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

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pack e.g. food, cigarettes, pharmaceuticals or cosmetics. Additionally a wide range

of specialty boards exists like artboard, gypsum board, playing-card board, book-

binders board etc. As the requirements for these board grades vary a lot the furnish

and the board machine design may differ accordingly. Some of the basic required

properties are:

• Mechanical properties like bending stiffness (bulk, Young’s modulus), to protect

the packed goods against damage.

• Flatness, plybond, creasability, punching, to ensure good runnability in convert-

ing and packaging lines.

• Resistance against moisture, gas and flavor, to protect the goods from quality

changes.

• Freeness from impurities like micro-organisms, toxic or mutagenous substrates,

taint or odor, to protect the goods against contamination.

• Brightness, gloss, roughness, printability of the surface, to ensure appealing

information, identification and promotion of the packed goods.

Paperboard mainly has a multi-ply structure to achieve the different product re-

quirements in the most economical and environmentally friendly way e.g. by ap-

plying various furnishes in the different plies. The surface plies (top and back

plies) have to ensure the strength properties (bending stiffness) and, together with

the coating, the required surface quality. With the inner plies (undertop and filler

plies) the bulk is optimized (affecting bending stiffness). Most cartonboards are

on-machine coated and calendered to obtain appealing surface characteristics.

Depending on the product and the geographic location of the board producer,

virgin or recycled fibers or both fiber types are used as raw material. In multi-ply

production the individual plies are formed on separate forming units and couched

together in wet condition. Forming of two layers in one forming unit is also done.

Different forming principles as described below have been and are applied either

uniquely or in combination with others.

In suction formers 3 to 11 cylinder formers in series with suction chambers

produce webs of about 25 to 100 g m

–2

each which are couched on a transfer felt,

thus building up the baseboard. The maximum speed is about 350 m min

–1

, the

maximum width is about 5 m. Cylinder formers without vacuum application are

limited to a maximum speed of about 150 m min

–1

In fourdriniers and hybrid formers each ply is formed on a separate fourdrinier

wire. Hybrid formers are used e.g. to produce the filler ply with the widest basis

weight range and highest basis weights which require extended dewatering ca-

pacity. The headbox and wire section have to be adapted to the high throughput

ratio and to the high surface quality requirements for coating and printing. The

maximum speed of a fourdrinier and of hybrid formers is about 1000 to

1200 m min

–1

As the fourdrinier wire sections have sufficient speed potential gap formers are

seldom used in cartonboard production. Nevertheless gap former installations can

be found for certain applications of specialty board like gypsum board.

6 Paper and Board Manufacturing322

The press sections of modern machines for cartonboard are designed for high

dewatering capacity, high bulk, smoothness and good runnability. This leads to

press concepts with typically three press nips, e.g. a first double-felted suction

press nip followed by a double felted shoe-press nip and finally an unfelted

smoothing press or a single felted roll press to enhance surface smoothness. Be-

tween the nips a closed web run ensures stable sheet transfer, independent of

speed and basis weight.

The dryer section consists of up to 100 drying cylinders in the pre-dryer section

and about 16 in the after-dryer section. One or two single-tier groups are followed

by double-tier drying groups. For good flatness of the finished product the top and

the bottom cylinder rows are heated separately to control curl tendency.

Starch or size can be applied in a film press or a pond size press to enhance

surface strength and bending stiffness. In this case an after-dryer section is re-

quired for baseboard drying.

Before coating, the roughness of the baseboard has to be reduced by calender-

ing. In modern board machines this is done by a heated hard-nip calender. As an

alternative soft-nip calenders can be used for reduced densification. The latest

development is the shoe-nip calender which combines long dwell time, low spe-

cific pressure and high roll surface temperature to achieve minimum roughness at

lowest densification. In Europe for machines below about 650 m min-1 the use of

a MG (machine glazed) cylinder is state of the art for high bulk, low roughness

board production. As the entering web dryness (about 60 to 70%) and exiting

(about 70 to 75%) web dry content is crucial for smoothness and bulk of the

baseboard the MG cylinder is often the bottle neck for machine speed increase.

Most likely it will be replaced in future by the shoe-nip calendering technology

(Section 6.9).

Fig. 6.83 State-of-the-art machine for board production

(source: Voith).

