Holik H. (ed.) Handbook of Paper and Board

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Fig. 6.76 Comparison of new generation reels (source: Voith).

controlled by force sensors as part of a force controlled system instead of the

pressure in the hydraulic cylinders. One disadvantage is still given: the change of

nip load application from the primary to the secondary arm.

The next two reel types have a movable reel drum to generate the nip load. The

advantage is that there is no change of nip load system as in type A. Type B has a

drum movable in the vertical direction. Here the weight of the drum has to be

compensated as there is a large load component in the vertical direction but only a

small part of it is used for nip load control. This reduces the accuracy of this kind

of control system. Type C has a drum movable in the horizontal direction. Here the

force direction of the nip load is orthogonal to that of the drum weight. Using low

friction carriages, line force deviations due to friction are low.

With this system very low line forces can be realized with high accuracy. For

example, now the sensitive NCR papers can be wound up with a nip allowing

about double the speed.

6.10.3.3 Oscillation

The reel spool in the secondary arm is way-controlled. It has to compensate for the

growth of the roll diameter. The reel drum is force controlled and follows the

movement of the nip. When the secondary arm gets an overlay of a sine movement

in the machine direction counteracting at FS and DS, the drum follows without

changing the nip load and its CD uniformity. This requires a vertical web run

before the reel drum, in order not to influence web tension over the width. The

result is an oscillated paper roll (Fig. 6.77), realized with only a few small further

hardware components. This feature is very useful for “hard” paper grades with

6 Paper and Board Manufacturing312

high Young’s modulus. Deviations in CD caliper profile cannot accumulate so

easily which allows one to wind larger diameters without winding defects.

6.10.4

Reel Drum Design

The structure of the wound paper roll is also influenced by the design of the reel

drum. As shown in Fig. 6.78, two kinds of air bubbles may occur at the reel. The

boundary layers below the web and on the reel drum are mainly influenced by the

Fig. 6.78 Influence of air boundary layers on winding result

(source: Voith).

Fig. 6.77 Oscillation in reeling (source: Voith).

6.10 Reeling

313

web speed. Low paper porosity leads to air accumulation (air bubble) between the

web and the drum before the nip. The web is no longer firmly fixed in the cross

machine direction and can float in CD which will lead to uneven edges. A critical

situation occurs when the air bubble gets too big and even goes through the nip.

The resulting wrinkles cause broke. The air bubble problem can be avoided or

reduced by two measures. First, narrow grooves in the drum surface allow the air

to pass the nip. Practical experience has shown that for higher speeds and lower

paper web porosity through-drilled drums have the highest efficiency. Here the air

is not compressed as it goes through the drills into the drum and can leave below

the drum.

Deviations in the CD caliper profile lead to a noncylindrical roll shape. This

means, that the nip is not closed over the whole width and air can be easily wound

into the roll between the outermost two layers. When paper porosity is high, the air

can escape before coming again through the nip. If not, the air will also accumu-

late between two layers of the paper web in the parent roll before the nip. Some-

times there exist more bubbles between several layers. If the bubbles are almost

stationary and not too large, then generally no problems occur but when they are

pulsating irregularly in the cross machine direction, they can cause wrinkles and

broke.

With the groove design on the reel drum the wind-in of air cannot be avoided,

but the air can be moved steadily to the edges resulting in a stable balance of air in

the roll. Here a pressure relief groove or wide grooves will be applied. The groove

has to be a spiral, changing its direction in the middle of the drum. The arrow of

the groove has to show in the web run direction otherwise the air would be trans-

ported towards the machine center.

A possibility to reduce the amount of wound-in air in the parent roll is to use a

rubber covered reel drum. This drum can adapt to the uneven surface of the roll to

a certain degree and thus close the nip over the whole width. At the same time line

force distribution with the rubber covered drum is more even over the whole

machine width thus avoiding the high pressure areas that can occur with a steel

drum.

6.10.5

Turn-up Systems

6.10.5.1 General

The continuous process of paper production is interrupted at the reel, when the

required roll diameter or web length has been reached. Now the paper web has to

be cut, a fresh reel spool started and the finished parent roll discharged. So first the

empty reel spool has to be accelerated to machine speed. With a standard Pope

reel, the empty reel spool will now be lowered, till the nip is closed (Fig. 6.75). The

reel is now ready for the turn-up which will be described separately.

Some new generation reels show a different sequence. They first open the nip by

moving the parent roll (Fig. 6.79). In order to avoid too much air being wound in

an air squeezing element like a small roll or a brush on the parent roll is applied.

