CALENDARING

What is “calendaring”?

In calendaring, textile fabric or steel cord is coated a film of rubber (“skim stock
rubber”) on both sides and into the material. Calendared textiles are used for casing and
cap plies and chafers. Calendared steel cord is used for belts.
Because the adhesion of rubber to the fabric or steel is important to final performance,
the calendaring process is an important step. Especially important are the tension,
temperature and humidity of the fabric to be calendared. The temperature of the rollers is
controlled via steam and water.

The rubber that coats the fabric is different from the one used to make the tread
or that which coats the bead cords.

What are the functions of calendaring?

1. Rolling of rubber mix
2. Coating of rubber mix onto the substrate
3. Frictioning of rubber mix on the substrate

1. Rolling of rubber mix
For simple products 2 – roll calendar is used. A major difference between this
and the other calendars is that here no fabric is being coated (as the geometry
of the roll suggests). But the rubber mix is rolled and a continuous sheet of
rubber is obtained.


Usually, maximum thickness for good quality is 3 – 4 mm. For getting
patterned sheets “profile calendars are used”.


2. Coating of rubber onto the substrate.

The rubber mix is coated – on both sides of the fabric (steel cords and synthetic
fibers like nylon, polyester), inside the fabric so that adhesion between the cords and the
rubber is good.
It is worth mentioning that in a 3 – roll calendar the coating is only done on one
side of the fabric while in a 4 – roll calendar a double side coating is obtained. If a double
side coating using 3 – roll calendar is required then two 3 – roll calendars have to be used
in tandem, each calendar coating the fabric on one side.

3. Frictioning of rubber mix on the substrate.

Good adhesiveness is achieved by frictioning. In frictioning, the two rolls rotate at
different surface speeds (or the rolls are of different diameters). Thus the rubber is
“fractioned” into the fabric.



What all go in for calendaring?

Milled soft rubber from mills, Fabric (Nylon, Steel cord, Polyester) from Dip
unit.

What are the different types of “calendaring”?

Calendaring is classified based on what is being calendared.

1. Fabric calendaring.
2. Steel cord calendaring.

1. Fabric calendaring

As stated above, the tension, temperature and humidity are critical before the
fabric cords are calendared with the rubber compound. That is why the fabric cord is kept
in a temperature – and – humidity controlled room once it arrives in the shop floor.

The textile is arranged in a flat parallel manner. Under proper tension they are
continually pressed through two steel rollers, and simultaneously the rubber compound is
added to the opening area between the rollers. A thin layer of rubber is applied into, on
top of and the bottom of the fabric. A continuous sheet of cord – rubber composite goes
through many more subsequent rollers to ensure proper penetration of rubber between the
cords and desired adhesion between rubber and the fabric.

Quality is quantized in terms of thickness of the sheet, spacing between cords, number of
cords and penetration of rubber into the space in between cords. The sheets are cut at
required angles (so that the cords are set at predetermined angles across the sheet).

Even inner liners are calendared the same way into sheets of required thickness
and then cut into appropriate widths for use in tire construction.
The calendared fabric is made into rolls with interleaving layers of woven fabric
liner to prevent them from sticking together.

2. Steel cord calendaring (radial tires)

High tensile brass coated steel cords (multi strand) are used in the construction of
tire belts. The brass coating is done to make rubber adhere to the steel. The steel cords
come in various arrangements such as in cross section of pairs, triplets and so on.

The storage of these brass coated steel cords should be in a temperature and
humidity controlled environment (the creel room). Even the distance between the creel
room and the calendar must not be too large. Otherwise, corrosion due to moisture may
cause the steel belts to lose adhesion with rubber.
The steel wire passes from the creel room on rollers through aligning combs into
the calendar where the wires are coated with a thin sheet of skim stock rubber.

A pre – set number of steel cords under proper tension are continually pressed
through two steel rollers, and simultaneously the rubber compound is added to the
opening area between the rollers. A thin layer of rubber is applied into, on top of and the
bottom of the fabric. A continuous sheet of cord – rubber composite goes through many
more subsequent rollers to ensure proper penetration of rubber between the cords and
desired adhesion between rubber and the fabric.

As above here also quality is quantized in terms of thickness of the sheet, spacing
between cords, number of cords and penetration of rubber into the space in between
cords. The rubber should also penetrate the steel cords for maximum adhesion. Maximum
adhesion also means least rusting. Then the steel cords are cut at specified angles and
widths for use in tire building.