high level of vacuum)

12 34

Papierzentrum Gernsbach

7 –6

7. Forming, Fundamentals

Individual Fourdrinier components
Breast Roll and Forming Board

Forming Board

Breast Roll

Breast Roll
The stock passes from the flow box slice onto the endless forming fabric. Water will be
removed out of the suspension, creating a newly formed web of paper.
The breast roll has two functions:
a. Return of the forming fabric to the forming section.
b. Assist initial dewatering. The Gap in between the breast roll and the forming board

can be used for the initial dewatering of the suspension.

Today’s breast rolls are constructed from a steel tube covered with a hard wearing rubber
compound. This combination gives the roll adequate strength to take the tension of the
wire and reduce wear on the roll surface by synthetic forming fabrics.

Papierzentrum Gernsbach

7 –7

7. Forming, Fundamentals

Hydrofoils (foil)
 Pressure pulses are on a lower level compared to the table rolls and they can be

controlled to certain limits
 Ability for dewatering is lower, but more foils can be used in the same space one ta-

ble roll is using. So the dewatering ability could be increased up to 30%.

A stationary surface (foil) over which the forming fabric passes over.
- Very gentle dewatering effect, in respect of sheet forming!
- (Part of the) Suspension is still fluid.

Aim:
- Support of the forming fabric
- Peeling off the water underneath of the forming fabric (a) at the leading edge of
the foil (working as a Deflector)
- Creating turbulences in the stock suspension (part of the suspension is still fluid)

Dewatering Effects:
The dewatering effect is depending on: fabric speed, length of the suction zone and the
foil angle

a. Peeling off the water (front side in machine direction)
b. Vacuum (back side in machine direction)

- Vacuum is created because of the run of the fabric and the foil angle (dy-
namic principle), below 100 m/min wire speed not very efficient.

- Foil angle usually between 0,5° (after the forming table) ... 3,5° (in front of
the suction box)

Papierzentrum Gernsbach

7 –8

7. Forming, Fundamentals

Foil Box:
A number of foils (up to app. 6) can be arranged on top of a box. This is due to space
savings.

Foils are constructed of high density polyethylene or ceramic for greater wear resistance.

Foil box and Vacu Foils

Several foils arranged in Connection to the vacuum pipe
one box-shape element
Drainage of foilbox is
higher stiffnes, supported by low vacuum
less vibrations, (1 – 5 kPa)
narrow spacing possible
Papierzentrum Gernsbach
( turbulence)

Figure Voith Paper

7 –9

7. Forming, Fundamentals

The Dandy Roll

The function of a dandy is to

 imprint a watermark (if necessary) and to

 consolidate the web of paper (improve the closeness and formation)

The dandy is a hollow metal roll with a stainless steel cover and a (watermark soldered
or sewn into the) wire cloth. On slow machines the dandy is driven by the machine wire.
Higher speed machines use an independently driven dandy.

Improve formation, working principle

Free surface

Dandy roll

Wire

Surface of the Velocity
Formed fibre profile
mat

p

+
-

Papierzentrum Gernsbach

7 – 10

7. Forming, Fundamentals

Activities on the wire
 The wire cloth of the dandy roll dives slightly into the stock suspension (gentle com-

pression of the stock suspension!).
 Fibres are gently pulled down to the fibre mat, water and fines are flowing through

the mesh to the surface of the suspension. As a result, a low consistency area is
created on top of the stock suspension. Because of the low consistency and a rela-
tively high fine content, the surface properties of the paper web can be improved
(e.g. formation, smoothness, etc.).
 Due to the action of the dandy roll, created flocks in the suspension are destroyed
and the individual fibres can be formed again.
The dandy is located on top of the wire between the wet suction boxes prior to the dry
line. A sheet moisture content between 94-96% is considered optimum.
Figure shows the dry line

Papierzentrum Gernsbach

7 – 11

7. Forming, Fundamentals

Wet Suction Boxes
The principle of water removal by a vacuum box is the difference in pressure around the
web and the vacuum maintained inside the box.

Figure shows a wet suction box for a low vacuum level (app. 0.1 bar, 10 kPa)
The boxes themselves are rugged in construction with a hard wearing slotted or drilled
top. It is important that the suction boxes are aligned straight.
- Vacuum is usually connected to an outlet at the back side of the machine. The water

and air mixture pulled from the box by the vacuum are passed to a separator.
- The water flows down a barometric leg to a seal pit while the air is removed by the

vacuum pump.
- As the sheet travels over the boxes, water becomes more difficult to extract hence

progressively higher vacuums must be used (wet suction boxes between 0.04 to 0.1
bar, dry suction boxes up to app. 0.55 bar).

