The Art of Owning & Operating Rubber Calenders

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The Art of Owning
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CALENDERS

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INSIGHT
July 2016
8
COVER STORY
The Art of Owning
and Operating
Rubber Calenders
A cover story that you will like for all the right
reasons as it covers the different facets of
rubber calenders.
Machinery
World
Rubber Tyre Knowledge On-The-Go Special Supplement
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Calendering is one of the oldest rubber processing
technologies. This is a mechanical process by which rubber
is pressed into textiles (cloth, fabric, tire cord) forming
composite sheets.
Rubber Calendering is classified into two based on what you
are calendering. Your guiding principle for calender line
operation should be consistency and continuity.
The machinery that helps you do rubber calendering is
known as a Rubber Calender and it performs various
functions. This is a heavy-duty machinery consisting of
two or more rolls that revolve in opposite directions.
Today, you have a wide range of new and used calender
purchase options.
Choosing the right Rubber Calender is important; learning
to operate it smartly is equally key for your success with
this machine. This edition covers all these aspects in a
descriptive manner.
In addition, we have the ‘Insight’ section that is an
infographic on ‘7 Proven Strategies To Methodically Grow
Your Business’. I hope you enjoy this issue.
As always, I welcome your feedback on
rubbermachineryworld@gmail.com.
Best Regards
Prasanth Warrier
| Editor’s Note |
The Art of Owning Operating Rubber
Calenders
Choosing the right
Rubber Calender
is important;
learning to operate
it smartly is equally
key for your
success with this
machine.
7
rubbermachineri
rubbermachineryworld
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grp/home?gid=8252803
rubbermachineri
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Rubber Tyre Machinery World does not verify/vouch for their genuineness, hence will not be liable for any misrepresented data)
Rubber Tyre Machinery World | JUL 2016

8
| CALENDERS |
The Art of Owning and Operating
Rubber Calenders Prasanth Warrier
Image: Rodolfo Comerio
Calendering is one of the oldest rubber
processing technologies. This is a
mechanical process by which rubber is
pressed into textiles (cloth, fabric, tire
cord) forming composite sheets.
The machinery that helps you do rubber
calendering is known as a Rubber
Calender. The word 'calender' itself is a
derivation of the word kylindros – the
Greek word that is also the source of the
word 'cylinder'.
Despite its development in 1836,
calendering as a process became
popular only after the 1930's
(almost 200 years later)!

| CALENDERS |
Delayed rise to popularity
In 1836 Edwin M. Chaffee, of the Roxbury
India Rubber Company, patented a four-
roll calender to make rubber sheet. He
worked with Charles Goodyear with the
intention to 'produce a sheet of rubber
laminated to a fabric base'.
Despite this development, calendering as a
process became popular only after the
1930's because it was difficult to adjust the
desired gap between rolls. Consequently, it
was difficult to get an accurate rubber
sheet thickness.
Since the calender machines became easier
to adjust, features have evolved. Modern
calenders' can achieve tolerances around
±0.005mm and their lines are adaptive for
most sophisticated precision requirements
of your product.
Purpose of Rubber Calendering
In calendering process, you pass pre-selected
fabric and rubber through a series of rolls to
flatten, smoothen and sandwich the
materials.
Depending on your end-use, the calendered
sheets could have multiple layers
'sandwiched' together.
A Rubber Calender can help you get either
'supported' or 'unsupported' calendered
sheets.
- Unsupported' sheets contain only layers of
rubber that has been joined together.
- On the other hand, a 'supported' sheet has
textile fabric or steel cord coated with a
film of rubber (also known as 'skim stock
rubber') on both sides and into the material.
Supported rubber sheets give you higher
strength or tear resistance.
The adhesion of rubber to the fabric or steel
cord is critical to final performance. And the
rubber compound that you coat the fabric is
different from the one used to make the
tread or that which coats the cords.
Coating of fabrics has been done for almost
200 years. Steel cord and fabric cord topping
is a process in your tire manufacturing.
Specifications and tolerances for calendered
cords are tough. So you will find it difficult
to fulfill the thickness variation across the
calendered ply and the cord density.
For example, you use calendared textiles for
casing, cap plies and chafers in tyre industry.
And you use calendered steel cord for belts.
Calendering process is also well adopted in
plastic industry (say, PVC sheeting).
Image: JM Machinery
9

