paper roll quality and roll hardness
TRANSCRIPT
Pele Oy
Paper compressibility and roll hardness
Newsprint is compressible and bulky paper compared to LWC and coated woodfree.
During winding newsprint loses the original wound-in tension very fast and the rolls are easily too soft. High density papers are not compressed as much and rolls are harder.
0
5
10
15
20
25
0 50 100 150 200
Co
mp
ressio
n,
%
Radial Pressure, kPa
Newsprint
LWC
Coated woodfree, gloss
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Roll stresses
Every wound paper layer on the roll surface
increases radial pressure and compress the
inner portion of the roll. Compression reduces
the original roll periphery and thus
circumferential tension as long as there will be
balance between the outer tension band and the
spring-back of the inner roll body.
When the balance is reached, there is a
negative circumferential tension inside the roll.
This tries to buckle the layers, but paper
stiffness and radial pressure prevent the
buckling in a good roll.
The positive tension band of a bulky paper is
only some centimeters from the roll surface.
On glossy SC and LWC rolls this positive tension
band is much thicker and the highest pressure is
reached deeper in the roll.
Compressed diameter
after winding additional layers
Outside
diameter
Original diameter
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Pele Oy
Internal roll stresses in a good roll
In a good roll the radial pressure between paper
layers is highest against the core and then
constant or slightly decreasing to the roll
periphery.
If radial pressure P can be measured,
circumferential tension T can be calculated:
• T = - P - dP/dr x r
• where r = roll radius/core outside radius
• P = internal roll pressure, kPa
• T = internal web tension, kPa
In a newsprint roll P can be 300 kPa (3 bar) and if
the curve is flat in the middle (dP/dr = 0) tension T
is also - 300 kPa (= MD compression).
If paper caliper is 0.07 mm we get a negative
tension of - 21 N/m.
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Roll stresses and paper properties
The positive tension band on top of the
roll is only some centimeters thick. Inside
this band paper is plastically strained and
under the band plastically compressed
during storage. This paper deformation
reduces all roll stresses.
In the following unwind process paper on
the roll surface has lower elongation and
dynamic stretch. This might have
influence on paper runnability and breaks
during flying splice.
Paper inside the roll is more compressed
and has better smoothness, lower bulk
and caliper, lower porosity and better
elongation and dynamic stretch
properties.
Main part of the paper is not similar as a
sample from the roll surface.
Can be runnability problem
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Pele Oy
Example of paperboard properties in a roll
Physical properties of paper are changed in a roll due to the stresses during winding and
on roll storage. Paperboard (here FBB) changes more than printing paper.
The picture below is an example of paper gloss, which is about 10 % higher (about 6 %-
unit) on the core compared to the roll surface.
This is from a two-drum winder. Belt supported winder gives more even result.
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Distance from roll core 0 750 mm
Glo
ss
, %
JUHO WALDÉN, 2014
Pele Oy
Winding and ”hardness memory”
Radial pressure in a machine reel is normally decreasing towards the periphery so that the surface is softer than the start of the winding. One reason is that the nip widens when the reel diameter grows. In addition, all paper reels have low radial pressure on the surface due to less and less pressing layers.
Paper layers on the machine reel surface are less compressed than inside the reel (they are thicker on the surface).
Winding is a very dynamic process. Web travel from unwind to rewind can be some 10 m and winder speed 40 m/s. The delay time from unwind to rewind is then 0.25 s. In this short time paper is without compression and it tries to expand to the original thickness. However, the time is so short that the thickness inside the machine reel is almost ”copied” to the winder roll and the compression due to winder parameters still reduce the thickness.
The result is that the first set from machine reel surface tends to be soft at start and hardness is increasing towards the roll surface. The last set tends to be more constant in hardness and especially soft starts or roll deformations are not as big problems.
Diameter
Hardness
after winder
3rd set
1st set
Diameter
Radial pressure
of parent reel
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Pele Oy
Roll density measurement
Online roll density measurements can be made in winding or unwinding supposing that
the original paper thickness is constant. This is possible by calculating paper caliper
inside the roll from the angular and surface speeds of the roll.
The picture below shows typical newsprint roll density distributions and how the first
set on the winder is softer than the other sets.