6.11 Paper and Board Machines

323

Virgin-fiber based cartonboard is mainly double coated on the top side and un-

coated or single coated on the back side. White-lined chipboard made of recycled

fibers is mainly triple-coated on the top side and uncoated or single-coated on the

back side, each layer with special tasks and coating formulations. Coating is done

by roll applicators or by free jet applicators for highest surface requirements and

machine speeds.

Final cartonboard post-calendering is mainly done with soft-nip calenders. The

Pope reeler can be used for reeling up to 3.5 m diameter, reelers with center torque

are applied for diameters of up to 4 m. A state-of-the-art board machine (Fig. 6.83)

with on-line coating and final calendering has a length of up to 350 m.

6.11.4

Tissue Machines

The bases for hygienic products are tissue grades with basis weights of 5 to

35gm

–2

. Consumer demands drive the tissue industry to ever more softness and

absorbency of the tissue grades used e.g. in the bathroom (toilet, facial) or in the

kitchen (towels). Softness can be increased by the proper selection of fiber furnish

and chemical additives or the proper machine concept and operation e.g. by low or

no wet pressing. On the other hand sufficient paper strength and economy of

production have to be regarded.

In former times tissue was made – and is still made in many places – on four-

driniers, later followed by suction breast rolls (Section 6.3.4) and since the 1970s by

gap formers. Since the 1990s the so-called crescent former is the most accepted

system. Today’s tissue forming section is a roll former where the headbox jet is

deposited in the wedge between two converging fabrics. One-, two- or even three-

layer headboxes are used, the latter for stratification of the tissue. The converging

fabrics can be either two forming wires or a forming wire and a felt (crescent

former). In this case the felt transfers the web directly to the dryer section. Web

formation is completed during the contact of the wire(s) on the forming roll. The

wire run in the forming section may be designed as a C-wrap or an S-wrap with the

headbox placed at the top or bottom position in both cases.

The dryer section in tissue machines consists of

either

• one large diameter tissue cylinder with an air impingement hood, where the web

is completely dried within less than half a second, and creped

• a tissue cylinder with air impingement hood where the web is only dried up to

about 70 to 80%, then wet creped and finally dried in a conventional after-drying

section

or today for high quality tissue production

• a through air dryer, which dries the web up to about 70 %, followed by a tissue

cylinder.

6 Paper and Board Manufacturing324

In a state-of-the-art conventional tissue machine (Fig. 6.84) the web is formed in a

gap former (wire plus felt), transferred (at a dry content of about 12 to15%) by the

felt to the dryer section where it is first pressed and mechanically dewatered (up to

about 38 to 40% dry content) by a shoe press acting against the tissue cylinder. In

older machines dewatering is done by one or two press rolls, one of them being a

suction press roll. The web is dried, by intense contact and impingement heating,

to about 95% dry content. Here the hood contributes up to about 60% of the

overall drying capacity. The web is then creped by a creping doctor and wound up.

During creping the web is reduced in length by about 10 to 20%.

In a tissue machine for premium soft tissue production (Fig. 6.85) the formed

web is first dewatered and pre-dried (to about 70 to 80% dry content) in a through

air dryer without mechanical pressing. Here the through dryer replaces the press

nip dewatering in order to preserve the bulk. On the other hand this results in

substantially higher energy consumption for drying. The web is then transferred,

pressed and glued to the drying cylinder. The pressing impacts the web, mainly

locally, according to a special surface pattern of the transferring wire. Coating

agent application ensures good sticking of the web to the cylinder surface to obtain

the required characteristics at creping. Web length decrease by creping in this case

is only about 5%.

Finally the web is calendered off line for web caliper control and for surface

smoothness. The line load is low in order not to lose too much bulk.

Compared to other paper machines tissue machines are compact as regards

their widths (maximum 7.9 m) and lengths which are about 45 m for conventional

machines and about 60 m for a machine equipped with a through air dryer. Tissue

Fig. 6.85 Tissue machine with through air dryer for extra soft

tissue production (source: Andritz).

Fig. 6.84 Conventional tissue machine (source: Andritz).

6.11 Paper and Board Machines

325

machines are the fastest paper machines with a maximum speed today of about

2200 m min

–1

(in the forming section). Maximum production capacity is about

100 000 tpa.

6.11.5

Specialty Paper Machines

Peter Mirsberger

The category of specialty papers defines a very wide range of different paper grades

with a wide variety of specialized end uses and thus different and particular quality

demands. These require special production technologies and specific know-how.