10
| CALENDERS |
Types of Rubber Calendering
Rubber Calendering is classified based on
what you are calendering, (A) Fabric
calendering and (B) Steel cord calendering.
Each of these is different yet similar in
operations and sensitive to environment
influence for your high quality product.
A. Fabric Calendering
The materials that go into your fabric
calendering are your softened rubber
compound (from mills) and Nylon or
Polyester from the Dip Unit.
In this process, the tension, temperature
and humidity are critical parameters of
your fabric cords before they are
calendared with the rubber compound.
And hence is the reason you will observe
that the fabric cord is kept in a room
(where temperature and humidity are
controlled well) once this raw material
arrives in your shop floor.
This textile is arranged in a flat and parallel
manner. Under proper tension they are
continually pressed through two rolls.
Simultaneously, you add the rubber
compound to the opening area or nip
between the rolls. As a result, a thin layer of
rubber is applied into top of and the bottom
layers of the fabric.
Then you make this continuous sheet of
fabric and rubber go through many
additional rolls to ensure that rubber is
Image: Maxxis Canada

| CALENDERS |
penetrated properly between the cords
achieving the desired adhesion between
rubber and the fabric.
The sheets are cut at required angles so that
the cords are set at predetermined angles
across the sheet.
Even inner liners for tyre manufacturing are
calendered the same way into sheets of
required thickness and then cut into
appropriate widths for use in tire
construction.
Finally, your calendered fabric sheets are
wound into rolls with layers of woven fabric
liner to prevent the surfaces of sheets from
sticking together.
B. Steel Cord Calendering
In the tire industry, steel cord calendering is
for the radial tires.
Here, the body plies and reinforcing strips
incorporate polyester cord that is coated in
an adhesive liquid. This cord is passed
between large, heated rolls of a rubber
calender machine. Woven fabric is similarly
prepared and calendered for the anti-chafing
strips.
Since your rubber compound will not adhere
to bare steel, the steel cord wires for the steel
belts are coated with a very thin layer of
brass. These high-tensile brass-coated
rubber-encased steel cords (multi-strand
cables) become the steel belts.
You may note that the steel cords come in
various arrangements like cross-section of
pairs, triplets and so on.
The brass-coated steel wires used in the
manufacture of tire components are also
extremely moisture sensitive. Hence, they
must be protected in a temperature and
humidity controlled environment – right
from procurement, to during shipment
and at the tire plant location. More
importantly, any exposure to moisture can
result in corrosion and a breakdown in
rubber adhesion when calendered.
At your plant, these wires are stored in an
environmentally controlled 'Creel Room'
until it is processed in your rubber calender.
You pass a preset number of steel cords
under proper tension from the creel room
on rolls through aligning combs into the
calender where the wires are coated with a
thin sheet of skim stock rubber. For
maximum adhesion, the rubber should also
penetrate these steel cords. Maximum
adhesion also means least rusting.
These steel cords are cut at specified angles
You pass preset number of steel
cords under proper tension from
the creel room on rolls through
aligning combs into the calender
where the wires are coated with
a thin sheet of skim stock rubber.
12