Papierzentrum Gernsbach

7 – 12

7. Forming, Fundamentals

Dry Suction Boxes
After the dry line, high vacuum boxes with flat covers containing circular holes or slots
increase the web consistency.
Its function is to extract water from the mix over the wire, with the aid of vacuum. As the
consistency of the paper web over the forming fabric increases, the web becomes
denser and the capillaries thinner, so the water removal requires a higher vacuum.
Final suction dewatering is made on the couch roll. The vacuum levels range from 10-
55 kPa in the dry suction boxes to 40-80 kPa in the couch roll.

Figure shows a dry suction box
− The top surface, in contact with the wire, is made of a plastic or ceramic material and

it is opened.
The contact surface of the dry suction boxes is open to communicate the vacuum
supplied to the box, to the aqueous suspension on the wire. The contact surface
can be open in different ways. There are three types of standard designs,
grooved, drilled, fish- or herringbone designs.

− One side of the dry suction box is connected to a vacuum collector.
− The bottom surface and the front edges are closed.

Papierzentrum Gernsbach

7 – 13

7. Forming, Fundamentals

The Couch Roll
“Couch”: French meaning to lay down.
The couch roll has three basic functions.
a. To drive the forming wire
b. To extract water

- dry contents can be reached up to 25 %, depending on stock and construction of
the wet end

- vacuum level up to app. 0.7 – 0.8 bar, 70 to 80 kPa
c. To assist in the transfer of the paper web from the wire section to the press section.
Following the couch roll, than the sheet is lifted off the wire transferred into the press
section by the Pickup roll.

Papierzentrum Gernsbach

7 – 14

7. Forming, Fundamentals

Construction
The suction couch construction consists of a drilled bronze or stainless steel shell,
It is drilled with many thousands of precisely spaced holes of about 4 to 5mm diameter.
T the vacuum affects the wire/web combination through these holes. Because the couch
roll rotates at the same speed as the wire, there is only low wire wear (a difference situ-
ation is a start-up or variation in production speed).
The hollow shell roll rotates over a stationary suction box. Vacuum is pulled on the suc-
tion box, which draws water from the web into the suction box. The vacuum box within
the roll is often divided into two separate parts: low- and high-vacuum chambers.
The effective dwell time consists of the time during which the web is in the open suction
area. The suction is not similarly pulsating as on the flatboxes but is rather a longer indi-
vidual suction event. The duration is typically a few milliseconds.
After the couch roll, the solids content of the web is approximately 15%-25%.

Properties:
 It has flat and smooth surface, small friction coefficient so as to help to lengthen the
service life of fabrics.
 It provides high strength, acid and alkaline-resistance, corrosion and wear re-
sistance. This results in a long useful life.

Papierzentrum Gernsbach

7 – 15

7. Forming, Fundamentals

Materials used in Construction of Foils, Forming Boards and Vacuum Box Tops
Materials used include wood, metal, rubber, plastic, silicon carbide, high density polyeth-
ylene and ceramics. High density polyethylene is now the most often used material and
ceramics although expensive, are popular particularly as inserts.
The most satisfactory type of covering material should conform to the following specifica-
tions.
a. Minimum wear
b. Does not retain foreign matter
c. Suitably lubricated by the water drawn from the sheet, reducing the drag on the wire

to a minimum

Papierzentrum Gernsbach

7 – 16

7. Forming, Fundamentals

A Fourdrinier Forming Section

A: Wire Drive Roll
The wire drive roll assists in driving the forming fabric. The drive applied to the wire drive
roll usually is independent from the couch drive (helper drive).

B: Return Wire Rolls
The forward drive roll is the last roll in the forward running of the wire which then passes
over or under a series of rolls before returning to the breast roll.

C: Stretch Roll
It is impractical to make a forming fabric the exact length of the fourdrinier wire run as it
would be difficult to fit on to the machine. They are made longer so that they can be easily
pulled on to the machine with the stretch roll taking up the excess length by being me-
chanically moved in the vertical plane.

D: Guide Roll (s)
A further impractical proposition is to perfectly align every roll, foil or vacuum box on the
machine. When a machine is initially installed instrument alignment checks are made but
it is not possible to spend so much time on shut down when equipment is replaced or
moved. To overcome this problem guide rolls are located at strategic locations in the wire
return run. They Guide roll moves in the horizontal plane to guide the wire.