Some of the required properties are very common and similar to commodity pa-

pers, but they are usually more distinct or with a closer tolerance. Such character-

istics include profile quality, formation, smoothness, strength, thickness, porosity,

and absorption. Some are very particular characteristics, such as wet strength,

electrical conductivity, pore size distribution, resistance to certain chemicals,

chemical reactivity, light proofness, heat resistance, and cleanliness.

Often specialty paper grades like photo, cigarette or label papers are cut into

small formats. Here the basis weight profile as well as the fiber orientation must

be as uniform as possible across the width so dilution water technology in the

headbox for CD profile control is standard on most paper machines for specialty

paper grades. Other important quality characteristics of specialty papers are MD/

CD tensile ratio and dimensional stability. Decor, inkjet and label papers for exam-

ple absorb a large amount of water during laminating, printing or gluing. Fibers,

however, swell in width more than in length when wetted. To achieve the best

dimensional stability the fibers of the sheet should be randomly oriented to obtain

a “square sheet”, i.e. the lowest possible MD/CD tensile ratio is required.

Not only are the quality requirements various but also the production of spe-

cialty papers covers a wide range of speed and basis weights. The basis weights

extend from 12 g m

–2

for tea bag paper up to 300 g m

–2

for specific photo base

papers. The speed range extends from 40 m min

–1

for banknote papers up to

1400 m min

–1

for thermal base paper, for example. Often, different grades are

produced on the same paper machine. Further characteristics are small tonnages

per grade, many grade changes and a dynamic grade development, i.e. new grades

are developed which can cope with new market demands, other grades disappear

from the market due to lack of demand. So flexibility is a key word – not only for

the production of these papers but also for the paper machine concepts.

The base sheet forming concept for specialty papers is the fourdrinier. It can be

used for the whole basis weight range and up to a machine speed of about

1200 m min

–1

. In order to achieve best results concerning formation and dimen-

sional stability Dandy rolls or hybrid formers and effective wire shaking units are

frequently used. New machines however can exceed the speed limit of fourdri-

niers, so gap former concepts are used in this case.

The basic raw material for specialty papers is wood pulp. There are however

grades like tea bag, plug wrap or filter paper where the required porosity, absorb-

6 Paper and Board Manufacturing326

ency or bulk can only be achieved by using special plant fibers like abaca, sisal or

hemp or even synthetic fibers like rayon. These fibers are much longer than wood

pulp fibers. Due to their length and the demand for best formation and uniformity,

even at lowest machine speeds, the inclined wire sheet forming technology is used

for theses grades (Fig. 6.86). Extreme dilutions are necessary in order to avoid

reflocculation of the fibers and headbox consistencies even below 0.01% are run.

Overlay papers for high abrasion resistant surfaces such as flooring laminates are

produced with embedded fillers (up to 30% aluminum oxide) on a three-layer

former.

Another “old fashioned” well proven sheet forming technology which is found

in specialty paper production only is the cylinder mould former (Fig. 6.87). This

sheet forming technology is still used for the production of cotton based banknote

paper. The paper production on a cylinder mould former enables the application of

three key security features:

• The use of cotton as raw material gives the paper a unique feel.

• The multi-tone watermark has a wide tonal range going from light through a

number of shades to dark.

• The security thread can be fully embedded or windowed, i.e. the thread appears

at intervals in the surface of the banknote.

These security features together with more than a 200 years old tradition and

know-how make the cylinder mould former and cotton-based paper still the best

choice for banknotes.

The press sections in specialty paper machines need to be highly flexible. For

paper grades where only one smooth side is required, multi-roll press concepts

with two or three press nips and a closed sheet run can be used. For increased

dryness and improved runnability these press concepts can be equipped with a

Fig. 6.86 Inclined-wire forming technology (source: Voith).

6.11 Paper and Board Machines

327

shoe press. For grades like inkjet or bible paper low roughness figures on both

sides are required so a bottom-felted straight through press is added in order to

reduce the roughness on the top side and to control the roughness two-sided-

ness.

Depending on the requirements, all available drying concepts are used for the

production of specialty papers. The most common concept is the double tier ar-

rangement. Fast machines are equipped with single-tier dryer groups for improved

runnability. When highest smoothness and gloss values on only one side or low

CD elongation at rewetting for best dimensional stability are required, the wet web

is dried on a Yankee cylinder. If high bulk or high porosity is a must, as it is for

filter or tea bag papers for example (Fig. 6.88), the web is dried on a through air

dryer.