| CALENDERS |
and widths for use in tire building process.
In ground reality, the distance between the
creel room and your rubber calender
(varying between 20 – 60 feet depending on
your layout) is usually not environmentally
controlled and, hence the wire may be
exposed to moisture prior to its being
encapsulated in rubber.
This problem is worsened by slowdowns,
temporary shutdowns, humidity spikes and
failure to adequately control temperature
and humidity within the creel room. Once
the belt wire becomes contaminated with
moisture, it becomes more difficult to
obtain proper adhesion of the rubber to the
brass-coated wire.
The strongest possible bond between the
rubber and the belt wire is critical in the
construction of your steel belted radial
tyres.
Judging Quality
So, how do you judge the quality of a
calendered sheet?
Well, you could quantify the quality of your
fabric calendered sheet in terms of your
pre-desired
- Thickness of the sheet
- Spacing between cords
- Number of cords and
- Penetration of rubber into the space in
between cords.
Recently, an industry friend confided that
many rubber good manufacturers look at a
Rubber Calender as a 'mysterious rubber
machinery'. Is it because of all this nuances,
I wonder?
Image: Cross-section of Radial Tyre
13

| CALENDERS |
Functions of Rubber Calendering
1. Skim Coating or Topping: Here, rubber
is coated on both sides of the substrate (i.e
your fabric or steel cords). This operation
strengthens the adhesion between rubber
and the substrate. Your process decides
whether you use a 3-Roll Calender or a
4-Roll Calender.
2. Frictioning: Here, you use calendering
process to force rubber into the fabric
weave. Frictioning imparts good
adhesiveness. You deploy 3-Roll Calender,
where the top and bottom rolls have a
lower speed than the middle roll. 4-Roll
Calenders cannot be used for Frictioning,
because obtaining a speed difference
between fabric and compound is difficult.
3. Rolling: Here, you just allow hot rubber
compound to pass through a 2-Roll Calender
and get a continuous sheet of rubber of
thickness 3-4 mm.
4. Embossing and Profile Calendering: If
you produce tread sections of cycle tires or
involved in hand building of foot wear, you
may choose a Calender with “patterns” on
the rolls to produce profiled treads or
patterned sheets. A step forward of Rolling,
here you press a heavy engraved roll
against hot rubber compound. You would
consider this if your sheet needs a 'design'
look. The engraved design of the rolls is
transferred to your rubber sheets.
Calendering operations need critical checks
to ensure you get high quality products.
Image: Rodolfo Comerio
14

| CALENDERS |
Types of Rubber Calenders
A calendar is heavy-duty rubber machinery
consisting of two or more rolls that revolve
in opposite directions. The classification of
Rubber Calenders is based on
1. The Number of rolls
2. The Position or Orientation of the rolls
Various rubber calender manufacturers
offer you different types. Today, you have
a wide range of new and used calender
4-RollCalender
» Also known as the “Modern
Calender”.
» Different Configurations.
» Used for Skim Coating.
ʻZʼTypeOffset
StackCalender
InvertedʻLʼTypeOffsetStackCalender
ʻIʼTypeorVertical
StackCalender
2-RollCalender
VerticalTypeCalender
InclinedorTiltedTypeCalender
3-RollCalender
VerticalStackor
ʻIʼTypeCalender
OffsetStackTypeCalender
» Also known as the “Universal
Calender”.
» Highly Flexible in usage.
» Used for Frictioning and
Skim Coating.
» ‘I’ Type is the oldest most
common configuration.
purchase options (see box below) for your
budget albeit of varying nature of quality,
features and durability.
Images from different sources only for representation purposes.
“Pelmar is there to offer a total
solution. Calendering lines sold
as is or refurbished, along with
mixing lines are amongst our
largest sales chunk”
Jacob Peled
Executive Chairman
Pelmar Engineering Ltd.
“The wide range of quality new
machinery made by Bharaj is well-
accepted worldwide. We offer
complete solutions.. I would say that
our Calenders are high in demand.”
Amardeep Singh Bharaj
Director - Sales Projects
Bharaj Machineries Pvt. Ltd.
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| CALENDERS |
8 Common Defects in Rubber Calendering
And Their Causes
A more significant aspect of rubber
calendering is operating this rubber
machinery itself.
You could have chosen your rubber calender
wisely but if you do not operate it smartly,
you are likely to have defects in the
calendered sheet.
And when you know the causes leading to the
defect (refer adjacent box), it becomes easy for
you to investigate them deeper, resolve and
continue production.
» Incorrect crowning of calender rolls
» Excessive wearing of bearings
» Improper operation of roll bending/zero clearance devices
» Roll temperature too high
» High feed bank on to the rolls
» Compound Stock warmed up for longer duration
» Insufficient break-down of your compound
» Inadequate temperature control
» Insufficient compound feed in the banks
» Roll temperature too high
» Large nip opening
» Stock warm up for long time
» High Compound in the nips
» Insufficient nip opening between rolls
» Excess compound in the nip
» Improper roll crown
» High compound viscosity
» Improper Roll temperature
» Improper running speed
» Low compound viscosity
» Improper warming of the compound
» High viscosity compound
» Improper warm up, high bank
» Non uniformity of warm up
» Lower roll temperature
» Lower temperature in one of the rolls
» High friction ratio
» High calendering speed
» Use of anisotropic fillers
Choose Wisely. Operate Smartly
Image of A 3-Roll Vertical Calender from
Bharaj Machineries Pvt. Ltd.
18