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660
665
670
675
680
685
690
695
100 300 500 700 900
Roll diameter, mm
Ro
ll d
en
sit
y, k
g/m
3
First set
Second and last sets
First online density measurements
Pekka Komulainen, 1977
Veitsiluoto, Finland
Pele Oy
Paper roll is a spiral
Paper roll is not a cylinder formed of paper rings but a
continuous spiral of paper with continuous MD tension
(plus or minus).
CD differences of web tension, strain and original web
length are important due to the spiral form.
Differences in web length and strain also have effect on
roll hardness in a spiral.
Often an accuracy of 0.1% is enough in papermaking. If
we have this difference in original paper length, it means
that after winding only 1000 m we would have the longer
part of the web one meter ahead of the shorter part. We
need web tension to even out these differences.
When crepe wrinkles inside the roll are formed, it is not
only one layer slipping but sometimes two or more layers.
This causes a longer total movement and more severe
damages between the slipping layers.
Due to the slipping, telescoping can be an additional
problem.
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Pele Oy
Example of the spiral web effect
If you measure drum speed difference in
a two-drum winder you will find, that the
front drum has slower speed in spite of
higher torque.
The explanation is that there is all the
time one web layer less on the front
drum compared to the rear drum.
Theoretical curve can be calculated. The
speed difference is higher in the
beginning and depends on the paper
caliper.
Some winders have speed difference
control instead of torque control.
However, if there are several paper
grades it will be very difficult to find
correct speed difference curve for each
paper grade.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
100 400 700 1000
Roll diameter, mmS
peed
dif
fere
nce, %
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Winding parameters for two-drum winder
1 Web tension, T
2a Rider roll load, N
2b Roll weight, N
3 Torque (Winding force), T
Roll diameter
Roll tightness
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= TNT
Pictures: Valmet
Pele Oy
Roll hardness in radial direction
Ideally the hardness should be slightly decreasing from the core to the periphery.
However, this is only possible when the winder has center drive and support.
More common than center drive winder is a two-drum winder, where roll’s own weight
increases nip force and normally some other optimization must be made.
Picture: R. Duane Smith
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Pele Oy
Typical two-drum winder hardness distribution
It is quite easy to get a tight and hard start to
the roll by using high torque.
When the roll diameter increases the roll nip
force decreases due to the geometry. In
addition, effect of torque decreases very fast
in the beginning.
The only way to keep roll hardness constant
would be fast increased rider roll load. For
most papers the rider roll load of large roll
diameters should be so high that there would
be wrinkles and bursts in the roll.
When the roll grows its own weight causes
very high nip load. To compensate this
hardening effect, web tension should be
reduced. However, too small web tension
together with high nip load can cause
wrinkles and bursts.
Roll diameter
Roll
Hardness
OK
Too hard
Too soft
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Pele Oy
Winding and CD profile properties
It would be easier to make directly sheeted paper without
winding like in a pulp dryer.
Total thickness of a paper pile under pressure is related to
original caliper and compression properties of the paper.
The diameter of a paper roll is related not only to the
caliper and compressibility but also to the original web
length and MD strain properties of the paper web.
To make good rolls we should have online measurements
of all the four variables. Sometimes we measure and
control only paper caliper without any information of the
three other profiles.
If there are CD profile related problems after winding,
papermakers usually argue that grammage, moisture and
caliper profiles are straight.
One should first think if these measurements are correct
and relevant and then look, how much worse the other
profiles become by controlling the basic profiles straight.
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Pile of paper
Pile thickness depends on
- paper caliper
- paper compressibility
Paper roll
Roll diameter depends on
- caliper & compressibility
- web length & strain
Pele Oy
Paper elongation and roll hardness
The picture below shows how sheet length or elasticity effect on roll hardness and
bagginess.
If web caliper is constant, longer part of the web forms softer roll or even bagginess
and wrinkles.
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Pele Oy
Web caliper, length and reel hardness
To make even reel hardness in cross-machine direction it is important to have correct web
length related to the roll diameter in every CD position.
For heavy calendered, high density papers constant caliper is more important (paper is not
any more compressible to even out reel diameter differences). For bulky webs length and
caliper differences are both important.
