Extruders - Special Supplement From Rubber & Tyre Machinery World

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World
Rubber & Tyre Knowledge On-The-Go Special Supplement
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EXTRUDERS

Rubber extruders have a varied field of application.
So, when you come across a rubber profile, strip,
hose, cable, wire, cord coating, tire tread, v-belt,
tube, or blank remember that they are only a few
handful of products manufactured using extrusion
process.
Rubber industry only had Hot Feed Extruders until
1950’s. Then came the Cold Feed Extruders and
other variations as we see them today.
Life today is mostly spent trying to catch up with
latest developments. However, in the process the
basics and fundamentals are sometimes overlooked.
In this issue, we bring to you a ‘collector’s edition’
on Extruders and Extrusion incorporating the
fundamentals, that I hope will find space in your
digital library.
My sincere thanks to Dr. S. N. Chakravarty, President
of Elastomer Technology Development Society, for
his inputs with the material for this edition.
As always, I welcome your feedback on
rubbermachineryworld@gmail.com.
Best Regards
Prasanth Warrier
| Editor’s Note |
Extruders And Extrusion
Extruders and the
process of Extrusion
has different
applications in the
rubber and tyre
industry.
3
rubbermachineri
rubbermachineryworld
+Rubbermachineryworld1
grp/home?gid=8252803
rubbermachineri
(Disclaimer: All the pictures and statements in this special supplement are sourced from web or shared by respective companies.
All copyrights belong to actual owner. Rubber Machinery World does not independently verify them nor will vouch for their
genuineness, hence will not be liable for any misrepresented data. The images are used here for representation purpose only.)
Rubber & Tyre Machinery World | FEB 2016

4
| EXTRUDERS |
Image of A Strainer Extruder - Pelmar Engineering Ltd.

5
Your Extruders And Its Extrusion
Process
Prasanth Warrier
With Inputs of Dr. S.N. Chakravarty
Rubber extruders have a varied field of
application. So, when you come across a
rubber profile, strip, hose, cable, wire, cord
coating, tire tread, v-belt, tube, or blank
remember that they are only a few handful
of products manufactured using extrusion
process.
Introduction
Extruders are machines, which shape rubber
to a profiled strip by forcing it through a die.
In the simplest form an extruder consists of
four basic components viz.
| EXTRUDERS |

(a) a device system
(b) a barrel
(c) a ram or screw for forcing the rubber
through the barrel and
(d) a head holding the die which ultimately
gives the desired shape.
The fundamental distinction between two
types of extruders is in their mode of
operation:
1. Continuous: Delivers rubber in a
continuous manner and has a rotating
member or screw; here the pressure is
produced by a screw.
2. Discontinuous: Delivers rubber in an
intermittent fashion and has a
reciprocating ram or screw. These type
of extruders are ideally suited for batch
type processes such as injection
moulding.
Screw Type And Ram Type Extruders
In Ram Extruders, a quantity of warm
compound is placed into the cylinder, the
die is attached to cylinder and ram pushes
the compound through the die to form a
profiled section.
Advantages:
• Extrusion can be carried out at lower
temperatures.
• Difficult compounds can be extruded.
• Easy to clean.
• Useful to short runs.
• Useful for compounds which need to be
strained through gauge for quality
products requiring completely
contamination-free material.
Ram extruders are not frequently used and
are good for very specialized extrusions.
Ram extruders are not
frequently used and are good
for very specialized extrusions.
| EXTRUDERS |
Image of A Ram Extrusion Line - Barwell
6

A Screw Extruder consists of five
components viz.
(a) Drive system
(b) A feed hopper
(c) A screw rotating within
(d) A barrel
(e) A head and die.
The basic principle is that the screw carries
material from the feed hopper by acting as a
conveyor or a hump providing pressure to
extrude or force the material along the
machine barrel through the head and the
die.
The extruder drive system comprises of an
AC or DC motor along with a reduction gear
unit for power transmission. The extruder
drive has to turn the screw at the desired
speed. It should be able to maintain a
constant screw speed because variations in
screw speed will result in throughput
fluctuation, which in turn will vary the
dimensions of the extrudate.
The purpose of the feed hopper is to receive
material and pass it down to the flights of
the screw. It is mostly supplied in the form
of strip.
The basic principle is that the
screw carries material from the
feed hopper by acting as a
conveyor.
| EXTRUDERS |
Image of A Screw Type Cold Feed Extruder - Pelmar Engineering Ltd.
8