| CALENDERS |
7 Do's (And Don'ts) In Calendering
Choosing the right rubber calender is
important, learning to operate it smartly is
equally key for your success with this
machine. (Refer adjacent box)
» Use lab approved fabric and compound only.
» Clean the mill and calender guides including the
equipment surrounding area before starting unit.
» Load single strip of compound to both the cracker
mills. Then feed these compound to your holding mill.
» Always keep minimum compound in calender nips
and mills.
» Apply correct bleeder yarn.
» Put identification tags before storing your calendered
fabric in racks.
» Close cooling water lines to mills when mills are not
running
» Do not over load your two-roll mills and calender nips.
» Do not use held up compounds and dipped fabric.
» Do not stop unit with fabric in the line.
» Do not throw sheeted-out compound on the floor.
» Do not store calendered fabric on floor.
» Do not use damaged liner / baggy liner.
» Do not run mills with cooling water lines closed.
Image: Reconditioned Calender
(JM Machinery)
Solving issues with Calender Gauge
Control
Calender control issues directly affect your
ability to properly control the rubber gauge
or to produce a rubber sheet of consistent
thickness.
While these sources are closely related, for
quicker understanding, you may visualize
and attribute the two main sources of
variations in calendered gauge (or
thickness) as,
A. Mechanical Induced – variations when
your calender is not operating under
any load and are at ambient conditions.
B. Force Induced – variations when your
calender is in operation or is
programmed to follow a set-process for
your product under manufacturing.
A. Mechanical Induced Variations
Mechanically induced Calender Gauge
variations relate directly to your rubber
calender's mechanical condition. Thus it's
basically a maintenance issue.
While there are many maintenance issues
that could affect your calender
performance, the following five directly
affect your rubber sheet's calendered gauge
1. Grind Profile of Calender Roll (Roll
Crowning)
You must periodically grind the calender
rolls to establish the proper profile. Your
selected crown profiles is based on the range
of your rubber compound hardness, type of
center compensation such as cross-axis or
rolls bending, and roll width.
19

| CALENDERS |
a) No Crown - No Load b) No Crown - Uniform Load
c) Uniform Load - Correct Crown d) Uniform Load - Correct Crown
by bending roll to frame
e) Uniform Load - Correct Crown
by bending reaction roll pair
f) Roll Crossing
The forces that separate the rubber calender
rolls to bend apart in the center are partially
compensated for, by grinding the center to a
larger diameter. When the grind profile is
incorrect it will be difficult or even
impossible for you to achieve a consistently
flat-calendered rubber sheet.
2. Eccentricity of Calender Roll or Run-out
at Ambient Temperature
Your calender rolls must be round and free
of run-out at ambient temperature. Any run
-out of the individual calender rolls will be
amplified as the high spots and low spots
periodically match up between the roll
pairs. You can easily check this with dial
indicators when the calender is running
without rubber and with a small nip gap
between the pairing rolls.
Source: GEA
20