Papierzentrum Gernsbach

7 – 17

7. Forming, Fundamentals

Twin Wire Systems
A: Hybrid Former
The traditional Fourdrinier can be built or modified to use two or more wires.
The simplest system is where a second wire is used on top of the single sheet, which is
produced in the normal way on a normal Fourdrinier.
Main principles of the Fourdrinier still apply, and then there are a few extra information
to consider, due to the second wire.
The operating principle of the hybrid former is to discharge a suspension of fibres and
water first onto the bottom wire, starting the dewatering and sheet forming process. Fur-
ther along the wire table a second moving wire on top is creating a nip with the bottom
wire. Sheet forming now is continued in between the two moving wires. The wires are
known as the top and bottom wire.
A vacuum box can be added at the top wire. This vacuum box is dewatering in an upward
movement.
The formed web is consolidated at the end of the unit by a suction transfer box, ensuring
the sheet remains on the bottom wire.

Figure Valmet (Metso Paper), Symformer MB
The main purpose of this second wire is
− to increase drainage and
− to speed up the process.
− It will also have other benefits such as making the sheet less two sided.

Papierzentrum Gernsbach

7 – 18

7. Forming, Fundamentals

Twin Wire System (Hybrid Former)

Breast
roll

Foils Vacuum box Rubber
covered roll
Pick-up roll

Forming Foil Forming Couch roll
board Box shoe

Figure Valmet

Papierzentrum Gernsbach

7 – 19

7. Forming, Fundamentals

Twin Wire Forming

B: Gap Former

Gap formers have been developed to overcome the limitations of the single wire Four-
drinier. They have been designed mainly for high speed, large volume type papers and
have achieved the following: -

- Produces a symmetrical sheet with single surface characteristics on both sides

- An even distribution of long and short fibres can be found throughout the sheet.

- Production rates increased beyond what any single wire machine could produce due
to the greater dewatering capacity

- Significant quality improvements, (less two-sidedness), therefore improved operations
later in process, e.g. Newsprint no longer super-calendered.

Twin wire (GAP-) formers guide the flow box jet into a closing nip, formed by two wires.
Main principles of the Fourdrinier still apply, and then there are a few extra items of in-
formation to consider, due to the second wire.

Gap Former, Working Principle
The flow of stock into a twin wire former can be from the side, downward or in an up-
ward direction.
- The jet of stock passes between two moving wires into a forming zone where rapid

dewatering takes place through the two wires.

- Dewatering pressure is created by the tensioned wires wrapping a radius which may
be in the form of a roll or a series of deflectors placed in a curve.

- The smaller the radius the higher the dewatering pressure taking into account the
tension of the wire.

Figure: Valmet (Metso Paper), GAP Former

Papierzentrum Gernsbach

7 – 20

7. Forming, Fundamentals

Figure Voith Paper: DuoFormer TQv

11 9 4 6
3 5

7
8

12

2
1
10

Description of the Duoformer TQv 2: Breast Roll
1: Tapered Header 4: Forming Blades
3: Forming Roll 6: Couch Roll
5: Forming Suction Box 8: Inner Wire
7: High Vacuum Suction Box 10: ModuleJet
9: Outer Wire 12: Stretch Roll Inner Wire
11: Stretch Roll Outer Wire

Papierzentrum Gernsbach

7 – 21

7. Forming, Fundamentals

Sheet Forming in principle
A. Impingement zone (Suspension on the wires)
 Gentle dewatering action!

 First fibres are fixed onto the wires  initial forming process
 The first fibre layer onto the wire is acting as a filter for the ongoing dewater-

ing process
 No flow back of the white water, to avoid that the formed fibrous layer gets

destroyed.

Figure Valmet (Metso Paper)

Papierzentrum Gernsbach

7 – 22

7. Forming, Fundamentals

B. Sheet forming zone and dewatering with low pressure difference Δp
 Ensure a sufficient turbulence in the stock suspension on top of / in between of the

developing fibre mat (s)
 If water is removed too quick, the fibre mat will become too dense!

Use of dewatering units which are able to create turbulences, e.g. hydrofoils

Figure: turbulence at a dewatering
foil

Figure Voith Paper, Dewatering with a low pressure difference Δp
 Using e.g. forming suction box.
1. Wet suction box
2. Forming suction box
3. Forming blades
4. Deflector strip

Papierzentrum Gernsbach

7 – 23

7. Forming, Fundamentals

C. Dewatering with a high pressure difference Δp
 Due to the ongoing dewatering action the sheet will become denser.
 Use of dewatering units with increased pressure difference:
 Wet suction box
 Couch roll
 High vacuum suction box

Vacuum Boxes
The principle of water removal by a vacuum box is the difference in pressure around the
web and the vacuum maintained inside the box.

