Everyone knows what a tyre is and what
it’s for, right? It’s a black donut made of rubber, placed on a vehicle so
the driver can transport himself and his cargo from point A to point B. The
tyre has to allow for easy steering, braking and cornering. It must provide
for a comfortable, safe ride. It needs to be durable. And that’s about the
extent of what most of us know.
Use the links below to look at the different stages of the manufacturing
process, they all referred to in the tyre manufacturing flowchart below:
SHOWING YOU WHAT WE'RE MADE OF
RUBBER COMPOUND FORMULATION
The two major ingredients in a rubber compound are the rubber itself and the
filler, combined in such a way as to achieve different objectives. Depending
on the intended use of the tyre, the objective may be to optimise
performance, to maximise traction in both wet and dry conditions, or to
achieve superior rolling resistance. The desired objective can be achieved
through the careful selection of one or more types of rubber, along with the
type and amount of filler to blend with the rubber.
In general, there are four major rubbers used: natural
rubber, styrene-butadiene rubber (SBR), polybutadiene rubber (BR), and butyl
rubber (along with halogenated butyl rubber). The first three are primarily
used as tread and sidewall compounds, while butyl rubber and halogenated
butyl rubber are primarily used for the innerliner, or the inside portion
that holds the compressed air inside the tyre.
The most popular fillers are carbon black and silica, and there are several
types of each. The selection depends on the performance requirements, as
they are different for the tread, sidewall, and apex. Other ingredients also
come into play to aid in the processing of the tyre or to function as
anti-oxidants, anti-ozonants, and anti-aging agents. In addition, the “cure
package”—a combination of curatives and accelerators—is used to form the
tyre and give it its elasticity.
RUBBER COMPOUND MIXING
Once the compound is determined, the next challenge is how to
mix it all together. The mixing operation is typically a batch operation,
with each batch producing more than 200 kilograms of rubber compound in less
than three to five minutes. The mixer is a sophisticated piece of heavy
equipment with a mixing chamber that has rotors inside. Its main function is
to break down the rubber bale, fillers and chemicals and mix them with other
The sequence in which the ingredients are added is critical, as is the
mixing temperature, which can rise as high as 160-170 degrees Celsius. If
the temperature is too high, the compound can be damaged, so the mixing
operation is typically accomplished in two stages. The curative package is
normally added in the final stage of mixing, and the final mixing
temperature cannot exceed 100-110 degrees Celsius or scorching may occur.
Once the mixing is completed, the batch is dumped out of the mixer and sent
through a series of machines to form it into a continuous sheet called a
“slap.” The slap is then transferred to other areas for bead wire assembly
preparation, innerliner calendering, steel and/or fabric belt/ply cord
calendering, tyre sidewall extrusion, and tyre tread extrusion.
FABRIC AND STEEL
Because tyres have to carry heavy loads, steel and fabric cords are used in
the construction to reinforce the rubber compound and provide strength.
Among those materials suitable for the tyre application: cotton, rayon,
polyester, steel, fibreglass and aramid
Fabric cord quality is based on its strength, stretch, shrinkage and
elasticity. The yarn used is first twisted and then two or more spools of
yarn are twisted into a cord. Before shipping the cord to the tyre factory,
the manufacturer pre-treats the cord and applies an adhesive to promote good
bonding with the rubber. The temperature, humidity and tension control are
critical before the fabric cords are calendered with rubber compound. For
this reason, fabric cord is kept in a temperature-and-humidity-controlled
room once it arrives at the factory.
Steel wire cord quality is based on tensile strength,
elongation and stiffness. It is manufactured from steel rod with high carbon
content; and while the steel wires used have different configurations, all
are brass-coated strands twisted together into cords. If the wire is used in
a multi-ply tyre rather than a belted tyre, the fatigue performance will be
important. If used in belted tyres, then stiffness is of primary concern.
Since the steel wire is brass coated, storage conditions are important to
maintain the steel wire to rubber bonding properties. Therefore, the steel
wires are also kept in a temperature and humidity controlled room once they
arrive at the
BELT AND PLY
To produce fabric or steel belts, the fabric or steel cord must go through a
calendering process—an operation in which the rubber compound is pressed on
and into cords. Because the bonding of fabric to rubber or steel to rubber
is critical to performance, the calendering process is an important step.
The calender is a heavy-duty machine equipped with three or
more chrome-plated steel rolls which revolve in opposite directions. The
roller temperature is controlled via steam and water. In this process, the
rubber compound is applied to the cords.