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| CALENDERS |
3. Condition of Calender Roll's Bearings
You must ensure that your Rubber Calender
roll bearings does not have excessive run-
out. Modern calenders employ precision anti
-friction bearings with very small radial
clearances. Excessive bearing clearance will
result in the roll shifting during operation
because the roll separating forces vary. This
could appear as calender roll surface run-
out and will adversely affect your ability to
control calendered gauge.
4. Condition of Calender Roll's Water
Passage
Most modern calenders have drilled rolls.
A drilled roll has water passages across the
roll face beneath their surface. These water
passages effectively forms a radiator-like
heat exchanger around the periphery of
your calender rolls.
In an event when these passages become
plugged, then non-uniform heat transfer
will occur. And this result in a thermal 'out-
of-round' condition for your calender roll
which subsequently has has the same effect
as if the rolls were machined 'out-of-round'.
You may avoid this situation by using only
treated water and ensuring that the rolls
are periodically flushed. You could also
check this condition by comparing the roll
run-out or eccentricity at ambient
temperature with that at normal operating
temperatures of your Calender.
5. Condition of Calender Roll's End
Actuator
Of the different variants - manual, motor-
driven and hydraulic nip gap adjustments
- electric screw actuators are the most
common type of calender roll end-
positioning-system. The screw and nut
operate under very high loads and thus,
you will observe, that they experience
significant wear even with proper
lubrication. As these actuators wear,
backlash develops.
When the backlash increases, the response
to small changes or corrections deteriorate.
Further with increased backlash the
number of corrections required would
increase, which in turn causes additional
wear.
Image: Three-Roll Calender (Left) and Spare Calender Rolls With Bearings (Right)
23

| CALENDERS |
Rubber calenders that have fixed speed
electric motors with mechanical brakes
require periodic maintenance, particularly
the brakes. Because sticky brakes will
adversely affect your gauge control results.
Thus, proper maintenance of all the above
systems is crucial to your rubber calender
performance and solves issues with
gauge control that are mechanically
induced.
B. Force Induced Variations
Force induced variations result from the
way your calender is operated. This is also
an outcome from previous process steps
such as your feed mill operation and
compound consistency from rubber mixing.
The separating forces generated during
calendering are quite high! For example, if
you have a 66 inch calender roll and
producing 60 inch wide rubber sheet, the
separating forces are in the range of 72,575
Kg (approx. 160,000 lbs) to 136,000 Kg
(approx. 300,000 lbs) at each actuator.
Such high forces literally stretch your
calender frame. Variations in these forces
will therefore vary the amount of frame
stretch and thus vary roll position and
calendered gauge of your rubber sheet.
Proper maintenance of all the
above systems is crucial to your
rubber calender performance
and solves issues with gauge
control that are mechanically
induced.
Image: From Web
24

| CALENDERS |
Here are five tips on how you could solve
issues of calender gauge control from force
induced variations.
1. Keep the rubber calender running.
Because stoppages cause your calender
rolls to become egg-shaped, that in turn
introduce significant gauge thickness
variation.
When you stop the rubber calender, the
heat loss from the rolls is not uniform
around their surface. Heat loss along the
face of the rolls near adjacent rolls is
minimal, while heat loss in other areas
that are open is higher. This differential
heat transfer leads to different
temperatures on your calender rolls. And
therefore different degrees of calender
roll expansion.
If you have a calender with rolls that is
24 -30 inches in diameter, even a few
degrees of expansion will result in
‘out-of-roundness’ of each roll that you can
measure. This is true when the calender
is empty but even more so when there is a
hot bank of rubber between the rolls. If
you stop the calender for longer period,
this condition worsens further.
Image: India’s first fully Automatic Tyre
Cord Calender Line from Bharaj Machineries
26