- usually used if app. 3 to 5 % consistency is achieved
- box with vacuum supply and a (wear resistant) cover with slots (high speed applica-

tions) or holes.
- vacuum level up to app. 65 (70) kPa

Figure shows a wet suction box for a low vacuum level (app. 0.1 bar, 10 kPa)

Papierzentrum Gernsbach

7 – 24

7. Forming, Fundamentals

Roll and Blade Formers, Equipment
Forming Roll
The first real dewatering unit is the forming roll, which can be equipped with a suction
chamber. The holes in the shell of the roll are drilled to get the maximum open area.
The forming roll can be covered with a shrink - fit wire (sleeve).

Figures Valmet: Forming roll with shrinking wire
Surface pattern of a forming roll shell.

Papierzentrum Gernsbach

7 – 25

7. Forming, Fundamentals

 The forming roll is one essential equipment for the dewatering process at a Gap
Former.
 If the suspension enters the Gap in between Breast and Forming Roll, it
consists of app. 99% water.
 If not enough water can be removed (or the dewatering pressure is too
high), the initially formed fibrous layer in between of the wires would be
destroyed again.
 To store / remove as much water as possible, the Forming Roll needs to
be a roll with a large open area (e.g. drilled/grooved/blind drilled roll sur-
face). Water removal from the roll is carried out by vacuum (1 or 2 vacuum
chambers) and / or centrifugal forces.

Remark: The initial sheet forming process is carried out. Fibres are placed on the
surface of the forming fabrics. In between those initially formed layers
still suspension may be available!

Some important parameters for the runability and the quality of the paper web are the
different vacuum levels in the dewatering units, adjustments of the loading elements and
the pressure in between the two wires (e.g. closing GAP along the forming roll).

Figure Valmet: Forming area

Papierzentrum Gernsbach

7 – 26

7. Forming, Fundamentals

Multi Foil (Blade) Shoe (Valmet)
The Multi-Foil Shoe is the dewatering unit after the forming roll. In the example below, the
“Shoe” is equipped with 2 chambers, supplied with vacuum. The vacuum level can be
varied on demand. The foils are covered with ceramics.

Figure: Metso Paper Multi-Foil Shoe
Creating Pressure Pulsations to improve formation (I)

Figure Valmet:
Multi Foil (Blade) Shoe
Depending on the vacuum level at the Multi Foil Shoe, pulsations can be created to im-
prove the formation and the dewatering action to both paper sides can be controlled.

Papierzentrum Gernsbach

7 – 27

7. Forming, Fundamentals

Loading (Blade) Element (LB Element)
Creating Pressure Pulsations to improve formation (II)
The Loading Element is equipped with flexible foils (loading elements).
The loading element foils are pressed against the cover of the suction unit. This causes
a pressure in the web between both wires. Due to this pressure the paper web is de-
watered.
 Due to the adjustable foils, pulsation forces can be applied to the forming area.
 With the pressure applied to the foils, Formation can be controlled.
Figure Valmet: Loading Elements

Papierzentrum Gernsbach

7 – 28

7. Forming, Fundamentals

Couch Roll with sheet transfer from outer wire

Outer
wire

Negative pressure

Removed
water

High Vacuum

Figure: Valmet –Couch (suction) Roll

Papierzentrum Gernsbach

7 – 29

7. Forming, Fundamentals

The Forming Fabric

Figure Voith Paper, Forming Fabric
The forming fabric is used:
− for the transport of the stock suspension / paper web
− for the initial forming process
− and for the dewatering / filtration processes.
The forming fabric should:
− be long lasting

 high wear resistance, high tensile strength, low soiling tendency
− provide high dewatering capacity and high retention
− provide low wire marking ability
Forming fabrics today are usually constructed from
 Polyester and Polyamide. They are woven from mono-filament synthetics similar to

fishing line.
Until the 1970s some phosphor-bronze metal 'wires' were used.

