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Calendering of Paper and Board

March 2016 Pekka.Komulainen@clarinet.fi

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Contents

Preface 02

Calendering principle 04

Selected Base Paper Challenges 10

Calendering Process 14

Conventional Calendering 26

New Calendering Methods 39

Research Demands 48

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Preface

Paper manufacturing is a long sequential process including pulping,

papermaking and finishing. Calendering is the last process of finishing

having a great effect on overall efficiency and product quality.

It is very important to have comprehensive knowledge of all parts of the

process to get maximum possible overall efficiency and good product

quality.

As an example in the following, I will present what is important in

calendering and how it has been developed during latest 20 years.

Helsinki, 4 March, 2016

Pekka Komulainen

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Calendering Principle

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Calendering principle

A calender is a series of pressure

rollers used to smooth and gloss a sheet

of material such as paper, cloth or

plastic film.

Important variables are:

Original paper properties

Calender itself with rolling contact

against paper surface

Calendering effects on paper properties

through:

Replication of roll surface pattern

Paper compression

Particle orientation

Flow of coating

Picture: Knowpap

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Calendered grades

It is difficult to make matt but

smooth paper, which would be

ideal for several purposes.

Glossy Paper Gloss 50-80

PPS <1

Silk or semimatt Gloss 20-40

PPS 1-2

Matt Gloss 10-20

PPS >2

Pictures: Jouni Marttila

Combinations Smoothness Gloss

Uncalendered, matt Low Low

Brush polished Low High

Special, silk finish High Low

Gloss finish High High

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Coated paper surfaces and calendering

Hunter Gloss, %

PP

S R

ou

gh

ness

, μ

m

1 Soft/Soft nip

2-nip Soft Calender

Multinip

Calender

Picture:Jouni Marttila

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Smoothening and glossing mechanisms

Pressing of highest tops (smoothness)

Pressure forces are important

Plasticity of total paper is required

Roll surface replication (gloss)

Smooth and clean roll surfaces

Only paper surface plasticity needed

Coating flow from tops to pits (gloss)

Plasticity of paper surface is required

Tangential forces are important

Particle orientation (gloss)

Plasticity of paper surface is required

Pressure and tangential forces important

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Importance of roll surface smoothness

The main glossing and

smoothening effect replicates

to the side against the hot iron

rolls.

Smoothness of the resilient

rolls also have effect on the

surface of the opposite side of

the coated paper quality.

In this example gloss is 8 %-

unit lower, when the rough roll

is in the bottom position

compared to the top position

(Nr 2 vs. Nr 7).

If new rolls are rough they

must be placed to the top

position.

Picture: Voith Paper

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Selected Base Paper Challenges

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Importance of fiber wall thickness

It is important to have several fiber layers in a thin paper to get good formation,

smoothness, opacity and gloss. This correlates with thin fiber wall.

To reduce roughening in offset printing it is also important to have thin fiber walls.

Area = Perimeter x Wall Thickness, A=P*T

Fiber volume = Area x Length, V=A*L=P*T*L

Coarseness = fiber weight/Length, C=W/L

C = Volume*Density/Length, C=V*ρ/L=P*T*L* ρ/L= P*T*ρ

Fiber grammage (g/m2) = Coarseness/fiber width = P*T*ρ/P*2 = 2*T*ρ = 3*T (in µm)

Fiber wall density ~ 1500 kg/m3

~ P/2 Fiber Fiber

Wall Thickness Grammage

µm g/m2

1 3

2 6

3 9

4 12

5 15

6 18

T Wall density ~ 1500 kg/m3

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Fiber wall thickness of Norway spruce

Average fiber wall thickness of Norway spruce TMP is almost 2 µm but there are

some fibers with wall thickness of 3-5 µm.

Reme, P. A., Kure, K.-A.,

Gregersen, O. W., Helle, T.,

1999 International

Mechanical Pulping Conference

Picea abies

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Coating and calendering quality

Uneven base + even coating (curtain) high roughness, gloss

mottling and uneven ink absorption after calendering

Uneven base + blade coating good smoothness but uneven

ink absorption

Even base paper + even coating (curtain or blade) ideal result,

even gloss and no print mottle

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Calendering Process

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Calender operation

Important calender control variables are:

Total nip impulse (linear load, number of nips, speed)

Web temperature and heating (gradient)

Web moisture and moistening (gradient)

Main controlled web properties are:

Smoothness

Gloss

Porosity/absorption properties

Caliper

Two-sidedness

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Nip impulse

Paper is viscoelastic. This means that not only the pressure but also the time of

pressure has effect on the calendering.

