Parts of Extruder

1. Hopper cylinder – It is made of cast steel and is jacketed for water-cooling.
The insides are made wear resistant by chrome plating or nitrited alloy steel. The
insides also have spiral undercuts for high feeding efficiency.

2. Rollfeeder – The feed roll is operated by the screw by the means of a gear and
pinion. The feed roll surface is hardened and an inside chamber for tempering fluid is
provided.

1. Forward cylinder – The cylinder is made of nitrited alloy steel (for high wear

resistance) and peripheral drilling for circulating tempering fluid is provided.
Sometimes, bi – metallic linings are given. The pins are made of high grade alloy
steel hardened fro greater wear resistance.

4. Screws – The twin screws are made of cast steel and are chrome plated. They
are usually bored for cooling. The diameter of normal screws is 60 – 200 mm with a
length of 5 – 20 times the diameter. The functions of the screw are manifold – mixing,
blending, homogenizing, dispersing, compounding, degassing.
Twin screw extruders are the most commonly used extruders. They can either be
intermeshing or nonintermeshing. Nonintermeshing extruders behave like two single
screw extruders with only minor interactions between the two screws.
Another classification is the direction of rotation of screws. Co - rotating screws have
both screws rotating in the same direction and the material exchanges from screw to
screw. On the other hand, the counter – rotating screws have material transported
through the extruder in a figure eight channel.
5. Feeding screws – They are made of high alloy steel and polished, nitrited or
satellite on their tips. They are also drilled for tempering fluid circulation.

6. Dies – Many special arrangements of extruder dies are used for composite
layered sheets. Two types of rubber enter the die from two extruder barrels and exit as
one sheet. The top and bottom layers are of different materials.



7. The mixing chamber – The chamber which contains the screws and where
the actual mixing takes place. This region is the region of maximum power
consumption as well as maximum temperature rise. The throughput of an extruder is
typically ~ 400 kg/hr.

8. Vacuum zone – A vacuum zone inside the cylinder is created by the use of
screws with high thread height. This is done to vent the rubber from inclusions of
gases to eliminate porosity.

9. Extruder tempering system – Usually it is a closed circuit type with
electrical heating. Both single – and twin – screw extruders may use hot oil
circulation systems for temperature regulation.
The sump temperature is set to 204 degrees, and the extruder barrel is heated or
cooled depending upon internal heat generation. Circulation may be constant (no
temperature controllers) or may be adjusted to control individual zone temperatures.

10. Gear reducer – The screw speed varies normally from 30 – 60 rpm. The gear
reducer reduces the motor speed to this speed. They are usually provided with forced
feed lubrication system.

11. Thrust bearings – They form an integral part of the extruder system.
Optimized design for long life at the maximum extrusion pressure and screw speed.



What are the control zones inside an extruder?

One important thing to note is that all components have bores or channels drilled
inside them for cooling purposes. This indicates that the temperature rise inside the
extruder is formidable and sufficient measures must be taken to prevent scorching. Also
excessive temperature makes the rubber to stick on the screw and make mixing inefficient. Too low a temperature gives uneven flow, bad surface, large expansion and
large shrinkage on the length of the product.

Also the mixing chamber or the barrel is divided into a number of zones
depending on whether it is cold (more zones) or preheated (fewer zones) rubber. Each
zone has its own temperature control circuit. The zones are roughly 375 – 450 mm long
i.e a 4.5 inch extruder(~112.5 m) will have 4 –6 barrel zones. The number of zones also
depends on the L/D ratio, the number of feed points for additives, and the type of material
being extruded.


Temperature control is complicated due to several factors. These include heating due to
shear rate (It is worthy to note that 80 – 100% of the heat produced throughout the
extruder can be generated by the screw shear alone) caused by screw speed, feed rate
changes, resistance offered by the die and difference in process gains for heating and
cooling. Say, the screw speed increases; the higher rate of shear in the screw channel
increases mechanical work, raising the compound temperature. To aggravate the
situation, cooling rate is reduced as screw speed and output rate increase. But suppose the
pressure is low a sin the case of low resistance die size, the amount of frictional heat may
not be sufficient to overcome the decrease in conducted heat, in this case an increase in
screw speed results in consequent drop in temperature.

At higher pressures, the temperature may show an initial increase before dropping and at
still higher pressures, the same screw and operating conditions can give an increased
temperature with increasing screw speed.

What all come out of extrusion?

The main tire components that come out of extrusion are sidewall and tread.
Other parts include the apex, sidewall and innerliner.


Behaviour of Rubber in Extruder

Inside the extruder,
• The pressure varies linearly from feed hopper to head
• Screw speed controls out put
• The tuber output rate of any die can be predicted if the screw
dimensions, speed and the pressures are known

Flow Mechanism

• The rotating screw inside the barrel induces a rotational
velocity to the stock in addition to the longitudinal movement
of the stock. The main effect of this movement is the
equalisation of temperature through out the stock because of
the rapid turnover and the effect of the wiping action of the
stock on the walls of the barrel and the screw.

• Drag Flow
The resistance to the forward movement induced by
dragging against the walls. This is the basic phenomenon by
which the material gets conveyed to the other end of the
screw.


Flow Mechanism inside the Extruder
• Pressure Flow
The high pressure at the head side and the relatively low
pressure at the feed end of the screw induces the screw to
attempt a back-flow against the drag flow. This results in the
back outs while running the extruder.

• Leakage
The backward flow through the clearance between the screw
and wall as a result of the increased pressure gradient from
the hopper to head.
Depends on Screw-Barrel clearance
Manifests undesirable extrusion characteristics
Increased residence time and therefore scorch
The negative flows as above reduces the output of the
extruder

Extruder output
Output of Extruder(Q) = Q(Drag) -Q(Pressure) -Q(Leakage)

Extruder Output Calculations

Theoretical out put
Output = Rpm of screw X Volume per flight
Volume per Flight = ¶L[D2/4- d2/4]

(Single flight Screw)
Where, L is the length of the flight
D is the Diameter of the screw
d is the diameter of the root

Actual out put

Output = Theoretical output - Back flow
Back flow = Pressure Flow + Leakage
Screw efficiency = Actual output
Theoretical Output
(Normally about 40-45% for Rubber extruders)

Extruder Output Calculations

COOLING OF TREADS

•To prevent scorching inside the tread
•To avoid soft extrudate-deformations while booking
•To prevent high stichout in building, leading to non
uniform gauge distribution
Cooling conveyors are used to cool the tread by
spraying chilled water on the tread through
nozzles provided both side of the cooling
conveyor.