Papierzentrum Gernsbach

7 – 30

7. Forming, Fundamentals

Manufacturing of a forming fabric
The fabric is woven similar to cloth, using a loom.
Weaving is done by intersecting the longitudinal (warp) yarns with the transverse (weft)
yarns.
A simple example:

Running Direction

Paper side

Machine side
Plain weave

warp yarn
WARP
Warp is a textile term used to describe the group of yarns that the papermaker knows
as the machine direction yarns.

weft yarn

WEFT

Weft is a textile term to describe the group of yarns that the papermaker knows as the

cross machine direction yarns. Weft Yarn

Warp Yarn

Running Direction

Papierzentrum Gernsbach

7 – 31

7. Forming, Fundamentals

The Forming Fabric
Forming Fabric, app. 50% used off

Forming Fabric used off

Papierzentrum Gernsbach

7 – 32

7. Forming, Fundamentals

Two Ply and Multi Ply Applications
Twin Wire Fourdrinier
The paper machine, see figure below, has two Fourdrinier Wires and two flow boxes.
The two Fourdrinier wires are running in opposite directions, with the two sheets being
combined / couched together at the Dandy roll.
The two top sides are joined together with the two wire sides being the outer surfaces of
the composite sheet.

Figure Voith Paper
Advantages of two ply and multiply applications:
a. Improved overall formation as the total sheet grammage can be split e.g. 150 gsm

total into two layers, one with 100 gsm and the second one 50 gsm.
b. Economical production of higher basis weights
c. No (less) two-sidedness
d. Efficient sheet building concept, e.g. paper- or cardboard:

− white layers outside (e.g. bleached chemical pulp)
− darker layers inside (e.g. recovered paper)
− increased stiffness of the product (calliper, type of furnish used) compared to the

same basis weight using only chemical pulp

Papierzentrum Gernsbach

7 – 33

7. Forming, Fundamentals

(Multilayer) Triplex Papers / Boards
To provide a more flexible system a paper may be made by forming three layers and
joining them together.

Bottom Fourdrinier with numerous Top Forming Units

Back Liner Middles Underliner Top Liner

Figure Voith Paper

Middle Underliner Top

Back

Figure Voith Paper, 4 Ply Fourdrinier

Papierzentrum Gernsbach

7 – 34

7. Forming, Fundamentals

Types of Board
The types of board available are known as:

a. Single ply b. Duplex c. Triplex d. Multiplex

Single ply boards A board consisting A triplex board con- This type of board
consists of one sin- of two layers and sists of 3 furnish has more than
gle layer made two furnishes. layers. The exter- three furnish layers
from one single fur- nal layers (top and but two or more
nish. back liners) may may have the
have the same fur- same composition.
nish composition.

The Structure of Boards

A board may be constructed as a single or multi-ply. Single ply boards consist of one
furnish layer only but other types may be more complex. The structure of multiplex
boards is a classic example of the various layers that may form the total web and the
names given to the various furnish layers are:

1: Top Liner 3: Middles
2: Underliner 4: Back Liner

1
2
3
3
3
4

Papierzentrum Gernsbach

7 – 35

7. Forming, Fundamentals

Cylinder Mould Machines

The traditional cylinder mould was invented in 1806 and was used as an alternative
method of producing single ply paper.
In 1870 the cylinder mould method of forming became the standard way of producing
board in a multi-ply form. Versions of the method are still used today but where paper is
concerned they are limited to speciality grades e.g. banknotes, security papers, filter pa-
pers.
This indicates that they are limited in use mainly due to their low production rates.

The original cylinder mould was of the contra-
flow vat type.

1: Stock inlet, area with high
turbulences

2: Flow direction of the inco-
ming stock and opposite
running direction of the cy-
linder

3: Immerse of the cylinder
mould into the vat

Figure shows a contraflow vat

The cylinder mould, covered by a wire mesh rotates in a vat of diluted paper stock.

Water associated with the stock drains through the wire mesh into the centre of the
mould leaving a layer of fibres deposited on the wire surface.
The drainage rate of the water from the fibres is determined by the stock properties (de-
gree of refining) and the difference in head between the level of stock in the vat making
circle and the water level inside the mould. This is known as the making head.
The water is removed from the inside of the cylinder by way of an adjustable weir lo-
cated at one of both sides of the unit. The water level can be changed by adjusting the
weir gate up or down hence changing the making head.
The fibre layer continues with the rotation of the mould to a point where it is couched
onto the underside of a felt by wrapping the mould by the felt and applying a light pres-
sure by a soft rubber covered roll.

Papierzentrum Gernsbach

7 – 36

7. Forming, Fundamentals

Today the principle has been adapted and uniflow, restricted vats, dry vats and vacuum
formers may be used for single ply paper production.

Figure: Uniflow vat
1: Inlet
2: Flow direction Cylinder mould
3: Return flow of the stock suspension
4: Turbulences (disturbing flow patterns) at the

interface of both flows
Problems associated with this type of vats were:
1. Poor formation.
2. Poor control of fibre orientation.
3. Poor control of Cross Direction Basis Weight Profile
4. Restricted in speed (approximately 100m/min)

Papierzentrum Gernsbach