Length Caliper
Thin Average Thick
Short Good Hard Rock-hard
Average Soft Good Hard
Long Very soft
Soft Good
When = π = 3,14 it means that roll hardness is even L
D
L
D
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Pele Oy
Roll Hardness Measurements
Pele Oy
Roll quality testing
Cross Direction Testing
Backtender's stick (wooden
stick)
Beloit Rho-meter
Schmidt hammer
Parotester
Tapio RQP
On-line backtender’s friend
Roll diameter profile
measurement
Valmet iRoll
ACA Systems RoQ
Radial testing
Cameron gap test
Smith needle
Improved needles or plastic strips
J-line measurement
Core torque wrench test
Tapio RQP
Wit-Wot Roll Analyzer
On-line density measurement
ACA Systems RoQ
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Pele Oy
Roll density vs. roll hardness
Roll density and hardness were measured from three different kraftliner grades.
Correlation is very good. Density was calculated from roll weight and compared to
average measured hardness.
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Ro
ll D
en
sit
y
Roll Hardness
Picture:
Linus Söremark
Pele Oy
Example of a radial hardness measurement
Normally roll hardness profiles are measured in CD direction. However, it is
also possible to use modern impact devices also for radial hardness direction
measurement (example below).
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Picture:
Linus Söremark
Distance from core
Pele Oy
Machine reel hardness profiles
The basic shape of the CD profiles in a machine reel is quite constant.
This picture shows relative hardness profiles of four sets measured on the
unwind roll surface from the winder.
Picture: Sami Hyötynen
Metso Paper Oy
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Pele Oy
Parent reel and roll profile forms
The shape of the curves from parent reel compared to customer rolls is very similar.
The average hardness of customer rolls is higher.
Soft spots in parent reel are about 20 units softer than in customer rolls.
D.M.S. WANIGARATNE et al.
APPITA 2010
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Pele Oy
Parent reel and roll hardness profiles
Customer rolls are harder than parent reel.
First set made from the parent reel surface is softer than the second set from the inner
part of the parent reel.
First set from parent reel surface
Second set from parent reel surface
Customer roll hardness profiles
Parent reel hardness profile
Pictures from:
www.doria.fi/bitstream/handle/10024/305
58/TMP.objres.156.pdf?sequence=1
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Pele Oy
Caliper and reel hardness
Reel hardness of 205 g/m2 packaging board correlates very well with caliper
profile.
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D.M.S. WANIGARATNE et al..
APPITA 2010-317
Pele Oy
Effect of supercalender on hardness profile
After coating the hardness profile of LWC paper is quite even (blue).
After supercalendering there is more short and long range variation (red with hard center).
Picture: Ilari Ikonen, 2010
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Pele Oy
CD caliper profile and reel hardness
Caliper as such is not very important. However, papermakers try to control the process
so that the shape of the CD caliper profile is straight (why?).
This does not guarantee a straight hardness profile and it is common, that edge rolls
have too soft ends after winder.
The second roll from the right shows a too large local hardness difference leading to
problems such as bagginess, bursts or corrugations.
Four rolls in a set, edge
rolls are soft at edges.
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Pele Oy
Roll hardness and paper caliper profiles
Typical CD hardness profiles of
uncoated magazine paper.
The profiles are from the top:
grammage, caliper and roll hardness.
It can be seen that caliper follows
grammage.
Final problem is the marked red range
of corrugation, where all profiles reach
the minimum.
Hardness is an accurate measurement
compared to the other measurements.
The biggest relative difference is in
hardness profile (over 10%).
Picture: Juha Turkki
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Pele Oy
Coated paper hardness profiles
The picture below shows machine reel hardness profiles of woodfree coated paper.
Profiles have been measured from winder unwind before winding each set of rolls.
There were totally five sets.
It is typical that the first set is always softer than the other four sets.
Picture: Teemu Pasi
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Pele Oy
Example of several hardness profiles
Example of 17 successive SC paper roll profiles from same CD position.
The stability of the curve form is quite good.
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Picture:
Sami Uhlbäck
2008
Pele Oy
SC paper runnability in gravure printing
The upper curve caused web breaks in printing (too much roll hardness variation).
The lower curve run without problems.
Picture: Sami Uhlbäck, 2008
30
Pele Oy
Local faults in hardness profile
Slack areas increase web breaks and paper waste.
In this picture a 20 mm wide slack area is found 100 mm from the roll edge.
This requires high resolution measurement.
Slack area
31 Picture: Sami Uhlbäck, 2008
Pele Oy
Caliper and hardness control
Caliper control with calender is directly seen on hardness profile.