6 Paper and Board Manufacturing314

After the parent roll is in its end position, the primary arm can be lowered to the

turn-up position and the primary nip is closed. The turn-up position of the primary

arm can now be varied between the position of a standard Pope reel and the

position on the rails, depending on the demand of the turn-up system and the

paper grade. The turn-up is easier and faster with self-threading concepts when

the empty spool is already partly wrapped by the web.

6.10.5.2 Nordic Turn-up

The most simple and oldest system is the nordic turn-up. The parent roll opens the

nip a little and then is decelerated. Thus a web loop is formed between the reel

spool/reel drum running at machine speed and the decelerated parent roll. Be-

cause of the vacuum between the outgoing web and the empty reel spool, the paper

web starts to wrap the spool. When it reaches the nip, at low basis weights the web

will be cut by the suddenly induced acceleration. For high basis weights, the web

will be cut by the high tear forces, because the paper on the parent roll and on the

new reel spool moves in different directions. For heavier paper grades wrapping of

the empty spool can be supported by an air stream from below. The disadvantages

of this system are the high amount of top broke and the high load of all compo-

nents, when the paper is torn in the nip, especially at higher basis weights.

Fig. 6.79 Turn-up preparation for two kinds of reel types

(source: Voith).

6.10 Reeling

315

6.10.5.3 Web-wide Cutting Knife

With the winding concepts, where the nip between drum and finished roll is

opened, the web can be cut with a web-wide knife operated in the open draw. As

the new spool is already wrapped by the web, the system is self-threading. The

system has a very high turn-up efficiency and is mainly applied for lower and

medium basis weights up to about 120 g m

–2

6.10.5.4 Air-supported Turn-up Systems (Gooseneck, Cobra)

The air-supported turn-up systems have their main applications at basis weights

up to 100 g m

–2

, or in special cases up to 150 g m

–2

. A blowpipe formed like a

gooseneck gave the name to one of these turn-up systems. A small cut of a few

centimeters in the cross machine direction will be created, e.g. with a needle, in

the middle of the web before the drum. The thus weakened web can be blown by

the gooseneck blowpipe to the empty reel spool. The tear in the cross direction will

be supported by two nozzles blowing from the middle to the edges between the

primary and the secondary nip.

For very small paper machines one or two blowpipes in the cross direction,

positioned at the edges after the primary nip, are sufficient. They cut the web from

the edges and transfer it to the empty spool. The so-called Cobra system can also be

used to support the gooseneck for large paper web widths.

The air-supported systems only need compressed air as a medium and they have

a quite simple design. As web cutting is not clearly defined the turn-up quality

depends on the paper properties and the machine speed. As a further consequence

some bottom broke may occur.

6.10.5.5 Tape Turn-up System

For higher basis weights and board and packaging grades the tape turn-up system

is often used. Here a tape of sufficient strength is placed before the drum, below

the web, across the machine width. One end of the tape is provided with an ad-

hesive which will connect the tape to the edge of the empty spool as soon as it

reaches the nip. A brake on the end of the tape builds up adequate strain in the

tape so that it can tear the web when it is wound as a spiral on the empty spool.

This system can provide defined tear forces acting on the web at the tape edge.

Although several tape types with different thicknesses are available, there are still

bottom losses due to the marking of the tape.

6.10.5.6 Turn-up with High-pressure Water Jet

One of these kinds of turn-up system has two high-pressure nozzles before the reel

drum which can be moved very quickly in the cross direction. These nozzles start

in the middle of the web at a distance of about 20 to 40 cm. The high-pressure

water jet thus cuts a center strip which is transferred to the empty reel spool after

the primary nip. After its successful turn-up, the two high pressure nozzles are

6 Paper and Board Manufacturing316

moved at high velocity towards the edges of the paper web which finishes the turn-

up of the full web width. The kind of center strip cutting and transfer to the empty

spool is the main difference in the existing systems.

On a new generation reel, where the secondary nip is opened, the next step is to

cut the center strip in the open draw after the primary nip with an air nozzle or a

small knife. The stripe is then self-threading and the turn-up can be finished as

described above.

Another high-pressure water jet system is similar to a gooseneck which only

turns up the center strip. This system can be used both with a closed and with an

opened secondary nip. With these systems the beginning of the center strip is not

immediately fixed at the empty reel spool, which can lead to considerable bottom

broke especially with the gooseneck-type system.

Figure 6.80 shows a newly developed system. A center strip is cut by two high-

pressure water jets as described before. For about one second a thin strip in the

middle of the center strip is cut by means of two further high-pressure water jets.

This thin strip is sucked before the reel drum and blown into the pulper. Now a

form sheet with an adhesive at one end can be connected to the empty reel spool

through this thin slot from underneath the web. This form sheet picks up the

separated center strip parts on the right and left side. As soon as the form sheet is

on the empty spool the turn-up is finished by moving the two other high pressure

nozzles to the edges. With this system every step in the turn-up is clearly time-

defined, and no component depends on the paper properties or the machine pa-

rameters. So high turn-up efficiency is given with almost no bottom broke as the

web is fixed on the empty spool accurately from the beginning. There is also no

limitation for higher basis weight applications.