Specialty papers can be divided into uncoated and coated grades. The uncoated

grades are either machine finished, like cigarette papers or base papers for sub-

Fig. 6.88 Machine with inclined wire former and through air

dryer for tea bag paper production (source: Voith).

Fig. 6.87 Cylinder mold forming technology for banknote

paper production (source: Voith).

6 Paper and Board Manufacturing

328

sequent value adding off-machine process stages such as for photo base paper.

Coated grades can be subdivided into two groups:

1. Where the coat is needed for a certain special function, such as thermal or

carbonless papers.

2. Where the coat is needed for improved printability, such as label or cast coated

papers.

In both cases the base sheet is normally pre-coated on one or on both sides. By pre-

coating, the functional coat weight can be minimized or the final surface quality

can be improved. The most suitable and economical applicator for the pre-coat is

the on line film press. Both sides of the sheet can be coated simultaneously and

different applications are possible.

The application of the functional coat is more demanding. For coated commod-

ity grades, the quality generally increases with increasing coat weight. For coated

specialty papers like thermal or carbonless paper, however, the quality does not

improve further once a certain coat weight for the functional coat is reached at any

point. The optimum concerning uniform coverage and minimum coat weight is

achieved with the curtain coater: a perfect layer of coating color is applied on the

surface without any mechanical impact on the paper. Since the first commercial

installation in the mid-1990s the significant advantages of curtain coating have led

to more than 20 units being installed within the last 10 years. Due to better flex-

ibility and time efficiency the functional top coat is applied off line.

There is no typical calender concept for specialty papers. Each of the different

specialty paper grades has its own demands with regard to surface properties,

smoothness, densification, caliper, porosity, transparency, etc. Calendering of spe-

cialty papers, therefore, requires specific customized calender concepts. Thermal

base paper, carbonless and label paper are pre-calendered usually with one soft or

hard nip calender. For coated inkjet papers, two soft nips are necessary – one for

each side. The smooth surface of decor papers is achieved using calenders with

two or four soft nips. Here the printed side of the decor paper is in contact with the

heated roll surface up to four times, supported by steam application as well. A

combination of one hard nip for pre-calendering and a multi-nip calender stack for

the final calendering is used for photo base paper. Silicone base paper has the

highest demands concerning densification. The paper has an initial moisture of up

to 16% before running through 14 nips of a supercalender, the rolls of which are

heated up to 180 °C and loaded up to 400 N mm

–1

References

Section 6.4.1

1 U. Weise, J. Terho, H. Paulapuro, Stock

and water systems of the paper machine,

in Paper Making Science and Technology,

Book 8, Paper Making Part 1, Stock Prepa-

ration and Wet End, Fapet OY, Helsinki,

2000, Ch. 5, p.1.

2 A. Gommel, Advanced Wet End Process –

A New Engineering Concept for Approach

Flow Systems, Proceedings 28

EUCEPA

Conference, Lisbon, 2003, Session 3, p.126.

3 P. O. Meinander, Compact Wet End Sys-

tems are Proven State of the Art – Experi-

ence of Sixteen Installations, Proceedings

References

329

Scandinavian Paper Symposium 2002, Paper

8, p.1.

4 G.Ortner, OptiFeed-Konstantteil-Konzept,

Wochenblatt Papierfabrik. 6, 2004, 6, 280.

5 A. Fladenhofer, Stoff- und Wasserführung

bei Karton- und Verpackungspapierma-

schinen – Weniger ist oft mehr, Wochen-

blatt Papierfabrik. 2001, 17, 1102.

6 A. T. G Giorgis et al., Concentric mixing of

hardwood pulp and water, TAPPI Journal,

May 2004.

Section 6.10

W. Kaipf, Sensoreeling Technologie, Wochen-

blatt Papierfabrik. 19/1999, 19, 1246–1250.

O. Rudolph, M.-O. Stenitschka, Moderne

Wickeltechnik für den All-Online-Produk-

tionsprozess, ipw 6/2001 Science and Tech-

nology. Das Papier pp. 82–87.

D. R. Roisum, The Mechanics of Winding,

TAPPI Press, Atlanta, 1994.