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Pele Oy
CD shrinkage profile in paper drying
This shrinkage profile is from a modern newsprint machine with single felted dryers.
One meter from the edges has 3-8% shrinkage while it is in the middle less than 2%.
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Picture:
Steve I’Anson
Pele Oy
Thickness scanner vs. iRoll CD profile (Valmet)
Hardness profile is a basic roll quality measurement. It cannot be replaced by
online caliper measurement. Valmet iRoll is a good alternative, but very
expensive. The following is what Valmet comments on caliper measurement:
There are four main issues with thickness scanner performance. These are:
1. Resolution is too low, in practice 1 micrometer or worse. The needed
resolution should be at least 10x better. Quality Control System (QCS)
suppliers sometimes state that they have better resolution but that is not true
in reality.
2. Requires constant care. There is a constant need for cleaning, calibration
and other maintenance and tuning. Without this the whole measurement
concept fails to work. In practice, it's not possible to provide this constant
service in a mill environment.
3. Caliper is not the right thing to measure. To assure good reel buildup and
runnability, the primary parameter is the hardness/diameter profile.
Thickness is only a secondary parameter that effects hardness and diameter.
4. Web breaks are caused by contacting caliper measurement scanners.
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Pele Oy
Continuous hardness measurement on winder
Due to the edge shrinkage on paper drying the edges are normally soft and loose under
the rider roll on winder. On a two-drum winder this is a demanding situation.
It is very dangerous to press edges more than the center on rider roll or otherwise wrinkles
will arise (loose web and high nip load at the same time). To avoid this, roll hardness
profile is normally slightly lower at edges or rider rolls are installed slightly higher at edges.
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Picture: Valmet
Pele Oy
Typical Roll Defects
Air to
roll
Uneven
hardness
profile
Pele Oy
Main problems of machine reels
The material is from 2004. The mills were printing paper mills. The most common
problems seem to be wrinkles, TNT control and CD profiles/bursts.
The main problems are connected together and could be reduced with new type of reels.
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Collected from several newsprint and
magazine paper mills
Pele Oy
Pressure from spool to paper
The problem: Wrinkles and bursts on reel bottom. Coated board: glossy
spots close to the spool. Bottom broke up to 2% = 2000 m of 100 km.
Problem areas
There is more problems at edges when the spool is thin, reel is wide, diameter is large, basis weight is low and paper density is high.
Thin groundwood papers are more sensitive. CD profiles are important.
Reel hardness, hardness distribution, air between layers and machine speed have influence.
Center drive, larger diameter reel spools and new type of reel changes can solve the problem.
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Picture: Valmet
Pele Oy
Main problems on printing paper winders
The most common problems seem to be dishing or uneven roll edge, CD profiles,
crepe wrinkles and loose cores or bad start.
0 2 4 6 8 10 12 14
Dishing/uneven edge
Profile
Wrinkles
Loose cores/bad start
Trim
Bursts
Splicing
SMB's
Spreading
Roll bouncing
Slitting
Vibration
39
Collected from several newsprint and
magazine paper mills
Draft only
Pele Oy
Product quality control by hardness profiler
The end product of a paper mill is the customer roll. Even
if paper is sheeted, it must first form a good roll.
Papermakers normally measure grammage, moisture
and caliper online. However, the correlation of roll quality
with online measurements is often very poor and the
evaluation of roll quality must be made by hand and
visually.
Roll hardness measurement reflects roll quality much
better than any other profiler from paper itself.
If the hardness measurement is easy, fast and accurate,
it could even replace in many cases online CD profile
measurements.
Mill information system can draw complete CD profiles
from individual roll measurements and send the
information to the laboratory and operators.
Normally the need to adjust CD profiles is only when
some changes are made and that would suit very well for
roll hardness measurements to control paper quality.
Severe roll hardness and
diameter variation of wrapped
LWC roll
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Pele Oy
Corrugations or rope marks
Roll hardness/roll diameter is different on
each side of rope mark.
Rope mark propensity can be found with
hardness measurement before it can be
seen.
