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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, %
PPSRoughness,μm
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
83
PPS
35
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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.
HunterGloss,%
News
SC-A
SC-B
Improved
News
LWC
SC-C
10
20
30
40
50
60
70
0 1 2 3 4 5
HunterGloss,%
NewsNews
SC-ASC-A
SC-BSC-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
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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
pressure,MPa
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
PPSs10roughness,µm
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