First, a pre-set number of fabric or steel cords under proper tension are
continuously pressed through two steel rollers, and rubber compound is added
to the opening area between the rollers. Then the rubber compound is pressed
into, on top of and on the bottom of the fabric or steel cords. A continuous
sheet of cord-rubber composite goes through several more rollers to ensure
good penetration and bonding between the rubber and cords. Quality is
measured by the thickness of the sheet, spacing between cords, the number of
cords and the penetration of rubber into the composite sheet. The composite
sheet is then cut into appropriate sizes, shapes, and angles depending on
the desired contour of the tyre.
The bead component of the tyre is a non-extensible composite
loop that anchors the body plies and locks the tyre on to the wheel assembly
so that it will not slip or rock the rim. The tyre bead component includes
the steel wire loop, apex or bead filler; the chafer, which protects the
wire bead components; the chipper, which protects the lower sidewall; and
the flipper, which helps hold the bead in place. The bead wire loop is made
from a continuous steel wire covered by rubber and wound around with several
continuous loops. The bead filler is made from a very hard rubber compound,
which is extruded so as to form a wedge. The bead wire loop and bead filler
are assembled on a sophisticated machine.
The precision of the bead circumference is critical. If too
small, tyre mounting can be a problem; but if too loose, the tyre can come
off the rim too easily under loading and cornering conditions. After the
circumference is checked, the bead component is ready for the tyre building
TREAD, SIDEWALL AND INNERLINER
Tyre tread, or the portion of the tyre that comes in contact with the road,
consists of the tread itself, tread shoulder and tread base. Three rubber
compounds are extruded simultaneously from different extruders and are then
merged into a shared extruder head. The next move is to a die plate where
the shape and dimensions are formed, and then through a long cooling line –
from 100 to 200 feet long – to further control and stabilise the dimensions.
At the end of the line, the tread is cut according to a specific length and
weight for the tyre being built.
THE TREAD AND SIDEWALL
Tyre components such as tread, sidewall and apex are prepared by forcing
uncured rubber compound through an extruder to shape the tyre tread or
sidewall profiles. Extrusion is one of the most important operations in the
tyre manufacturing process because it processes most of the rubber compounds
produced from the mixing operation and then prepares various components for
the ultimate tyre building operation.
The extruder in a tyre manufacturing process is a screw-type system,
consisting primarily of an extruder barrel and extruder head. First, the
rubber compound is fed into the extruder barrel where it goes through a
heating, blending, and pressurising process. Then the rubber compound flows
to the extruder head where it is shaped under pressure. The modern cold-feed
extruder is computer-controlled for accuracy.
The tyre sidewall is extruded in a way similar to the tyre tread component;
however, its structure and the compound used are quite different from tread.
Sometimes the sidewall extrusion process can be more complicated and four
extruders may be needed; for example, when building a tyre with white
sidewalls or with white lettering on the sidewalls.
The innerliner is just what it sounds like – the inner-most layer of the
tyre. Its main functions are to retain the compressed air inside the tyre
and maintain tyre pressure. Due to its low air permeability, butyl rubber –
or halogenated butyl rubber compound – is the primary rubber compound used.
Because this is a thin layer it is also produced using the calender. The
gauge control and no-defect surface finish are critical to retaining air
pressure. Innerliner calendering is also a continuous operation. The proper
length of innerliner sheet is pre-cut to be ready for the tyre building
Finally, the tyre is ready to be built by a highly robotised machine which
ensures quality and efficiency. All components – bead assemblies, calendered
plies, belts and innerliner, tread and sidewall sections – are assembled and
the building process begins.
A typical radial tyre is built on a flat drum in a two-stage process. In the
first stage, the innerliner is wrapped around a drum and the first body ply
is wrapped on top, followed by the second body ply. The bead assemblies are
then positioned, and a bladder on the drum is inflated and pushed in from
both ends of the drum, forcing the body plies to turn up to cover the bead
assemblies. The sidewall sections are then pressed on to both sides.
In the second stage of the tyre building process, another machine is used to
apply the belts, nylon cap, and tread on top of the first stage. At this
point, the tyre still needs curing because there is no tread pattern on it.
In this final step, curing occurs through a series of chemical reactions. In
addition, the sidewalls and tread are moulded. Tyre curing is a
high-temperature and high-pressure batch operation in which the uncured tyre
is placed into a mould at a specified temperature. After the mould is
closed, the rubber compound flows in to mould the shape and forms the tread
details and sidewall. The mould cannot be opened until the curing reaction
Tyre inspection is the last step in the tyre manufacturing process—an
important step in ensuring quality in both performance and safety. The tyre
Trimming of the mould flash and micro-vents
Visual inspection for appearance and to spot obvious defects
X-ray examination to check internal structure and to spot defects
Tyre durability, uniformity and weight balance inspection
After a tyre passes these rigorous inspections it’s time for the rubber to
meet the road! Our tyre is ready to be put into service.