Root causes for corrugations are:
• Differences in paper cross-direction profiles
of caliper, compressibility, tension and
elasticity
• Differences of nip load in cross-direction or
web travel from unwind to rewind
• Too hard winding
• Most problematic with high density papers
(glossy papers)
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Pele Oy
Bagginess
Bagginess can be hidden inside the roll and will show up when paper is unwound in
converting. Roll hardness and web tension profiles can easily show this kind of
bagginess propensity before it can be seen.
Typical for bagginess is:
Out-of-plane buckling (at low tension)
Bagginess is caused by CD profile variation (grammage, moisture, caliper etc.) together
with hard winding and low elasticity of paper
Typical for thin, hard calendered papers with coating or high filler content
Can be seen only inside the roll or on top of the roll
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Pele Oy
43
Cross machine direction burst
This burst is identified by a break in the sheet across
the roll face. This burst is generally found near the
core or in the outer few centimeters of the roll that is
wound too tight (hard).
The burst can be across the full face of the roll or a
partial burst just enough to break down the web
under stress.
If rider roll is pressing too much, this can be in the
middle of the roll radius.
Reasons are:
• Too hard roll
• Tight web together with high caliper
• High MD tension and nip pressure
• Low MD strength and stretch (low TEA)
• Very dry paper
Pele Oy
Crepe wrinkles and their reasons
Combination of low web tension and high nip load
Hard area on top of soft area or increasing hardness to the roll periphery
Low web caliper/grammage and low MD stiffness
Low coefficient of friction, slippery inside the roll (DIP is today well washed and COF is
quite high, the first DIP plants produced very slippery paper)
Hardness variation in CD profile, loose and thick web at edges
Uneven pressure from rider roll
MD
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Cross sections of crepe wrinkles
(David Mcdonald, 2014) Crepe wrinkles start from the roll edge
Pele Oy
J-line and crepe wrinkles
Internal slipping of paper layers under the roll surface
(decreasing tension) is measured with J-line
measurement.
Shooting or marking a roll radius and then winding
layers on top of that will move the marked layers
towards the winding direction (= loosening the original
web tension).
If the original tension is low and slippage high there will
be a negative tension in the machine direction resulting
in buckle of the layers (= crepe wrinkle).
Very bad combination is a high nip load, a low web
tension and a low COF. This is most common at the
machine reel edge, where caliper can be high and web
length long (=loose edges). In addition edges can be
dry and thus COF is low.
It is best to have curved CD profile = slightly lower
caliper at edges to avoid crepe wrinkles and other
problems.
Read more: Finishingnet.com
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Pele Oy
Sensitivity to crepe wrinkles
There will be slippage and possibility to get crepe wrinkles, when
- F > µ • p • A + S
- F = buckling force from nip action = f(N and nip width)
- A = area of possible sliding
- S = paper stiffness force
To avoid crepe wrinkles paper static COF must be high as well as roll hardness, but nip load should be low.
Low paper stiffness and caliper increase possibilities to get crepe wrinkles.
With a soft drum cover the buckling force and J-lines are smaller than with a hard winder drum. Possibility to get crepe wrinkles is very small.
Picture: Valmet
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Pele Oy
J-line measurement
When measuring J-line all possible safety
measures should be considered. A safe
method is to stop the winder, draw a radial
line to the end, and then start winding again.
After winding the J-line can be measured,
photographed and analyzed.
This method, however, is not correct,
because stopping and starting have effect on
winding forces and slippage of the layers.
The old, but not so safe method is to shoot a
chalked special string against the rotating roll
end and then look the line after winding (right
picture).
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Picture: J.K. Good
Pele Oy
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Effect of process on crepe wrinkle formation
Crepe wrinkle is a common defect at web edges. All parameters from raw materials to the winder have effect on crepe wrinkle formation – especially at edges.
In unwinding it is easy to feel the wrinkles by hand. If there are wrinkles already in the machine reel you can feel them on unwind reel at winder.
Main winder wrinkles must be checked at salvage winder.
To improve the situation all the papermaking process should be improved – not only winder parameters. At the edges the situation can be described as follows:
Higher CD
shrinkage
CD control
of B.W
Lower B.W.
to dryers
Faster drying
at edges
Large roll
diameter
Permanent
edge strain
Lower MD
E modulus
Strain on reel
surface
Loose
edges
Larger reel
diameter
Web run,
spreading
Higher rider
roll load
High nip
load
Dry edges
to dryers
Higher
caliper
Higher
porosity
Higher dryer
temperature Sheet
flutter
High paper
density
Low web
tension
Crepe
wrinkles
Low paper
static COF
Low paper
MD stiffness
Pele Oy
Effect of splicing on roll structure
Rolls having winder splices are never as good as
normal rolls.
