the art of owning & operating rubber calenders

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377 Proven Strategies To MethodicallyGrow Your Business

INSIGHT

July 2016

8COVER STORY

The Art of Owningand OperatingRubber CalendersA 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

http://rubbermachineryworld.com / http://tyremachineryworld.com

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

[email protected]. �

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

+Rubbermachineryworld1

grp/home?gid=8252803

rubbermachineri

(Disclaimer: The pictures and statements in our special supplements are shared by the respective companies and/or sources mentioned. 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 CalendersPrasanth 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-Roll�Calender��

» Also known as the “Modern

Calender”.

» Different Configurations.

» Used for Skim Coating.

ʻZʼ�Type�Offset�Stack�Calender

Inverted�ʻLʼ�Type�Offset�Stack�Calender

ʻIʼ�Type�or�Vertical�Stack�Calender


Vertical�Type�Calender

Inclined�or�Tilted�Type�Calender


Vertical�Stack�or��ʻIʼ�Type�Calender

Offset�Stack�Type�Calender

» 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.

Box: Excerpts from ‘Know Your Supplier’ Editions


16

1.�DISPLAY�Your�Products�Regularly�to�Buyers.�2.�COMMUNICATE�Your�Competencies�Differently.�3.�ATTRACT�More�Buyers�to�Your�Brand�Informatively.�

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media differently, Buyers get all information of Your Products &

Solution anytime and anywhere - on any smart gadgets they

use (Phone, Tab, Laptop or Desktop).

For more details and your customized advertising, please email us

at [email protected]

On-The-Go With Your Info

Reach�Machinery�Buyers�Fast.�Anytime.�Anywhere.

ADVERTISE On

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Rubber & Tyre Equipment - Accessories

| 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

» H igh feed bank on to the rolls

» Compound Stock warmed up for longer duration

» Insufficient break-down of your compound

» I nadequate temperature control

» I nsufficient compound feed in the banks

» Roll temperature too high

» L arge nip opening

» S tock warm up for long time

» H igh Compound in the nips

» Insufficient nip opening between rolls

» E xcess compound in the nip

» I mproper roll crown

» H igh compound viscosity

» Improper Roll temperature

» I mproper running speed

» L ow compound viscosity

» I mproper warming of the compound

» High viscosity compound

» I mproper warm up, high bank

» N on uniformity of warm up

» L ower roll temperature

» Lower temperature in one of the rolls

» H igh friction ratio

» H igh calendering speed

» U se of anisotropic fillers

Choose Wisely. Operate Smartly

Image of A 3-Roll Vertical Calender fromBharaj 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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* [email protected]

) (+6) 017-362.79.71

AU

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@2016

| 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

30

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• Mixing Mills – 16", 22", 26" and special sizes

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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. Ima

ge

So

ur

ce

: G

EA

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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