Fig. 6.80 Turn-up system with high-pressure water jet and

cutting form sheet (source: Voith).

6.10 Reeling

317

6.11

Paper and Board Machines

Herbert Holik

Since the earliest times of industrial papermaking fourdrinier and mould formers

have been used for the production of all kinds of paper and board. In the last

century the machines started to be more customized to the special requirements of

the individual product grade, based on quality, economy and operational require-

ments. When planning today a new machine or a major rebuild requirements have

to be defined and decisions made relating to e. g. capacity, furnish, essential prod-

uct properties, frequency of production changes, multi-ply, multi-layer or single-

layer production, machine width and speed, machine equipment, etc. The variety

of machines and their layouts are mainly related to the five main classes of paper

and board grades with their varying requirements and can be defined as:

• graphic paper machines

• packaging paper machines

• board machines

• tissue machines

• specialty paper machines

6.11.1

Graphic Paper Machines

The majority of paper machines worldwide produce graphic paper grades on four-

drinier wire sections or on hybrid formers. These kinds of conventional forming

section usually satisfy the quality requirements. Their disadvantages are the limita-

tion in machine speed of about 1200 m min

–1

and for some paper grades the

nonsymmetry in the web z-direction.

Modern paper machines for graphic paper production are – except for tissue

machines – the fastest paper machines. They comprise a gap former, a closed-draw

press section and a single tier or combined single/double tier dryer section and, in

certain cases, a size press or a coating station. The technological progress as well as

increased cost pressures have led to in-line multi-nip calendering positioned be-

tween the dryer or coater section and the reeler in SC and LWC papermaking lines.

The paper machine layout and operating conditions differ according to the spe-

cial requirements of the grades produced as regards

• woodfree paper such as copy paper or base paper for coated paper

• wood-containing paper such as newsprint and SC or LWC paper grades

with

• off-line or on-line coating and

• off-line or on-line calendering.

An example of woodfree paper is copy paper. The most important properties are

high bulk at a certain roughness, and good formation. Here the whole paper-

making process has to be considered to meet the high bulk requirements of the

6 Paper and Board Manufacturing318

market. In particular the wet pressing impact has to be limited in favor of a bulky

sheet. The trade-off between dry content, speed potential, energy consumption,

and paper quality is a matter of continuous research and development activity. As a

consequence, single-nip presses have been recently introduced for copy paper pro-

duction. The single nip press employs one shoe nip and is double-felted. This

design enables a no-open-draw web run into the dryer section. Despite the lower

web dryness the speed limit is currently well above 1400 m min

–1

. With increasing

machine speeds formation quality decreases. Here the application of lamellas in

the headbox nozzle can improve the formation quality to a certain extent. The

upcoming tendency for hard fine flocs in look-through is counteracted by optimi-

zation of the lamella geometry. With the lamellas the MD/CD ratio of strength

parameters is reduced, which results in higher strength in CD but also lower

strength in MD. These papers are in general single-layer products with one paper

machine producing copy paper with a three-layer headbox. Today’s speed limit in

copy paper production is about 1600 m min

–1

. The dryer section is a single tier

with a double tier part after the size press or the film coating. Double tier is

required in order to produce a paper which does not curl in the copy machine. For

coating base paper production a single tier dryer section is possible as the non-

symmetrical “frozen tension” during drying in a single-tier dryer section is elimi-

nated during coating by wetting.

Machines for wood-containing papers aim for high dry content after the press

section. This not only reduces energy consumption in the dryer section but also

improves machine runnability due to the resulting higher web strength at the first

free draw. High demands regarding surface smoothness such as for SCA paper

grades require a press section where both web sides come into contact with a

smooth roll surface without a felt in between. High surface smoothness has to be

aimed for, starting from forming and followed during pressing. Smoothness gen-

eration by excessive calendering results in blackening, especially when recycled

fibers are used and at lower basis weights. The dryer section is single tier for high

runnability at maximum machine speeds which are today up to 1950 m min

–1

Optimized fabrics support the efforts of the paper maker towards high quality

and economy. E. g. the wire design has to assist high filler retention in the forming

section and the design of the press felts has to ensure good surface smoothness

and long lifetime at constant quality production and high machine runnability.

Figure 6.81 shows a schematic of a modern paper machine for LWC paper pro-

duction started up in 2004. It includes a gap former wire section, a press section

with a double-felted first press nip and a second press nip with felt and transfer

Fig. 6.81 Schematic of a state-of-the-art paper machine for

LWC paper production (source: Voith).