Effect of pressure forces in calendering are related to pressure impulse, which is

about the same behavior as in wet pressing.

Impulse = Pressure x Time

Pressure = linear load / nip length

Time = nip length / speed

Impulse = linear load / speed

time

Impulse

= area

Impulse = Σ pressure x time = speed

Σ linear loads

Nip pressure

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Calendering effects on paper properties

Positive Effects:

Smoothness (rotogravure)

Gloss (coated papers)

Absorption and porosity (SC paper, release paper, cartonboard)

Linting tendency (offset)

Caliper control (specialty papers)

Two-sidedness control (printing papers)

Negative Effects:

Bulk and stiffness

Strength properties

Light scattering

Opacity and brightness

Blackening, mottling

Barring

Wrinkles and calender cuts

Runnability

Costs

Special Effects:

Widening of web <0.5 %

Drying of paper 1-10 %-unit

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Calendering of second side

Hot glossing surface

Paper before the second side hot roll

Elastic roll cover

Arch breaks down – smooth

surface roughens again

Glossing of second side

Hot glossing surface

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Effect of moistening on SC paper

Thick-walled mechanical fibers swell in moistening thus reducing paper smoothness and

gloss.

If these fibers are compressed in calendering, they easily spring back to the original form.

Fibers should be smaller and thin-walled to stay collapsed.

Thick fibers should be compressed at the wet end to get more permanent result.

Moistening with water like in offset printing.

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The influence of nip load on pore structure

When linear load is increased compaction of coated paper can be seen in

reduced number of large pores.

Rescxh et al. January 2010 Tappi Journal 20

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Soft calendering, gloss and smoothness

It is possible to improve gloss of coated paper by increasing steel roll temperature up to 190 °C.

PPS roughness decreases with temperature only up to 150 °C and is after that constant with increasing temperature.

Robert Rounsley, January 1991 Tappi Journal

No effect

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Influence of humidity on paper gloss

If paper is not in balance with the ambient air humidity gloss decreases.

To get a good printing result, correct and even moisture content is as important as gloss or smoothness itself.

In the picture gloss decreases 25% when air humidity is increased from dry air to 90% humidity. Gloss improves slightly when paper is dried again.

TOSHIHARU ENOMAE

AND PIERRE LEPOUTRE:

JPPS 23(7):J1-J7(1997)

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Coated paper and COF

Calendering reduces COF only slightly

compared to coating formulation.

Adding PCC to clay coated paper

formulation increases Coefficient of

Friction (picture).

Lubricants reduce friction of coated

paper.

It seems that too much friction and

shear force can cause dusting and fiber

picking to the calender rolls.

Too high COF can also cause vibration

problems on customer roll winder.

Picture: Toshiharu Enomae, Naoya Yamaguchi and Fumihiko Onabe

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TMP handsheets, PPS and density

Smoothening made only with calender is not retained after offset printing.

Forseth, T., Helle, T., Wiik, K., 1996 International Printing and Graphic Arts Conference

After remoistening

(simulated printing)

After

calendering

Before

calendering

Density 300 750

450 750

PPS

83

PPS

35

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Temperature gradient calendering

Whole paper deformed Only surface deformed

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Coventional Calendering

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Calendering methods

Machine calendering (hard rolls, MF)

Brush calendering (old method for cartonboard)

Soft calendering i.e. one nip per soft roll

• On-machine soft calendering or gloss

calendering with hard/soft rolls

• On-machine matt calendering with soft/soft rolls

Multi-nip calendering including intermediate soft

roll(s)

• Off-machine (Supercalender)

• On-machine (Janus, OptiLoad etc.)

Wide nip shoe calendering

Hot metal belt calendering (Valmet)

Aqua cooling calendering (Valmet)

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Early supercalender

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Typical hard nip calender

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Two-nip soft calender

This kind of soft calender is typical for copy and other uncoated woodfree papers.

Sometimes only one nip is needed, if base paper is not symmetrical.