| CALENDERS |
Your calender rolls run at different speeds.
This means that periodically the high and
low spots on the rolls match up. When high
spots of adjacent rolls match up, you get
thin gauge spots on your calendered sheet.
When the low spots match up a thick spot is
observed. So calender stops induced
variation results in an effective doubling of
any roll “out-of-round” or run-out error.
Typical short-term thermal out-of-round
gauge variations are ± 0.0005 to .002 inches.
Though thermal run-out is gradually
reduced as the calender runs, it takes 15 to
25 minutes for variations to be eliminated.
Experts recommend you to adopt the
following steps to minimize ‘thermal
run-out’ of rubber calender,
- Keep the calender running during warm-
up. If not done, severe thermal run-out can
be introduced. You need to remember that
the calender nip gap between adjacent rolls
will be reduced as the rolls enlarge with
their temperature increases. So, a good
practice is to open your rubber calender at
least 0.10 inches (2.54 mm) before starting
to increase the roll temperature above
ambient. Keeping the calender rolls turning
during warm-up and cool-down also
eliminates the possibility of warping the
rolls.
- Minimize calender stops.
- When you stop the calender for longer
time, remove the rubber from the banks.
This is a good operating practice for the
rubber compound as well.
- When leader is going thru the calender,
dropping off tension and allowing the
uncoated fabric or leader to go slack will
permit running the calender during delays
and personnel breaks. This will
dramatically reduce thermal run-out.
Image Source: Web
27

| CALENDERS |
2. Increase the temperature by 5 degrees
Celsius when you stop the rolls.
Because this helps to maintain a more
uniform roll surface temperature.
You need to realize that the TCU
(Temperature Control Unit) of your rubber
calender controls the temperature of the
water exiting the rolls and not the 'roll
surface temperature'. This is an important
distinction.
Visualize these two scenarios –
Scenario 1 – When the calender is not
processing rubber (i.e. during warm-up and
when the calender is stopped), the roll
surface is losing heat to the atmosphere.
Here, the TCU is actively heating the water
loop. In this condition the roll surface
temperature is below the water temperature.
Scenario 2 – When the calender is running
and processing rubber, heat is being
generated. Here, the TCU is cooling the water
loop. In this condition the roll surface
temperature is above the water temperature.
From the above, you will realize that for a
constant water circuit temperature, the roll
surface temperature swings (above below
the water temperature) between the
calender normal running condition and
when the calender is stopped. This
difference in roll surface temperature
means that your rubber compound is being
processed under varying conditions.
Shrinkage and other properties of your
processed compound will therefore vary.
Image Source: Pelmar Engineering Ltd.
28

| CALENDERS |
To minimize the differences from such
processing condition variations, the roll
temperatures should be increased whenever
the calender stops. The temperature
increase should be gradual. Your specific
value should be experimentally determined
by comparing roll surface temperatures in
normal operation and after the calender has
been stopped for 20 minutes.
3. Limit the temperature rate of change at
all times to a maximum of 3 degrees per
minute.
Because faster rates can damage or warp the
rolls.
The control system on the TCU should be
designed and operated to limit the rate of
temperature change for each calender roll
water circuit to 3 degrees per minute. Faster
rates of change do not allow for equalization
of mechanical expansion throughout the
rolls, bearings, and calender frames.
Differential expansion can stress various
components of your calender. In extreme
cases, they can actually crack or warp the
rolls, or even damage the bearings.
Heating usually is limited by the available
heating capacity but the cooling system
frequently has the capacity to rapidly cool
the rolls and therefore also has the potential
to cause damage if not limited to 3 degrees
per minute.
Again, it is important to keep the calender
running during warm-up and during cool
down. Stopping the calender and turning off
the TCU circulation pumps before the
calender reaches ambient is not considered
good practice.
If the temperature control system has rate
of change limits properly set, cooling down
is easy, since all the operator must do is to
lower the temperature set points to ambient
and then 60 to 90 minutes later return back
to the operator console to turn off the TCU.
4. Minimize speed changes.
Because changes in your calender speed
result in gum wall gauge thickness
variations that are undesirable.
When the calender is running there are
forces generated that try to separate the
rolls, as explained earlier. And these
separating forces are proportional to
calender speed (although the relationship
is not linear).
The hardness of your rubber stock and
bank temperature determines the
magnitude of separating forces. The
thickness of the gum wall being calendered
also affects the separating force – thinner
gum walls generate higher separating
forces. Separating forces of 160,000 to
300,000 lbs at each actuator literally
stretches the calender frame. When the
forces vary, the amount of stretch varies.
This further results in the roll-gap varying
that in turn causes the gum wall gauge to
vary.
Hence, speed changes should be avoided
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| CALENDERS |
to the extent practical. A speed reduction
from 45 mpm (meters per minute) to 9 mpm
can result in a gauge decrease of approx.
0.002 inches. Similarly, the vice-versa holds
for a gauge increase with roll speed increase.
Calenders are of different types. And
different calendars have different degrees
of frame stiffness so the quantum of the
gauge change for a given speed change will
vary accordingly.
5. Mill management
Maintain consistency.
You can substantially reduce gauge
variations by feeding the calender
consistently.
A consistent, uniform, dwell times on the
cracker and feed mills, as well as consistent
bank size, will give consistent rubber stock
temperatures and work history. The stock
temperatures, bank sizes, and work history
directly influence calender roll-separating
forces.
When you minimize variations in these 3
important process parameters, you will
minimize variations in roll separating
forces, and this in turn will minimize gauge
variations of your calendered sheet.
Further, good mill and bank control
practices also give improved uniformity for
other final calendered gum and/or fabric
properties.
When you feed to the calender, you must
also ensure that the rubber bank does not
have areas that could stagnate. Because if
rubber remains in the bank on the calender
for an excessive amount of time, it will
partially cure and become much harder.
Edge trim process that returns to the bank
and stagnates is a common source of this
problem.
Image Source: Pelmar Engineering Ltd.
32

