EXTRUSION

Extrusion can be considered one of the very important steps in the tire
manufacturing process as it directly converts the mix compound into the various final
components that go into the tire building.

What is extrusion?

Extrusion is the process by which a polymer (here rubber) is propelled
continuously along a screw through regions of high temperature and pressure where it is
softened and compacted (or compressed), and finally forced through a die (slit).

Why extrude at all?

Extrusion is a primary requirement in the manufacture of the tire. Certain tire
components like the tread and the sidewall have specific profiles that can only be
got through the process of extrusion.

What all go in for extrusion?

The softened rubber from the feed mill or homogenizing mill goes into the
extruder.

The two major tire parts that are formed by extrusion are the sidewall and the
tread. These parts are constructed by forming two specific compounds into tread profiles
by feeding the rubber through an extruder. Other parts include the apex. Extruders
produce continuous lengths of treads, which are cooled and cut into specific lengths.

What exactly happens in the extrusion process?

Typically, extruders are barrel shaped. It is a screw – type system consisting of an
extruder barrel and an extruder head. The compound is forced into the extruder barrel
where heat and pressure are applied and the rubber mix compound undergoes blending.
Then it flows through the head where the shaping of the tread takes place under pressure.

Simplex and duplex extruders are used for sidewall extrusion. Triplex extruders
for tread and sidewall extrusion and rollerhead extruders for innerliner extrusion.

Actually the extrusion process is not as simple as it seems. The screws have to
provide a consistent through – put and to build the pressure required for the downstream
extrusion tools that shape the part. One of the most important aspects is thermal balance.
The temperature profiles must be maintained to ensure even heating and homogenization,
and also to prevent scorching which happens in the case of temperature rise. A set of air
blowers is placed beneath the extruder barrel for cooling purposes.
It may be thought that the screws only provide the function of extrusion but as in
some cases certain additives are mixed in this stage the screws have to provide significant
amount of mixing as well.


As mentioned above, the extruder is not the only machine capable of producing
the tread. The profile calendar is a costly alternative. The rolls are profiled to give the
shape to the tread that normally will be given by the die. It has several advantages
associated with it like high line speed and low-pressure extrusion resulting in low die
swell. But it has a serious disadvantage that it takes more time to change rolls in it as one
has to change the profile. Also it cannot produce undercut patterns.

An innovative idea to reduce die swell is to use a single roll roller die. In the head
section of the single roll roller die the pressure builds up the same as in a conventional
extrusion head. But here a perform die allows the pressure to be released when the
compound reaches the roll. Low pressure equals low die swell and this produces very
accurate extruded strips


Extruders can be both cold and hot – fed systems. The final shaping is done by
continuous extrusion through the use of dies.

Tire tread extrusion

The tire tread is the component of the tire that comes in contact with the road.
The parts of the tire tread are the tread itself, the tread shoulder and the tread base.
Sometimes even three different rubber compounds are used in the making of the tread
profile. In this case, three extruder barrels share a common head. Three different rubber
compounds are extruded from the three different barrels and combined in the shared
extruder head.

The next step would be to pass this mass from the head into a die plate where the
tread gets its shape and dimensions. Next come the cooling line where the tread is cooled
so as to stabilize the dimensions. The cooling line may be as long as 30 – 50 meter.
Before the tread skiver cuts the tread into required lengths, it is weighed in – line.
An important aspect when it comes to tread cutting is that it must be cut to precise
lengths, not so much as in the case of sidewalls or so as they are cut on the tire building
machine.

Tire sidewall extrusion

The tire sidewall extrusion is very similar to the tread extrusion but the rubber
compounds used in the sidewall are different. Sometimes, the sidewall extrusion can be
more complex than the tread extrusion, requiring as many as four different rubber
compounds.

One of the important factors in final extrusion is the die pressure. Not only this,
the mixing and extruding pressure create internal stresses in the compound. This leads to
uncontrollable dimensional change in the extruded mass as it leaves the die. Usually in an
extruder the die pressure is uncontrolled and so is the die swell that results in the extruded compounded. This problem can be countered by engineering the flow channel or
providing for a pressure release





EXTRUSION
Is the process by which a rubber compound or plastic formulation is given
a definite continuous shape.
Examples of extruded items are tire treads, side walls,channels,
beadings, PVC pipes, latex threads.
Note the terms “definite” and “continuous” –Definite because the orifice
or the die decide the shape of the article and Continuous because the
product is practically endless.
.
“Extruder” is the equipment which carries out the process of
extrusion.