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www.mhibeloit.com

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On-machine calendering

Pre-calender

before coating

Picture: Voith Paper

On-line calender

after coating

Hot iron rolls

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Supercalender

Soft rolls can be paper filled or synthetic covered rolls

To make glossy paper about 10-12 rolls are needed

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Roughness, gloss and calendering

Multi nip calendering is required for WFC, MWC, LWC and SC-A grades. Instead, other grades illustrated in the graph below, can be calendered using soft nip calender. A hard nip calender is not very suitable for offset paper due to mottling tendency.

Hu

nte

r G

loss, %

News

SC - A

SC - B

Improved

News

LWC

SC - C

10

20

30

40

50

60

70

0 1 2 3 4 5

Hu

nte

r G

loss, %

News News

SC - A SC - A

SC - B SC - B

Improved

News

LWC

SC - C

FCO

WFC

MWC

Soft

Calender

Multinip

Calender

µm

PPS 10 Roughness,

Multinip

or Soft

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Cross Direction Profile Questions

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Supercalender profiles and temperature

In a cold supercalender edges may have higher pressure than the middle.

When supercalender heats up the hotter middle part presses more than colder edges.

It is important to take nip impressions when calender is warm.

ELISABETH H. JONES AND ROBERT H. MOORE, TAPPI Journal, Feb. 1997 34

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CD-moisture control

Conventional SC paper is overdried to 2-4% and then moistened to 8-10%.

Overdrying evens out the profile, because moist parts have better heat conduction and easier evaporation.

On-line calendering requires good moisture profiles without overdrying.

If profiles are not good, final moisture must be reduced (average printability and runnability are then suffered)

Moisture

10%

5%

1. Higher steam cons.

2. Less water sold

3. Lower runnability

Front Back

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Moisture level and CD profiles of SC raw paper

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Max 6.07

Avg 5.00

Min 4.02

Max-Min 2.05

5%

Max 7.21

Avg 5.93

Min 4.34

Max-Min 2.87

6%

Max 9.37

Avg 7.66

Min 5.60

Max-Min 3.77

8%

Max 3.49

Avg 2.96

Min 2.56

Max-Min 0.93

3%

Picture: Valmet

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Example of grinding tolerance effect on paper

Calender roll diameter 500 mm. Best possible grinding accuracy 1 µm = 0.0002 % of

roll diameter but 2 % of paper caliper (magnification 10 000).

From a 1000 mm roll diameter 2% is 20 mm (very high difference). Paper maker’s

demand is always higher than any maintenance can offer. It is always feasible to grind

rolls to the best possible accuracy.

Accuracy cannot be better than measurement. If the measurement accuracy is 0.01

mm the result is ten times worse (quite common).

Paper caliper 50 µm

+0.5 µm

Roll surface

+0.5 µm

49 µm = 2% lower caliper

Cross machine direction

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Moisture streaks on machine reel

The wide temperature variations seen in the left IR picture, caused by evaporative cooling,

correspond to variations in CD moisture.

Picture on the right show severe moisture streakiness. This is so narrow that it is not

shown with standard scanning measurements.

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New Calendering Methods

How heavy is your iron?

>7 kg < 1 kg

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Metal belt calender

Dwell time under heated belt and nip is extended. Heated steel belt is only 0.8 mm thick.

Three rolls are oil heated. Surface temperature of rolls and belt is 150 - 200 ºC.

Metal belt precalender improves final smoothness and printability

• less re-roughening during coating

• uniform coating layer

• low final PPS roughness

• low mottling values after printing

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0

5

10

15

20

25

0 20 40 60 80 100

time, ms

pres

sure

, MP

a

Extended

calendering zone

hard nip

soft nip

Metal belt calender

Metal belt calendering

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Results after final calendering

Metal belt precalender gives lower PPS roughness at same bulk level.