| CALENDERS |
Such cured and hardened compound will
introduce gauge variations as well as
negatively influence other properties of
your calendered material. In extreme
cases, burnt lumps will appear in the
calendered gum.
Presence of hard stock makes it difficult
to properly penetrate the cord and may
cause cord disturbance and improper
cord distribution.
In such cases the cushion force causes
lateral movement in the nip and this
further results in cord displacement that
is visible. Paired cords are often the first
indicator. You will observe that the
calendered thickness will be heavy and
there will be cord displacement or
irregularities. However, the weight of test
samples will not be significantly affected.
Localized cord disturbance in cord
distribution does not usually affect fabric
weight, whereas redistribution of the cord
does. Overall cord distribution problems
can affect fabric weight in the case of fabric
calenders.
For example, incorrect cord distribution
may reduce the average EPI (Ends Per Inch)
in the center of the fabric and increase the
EPI (Ends Per Inch) on the outer edges. This
will affect fabric weight and thickness – the
weight of the center would be light, while
the outer edges would be heavy.
In steel cord calendering, visible cord
A) Uniform Cord Spacing - Uniform Top and Bottom Rubber Coat.
B) Cords spread at the edge of the sheet.
C) Cords paired. Top or Bottom Rubber Coat is heavy in the center.
ImageSource:GEA
33

| CALENDERS |
displacement and cord pairing after the
cushion nip is to be expected in the areas
where the cord EPI count is excessive.
Training
Your guiding principle for calender line
operation should be consistency and
continuity. If all the inputs to your
calendering process are constant, and your
installed machinery is functioning correctly,
you can produce quality goods meeting tight
specification tolerances. Your operators need
to acquire knowledge on how the rubber
calender works, how changes in process
variables affect outcomes, and how changes
in machine and/or process conditions can
beat those changes. Training them to run the
calender under complete manual control
Your guiding principle for
calender line operation should
be consistency and continuity.
will make them know enough to understand
the importance of maintaining as close to
steady-state operation as possible. Further,
this knowledge will also help them to
operate the line without the gauge or the
automatic set-up, if they ever need to do so.
Summarizing, the nuances in the world of
rubber calenders is an interesting subject
and as you understand its operations
deeper, the 'mysteriousness' of this rubber
machinery disappears faster.
Image: Bharaj Machineries Pvt. Ltd.
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