6.11 Paper and Board Machines

319

belt, a pre-dryer section followed by a soft calender for preliminary calendering, a

coating station, an after-dryer section, a ten-roll multi-nip calender and the reeler.

A winder follows which converts the large parent rolls into suitable-size rolls.

6.11.2

Packaging Paper Machines

The main products of packaging paper machines are grades which are the base for

corrugated board. Corrugated board consists of two outer layers (testliner or kraftli-

ner) which are glued to a corrugated inner layer (corrugating medium or fluting) to

obtain high bending stiffness. Testliner and corrugating medium are made from

recycled fibers, kraftliner and fluting from virgin fibers. Corrugated containers

must safely transport the packed product e.g. from the origin to the super market.

Here they inform the customers, and the staff, about the content and its character-

istics, and therefore should allow an appealing printed display on the surface. So

the main quality requirements are the box compression resistance (ability to pile

the boxes) and good printability. As a result the raw paper must show sufficient

strength, measured as SCT (short span compression test) or RCT (ring crush test)

for liner and CMT (Concora medium test) for corrugating medium or fluting.

The various applications require a wide basis weight range of about 70 to

450gm

–2

, to be run on one single machine. In Europe emphasis is put on 70 to

130gm

–2

. Here the main furnish is recovered paper. By its adequate treatment in

stock preparation and by thorough cleaning of the fabrics and rolls in the paper

machine good paper quality as well as good machine runnability at high machine

speeds are achieved. Machine speeds were 500 to 800 m min

–1

in the 1970s and

1980s. Today the maximum operating speed is about 1450 m min

–1

, with design

speeds of up to 1800 m min

–1

. The driving force for the speed increase is the

emphasis on lowered basis weights and low cost production.

High machine speeds require a twin wire gap former to ensure high uniformity

of the product and sufficient dewatering capacity. Twin wire gap formers are availa-

ble in different configurations. One is shown in Fig. 6.82 which depicts a state-of-

Fig. 6.82 State-of-the-art paper machine for packaging grades

(source: Voith).

6 Paper and Board Manufacturing

320

the-art packaging paper machine with the headbox of the gap former in an upper

position. Lamellas in the headbox nozzle help to optimize fiber distribution. A two-

layer headbox with different fiber fractions in the two layers is an option to in-

crease paper strength properties.

The press section with three nips ensures high dry content of the web on enter-

ing the dryer section. As the paper web is always supported during its transfer

from the wire to the dryer section, without any free draw, machine runnability is

high, even at the elevated machine speeds and lowered basis weights. For runn-

ability reasons the pre-dryer section is single tier. Safe web run is supported by air-

flow based stabilizers. For high machine speeds a film press replaces the pond size

press as the latter starts to encounter unacceptable pond turbulences at speeds

above 800 to 1100 m min

–1

. Surface sizing increases the strength properties of the

final sheet significantly. However the web strength in the machine is reduced

dramatically by rewetting during sizing. For low basis weights, high moisture pick

up at the size press and high speed production an air-float dryer may be used after

the film press, followed by single tier and double tier dryer groups.

In reeling one main requirement is a large paper roll diameter, for two main

reasons: First to reduce paper loss. A certain loss at each reel is unavoidable, so

less reels with larger diameters for the same production reduce the overall loss.

Secondly, to allow one single winder to cope with the paper machine output. This

can be reached by reducing the nonproductive time for parent roll change in this

noncontinuous process step. Today Jumbo rolls with diameters of up to 4.5 m are

produced.

There are other packaging products on the market, e.g. white top liner and sack

paper. White top liner has a white surface to cope with the high demands for an

appealing display. For its production two webs are formed in two separate forming

units – one white and one brown layer and then couched together ahead of the

press section.

In the case of sack paper high strength and sufficient elongation at rupture in

both directions (MD and CD) of the paper are required. These special properties

are important for instance during the filling of the sacks. Other applications are all

kinds of wrapping. For the highest elongation of this kind of packaging material

the “Clupak process” introduced in the 1960s is applied. Here the still wet web is

pressed in a nip where a compressible belt runs with the web. The belt deforma-

tion in the nip and the shape regain of the “extensible unit” after the press nip

results in micro-creping with up to about 12% elongation (MD) at rupture. Micro-

creping can also be obtained by passing the web through a nip with a hard and a

soft covered roll.

6.11.3

Cartonboard Machines

Cartonboard is a product with a wide range of basis weight of about 160 to more

than 600 g m

–2

. Cartonboards have the highest share, about 80 %, of the total pa-

perboard production. Cartonboard is mainly used for production of carton boxes to

6.11 Paper and Board Machines 321