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0.76 0.78 0.80 0.82 0.84 0.86 0.88

Paper Bulk, cm³/g

PP

S s

10 r

ou

gh

ness, µ

m

no precalendering + blade coating

hard nip + blade coating

soft nip + blade coating

2 shoe nips + blade coating

metal belt calender + blade coating

Precalendering + Coating

Final calendering

5 nips, 145 °C, 255 kN/m

Picture: Valmet

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Precalender concept Bendtsen roughness, ml/min

Before After Difference

Hard nip, (150 °C, 150 kN/m) 83 120 +36

Soft nip (150 °C, 300 kN/m) 78 111 +33

Metal belt calender (150 °C, 70 kN/m) 90 114 +24

Metal belt calender (150 °C, 100 kN/m) 65 87 +23

Results of rewetting test

Bendtsen roughness of calendered paper before and after surface moisturizing:

Less re-roughening when moisturized for metal belt calendered paper

More stable surface with metal belt calendering

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Metal Belt precalender

Metal belt precalender + multilayer curtain coating gives high gloss and

excellent printability

• glossing pigment in top layer

• more uniform coating color layer

• PPS s10 roughness level is comparable to “hard nip precalender + blade

coating” (common technology today)

Multinip final calender gives clearly higher gloss and lower PPS than a

two-nip soft calender

Metal belt precalender and/or murtilayer curtain coating can not fully

compensate for a lighter final calendering

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Valmet metal belt calendering

OptiCalender Metal Belt is easy to use. Linear load is the only control parameter that

needs adjusting in the calendering process.

Two-sidedness can be controlled with temperature. Also the operating window is

large. The example in the figure below shows the comparison of coated board

calendering (the quality level and bulkiness).

Picture: Valmet

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Valmet Aqua cooling calender

With conventional roll nip, the best calendering results are achieved when the web is cool

and stiff, like in pilot trials.

With aqua cooling technology, pilot-type conditions are brought to production-level, by

cooling down the web before the calendering nip.

The first system is delivered to Stora Enso Inkeroinen board machine to produce folding

boxboard. With aqua cooling technology it is possible to get almost half of the metal belt

calendering benefits with significantly lower investment costs.

Picture: Valmet

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Calendering effect of Aqua cooling

The picture shows cooling effect of 200 g/m2 FBB to Bendtsen roughness and bulk.

Speed is 600 mpm, thermo roll surface 200 °C, and nip loads 30 and 150 kN/m.

Compared at the same roughness level (150) after calendering, bulk was 1.72 at an

incoming web temperature of 65 °C, while bulk was 1.78 at an incoming web temperature of

36 °C after cooling the web from 65 °C. This means that extra 3.5 % bulk can be obtained.

Picture: Valmet

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Research Demands

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Old truth is still valid

I had a presentation in Turku, Finland 27 October, 2004

The title of the presentation was “Calendering Processes -

Future Scenarios and Research Demands”.

The following two pages are a copy of that presentation.

Now, after more than ten years, it is interesting to note, that

there has been huge progress in almost all of those areas

which I listed.

The only area where I desire more research is calender nip,

including forces and their effect on smoothness, gloss and

bulk. Not only pressure force but shear forces, friction forces

and the length of slip and stick areas in the nip of elastic roll

material.

Important would also be the effect of drive torque and

Poisson’s ratio to these phenomena. Old studies show that

smoothness is different when iron roll drives elastic roll

compared to the opposite when elastic roll drives iron roll.

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Research demands

More comprehensive research

Trials with variables in papermaking, coating and calendering.

Effect of friction and shear forces as well as Poisson’s ratios of soft rolls

How to simulate on-machine calendering?

More mill research to get correct incoming web properties and CD profiles

Two-sidedness control

Total process control in papermaking, coating and calendering

How to get even-sided paper at the same time in gloss, smoothness and oil absorption.

Interactions between (shoe) pressing, drying and calendering

Calendering effect on the soft roll side

Improved bulk and stiffness

How to get better gradient effects with not only temperature, but also with moisture and base paper (raw material layering, press section gradient, drying section gradient)

Hot calendering without water evaporation

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Future calendering scenarios

Reduced calendering costs

More on-machine calendering

Simple, high-capacity calenders

Better two-sidedness control

Simultaneous glossing and smoothening of both sides

Multivariable control for gloss, smoothness and oil absorption

Improved bulk and stiffness

Heavy calendering of base paper – light calendering of coated paper

Calendering chemicals and ”chemistry”

Web cooling, variable dwell time moistening

Three-layer base paper (or otherwise different surface than in the middle)

Air conditioning around calender

Better CD-profiles and winder rolls

Additional CD profile measurements of temperature, moisture, porosity,

blackening, web tension and roll hardness

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Thank You

for Your Attention