If the splice is made because of web break in the
machine reel, the winder must be decelerated
before the splice, which increases web tension.
During the splicing the web can be loose, which
decreases roll hardness.
After the splice winder is accelerated, which
reduces roll hardness.
On top of this loose part nip effect is higher and
roll hardness is increased again.
On a rewinder (salvage winder) dynamic forces
are smaller and spliced rolls can be better.
Roll hardness
Roll diameter
Splice
49
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Transport and roll storage
Sometimes rolls are stored
horizontally on each other.
If rolls are too soft they will form
out-of-round.
On a two-drum winder soft rolls
always have harder surface, and
they easily get buckled and show
starring on the end.
Internally soft rolls are formed in
the positions where rider roll is
not contacting the rolls.
50
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Roll ridges and hardness
Rolls with hardness variation can look quite nice on the winder, but after
storage and transport there can be severe bagginess on the hard areas.
This kind of bagginess is higher on the roll surface, where the nip load
and diameter differences have been highest.
Original picture: Roisum
51
Pele Oy
Starred roll and hardness
Stars or paper buckling in the machine
direction is caused by negative MD
tension inside the roll.
The basic reason is increasing roll
hardness towards the roll surface.
Starring could occur later when the roll
gets external forces and impacts in
handling, transport and storage.
It is very important that rolls are not
stored like in the picture.
52
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Dished roll or telescoping
Reasons can be:
Bad spreading (D-bar), misalignment of
incoming webs
Misalignment of winder rolls
Unsymmetrical diameter and hardness
of paper roll (CD caliper or tension
differences)
Soft cores in CD direction
Core chuck pressure is too low
Air entrainment with high speed
Low paper-to-paper COF (Coefficient Of
Friction)
53
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Why rewinder (salvage winder) makes hard rolls?
If only one roll is pressed with rider rolls there is good contact. On the main winder edge rolls may be smaller in diameter and not pressed at all at the end of winding.
Bulky paper like newsprint is compressed plastically and deformed after main winder. Thinner and denser paper makes harder rolls. Especially roll surface is made on rewinder from the hardest part of the unwinding roll. Hardness is easily increased towards the roll surface at the rewinder.
Sometimes web tension control has same total force for wide and narrow rolls. Then the narrowest rolls can have very high tension (= force/width).
Drum radius of a two-drum rewinder is normally smaller than that on the main winder (e.g. 550 mm vs. 850 mm). Narrow nip – harder roll from own weight.
Winder speed of a rewinder is about half or the main winder speed (e.g. 1200 vs. 2400 m/min). Higher speed – more air into the roll, higher centrifugal force and faster nip impact all reduce roll hardness on the main winder.
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Optimum roll hardness
Average hardness
Hard
ness v
ari
ati
on
To
o s
oft
ro
ll
To
o h
ard
ro
ll
Too much hardness
variation
Good roll quality
Soft roll starring
55
If the roll is too soft, especially in the center, there will be handling damages such as
starring and out-of-roundness. Additionally, there can be crepe wrinkles or telescoping.
If the roll is too hard there can be bagginess and bursts.
Too soft or too hard rolls tolerate less roll hardness variation than rolls with optimum
hardness. There is always some CD profile variation in the hardness and it is important
to find the optimum average hardness level to avoid roll broke.
Pele Oy
Roll hardness and roll deformation
It is clear that harder rolls keep roundness better in handling than soft rolls.
Permanent deformation grows very fast when the roll is softer.
However, it is evident that the roll should be hard inside but softer on the top.
Unfortunately two-drum winder tends to make the opposite.
56 Picture: D.MCDONALD, J. HAMEL AND A.MÉNARD, 2005
Pele Oy
Conclusion
The following four things are most important to be remembered about roll quality:
The end product of papermaking process is a high quality paper roll and its quality must be followed.
The highest cost broke of the papermaking process is at the end.
All faults should be corrected where they first appear (in the process phase and position). It is never
possible to totally compensate earlier fault with later correction.
Winding cannot make paper better, but in many cases product quality can be much lower after
winding.
57