9
Extruder Screw
The screw rotated within the Barrel, the
usual clearance is approximately 0.40 mm.
Normally, the barrel is fitted with a
detachable liner in the form of a sleeve
which is highly wear and corrosion resistant,
made of hardened steel. The barrel is made
double walled for steam or water circulation
so that a constant temperature is maintained
in the extruder head.
A conventional extruder screw has three
geometrically different sections.
1. Feed section (closest to the feed opening)
generally has deep flights and consists of
approximately 1/5th of the length of the
screw. The material in this section will
mostly be in solid state.
2. Metering section (closest to the die)
usually has shallow flights and consists of
approximately 2/5th of the length of the
screw . The material in this section will
mostly be in molten state.
3. This section connects the feed section and
the metering section, is called as
compression section. In going from feed
section towards the metering section, a
compression of the material in the screw
channel takes place, which is essential for
the proper functioning of extruders. The
compression streamlines flow, helps to
eliminate air and also ensures a constant
pressure in the head.
Screws are made of steel alloy forging, heat
treated to a machinable hardness and hard
chrome plated. The flight lands are surface
hardened to approximately 600 Brinnel.
The screw is internally bored to facilitate
water-cooling.
The extrusion process has to be effective.
What does this mean to you? Your extruded
profile temperature must be within
required limits to prevent scorching.
A conventional extruder screw
has three geometrically
different sections - Feed,
Metering and Compression.
| EXTRUDERS |

Core Diameter
The key screw design elements are Pitch,
Flights and Core Diameter.
Flights: The built up part in between
which the rubber compound traverses.
Pitch: Distance between two consecutive
flights
Core Diameter: Diameter of the Core
Shaft
Each manufacture has their own design of
screw. However, the basic function remain
same. An adjustment can be made for
volume occupied by the flight lands. Screw
design has a direct bearing on out put and
quality of Extrudate. Output rate depends
upon the screw design and geometry of
extrudate.
In general, you could increase output by
either or a combination of the following
- Decrease the Flights
- Increase the Pitch
- Increase the Core Diameter
Reputed manufacturers offer different
screw designs to process your specific
rubber compound requirements. One of the
basic characteristics of all the extrusion
process is the clearance between Barrel and
screw. This is generally
permitted to a limit in order to control
• porosity
• extrusion temperature
Reputed manufacturers offer
different screw designs.
| EXTRUDERS |
Schematic of a screw inside barrel and the different terms. Image from Web.10

Extruders are usually designated by the
diameter of the extruder barrel. The ratio
of relative output of extruders varies as
the square of the screw diameter . Thus
the output of a 60 mm extruder will be 2.2
times that of a 40 mm extruder.
The extruder is operated in such a manner
so that temperature is gradually more from
feed to discharge, the die being the hottest
part.
When you choose a rubber extruder, you
should deliberate and discuss extensively
upon the design, material technology and
manufacturing accuracy of screw, barrel
and die-head.
Very few manufacturers world-wide can
guarantee you a well-designed and
precisely manufactured rubber extruder.
An additional designation often used is the
length to diameter (L/D) ratio . This is an
important factor in the selection of
extruders to match process requirements.
The length of a rubber extruder depends on
whether it is a hot feed or cold feed type.
Hot feed extruders are usually very short
about 3D to 5D while cold feed ones range
from 12D to 24D.
Hot Feed Extruder (HFE)
Rubber industry only had Hot Feed
Extruders until 1950’s. HFE’s extrude your
rubber compounds at reduced
temperatures.
The screw depth of a HFE is relatively
larger and you get a consistent output due
to its short screw design. L/D ratio is mostly
in the range of 4:1 to 6:1 which keeps your
rubber compound dwell time and its
temperature increase to a minimum.
In many cases the screw has increasing
pitch.
When you choose a extruder,
you should discuss the design,
material technology and
manufacturing accuracy.
| EXTRUDERS |
Image:VMI
12

Each HFE has an hopper and feed roller
section with spiral undercut liner that
allows your compound to enter the
extruder easily. The feed roller on a hot
feed extruder allows your compound to
pass the scraper knife, directed around the
roll and then fed back into the hopper. The
feed roll bearings are placed in positions to
prevent contamination. You can vary the
output by changing the screw speed using
variable speed drives.
Despite these advantages, the HFE’s are
getting outdated in many applications.
Because the rubber that is fed into a Hot
Feed Extruder needs to be pre-heated or
warmed using two-roll mills to achieve the
required degree of viscosity and
temperature that facilitates smooth flow of
rubber, its compaction and extrusion
through the die.
And that made experts consider Cold Feed
Extruders.
Cold Feed Extruder (CFE)
Cold Feed Extruders are designed and
manufactured with specially designed
screws best suited for cold feeding of
rubber.
While manufacturers offer L/D ratio up to
24:1, the most preferred by end-users is
generally in the range 12:1 to 18:1.
In most cases, the screw has decreasing
pitch. The screw has increasing root
diameter with more flight depth at feed end
which increase from feed to discharge. And
of course, there are various other designs
available.
You can discuss with your manufacturer
and avail various options of screws for a
wide range of compound and extrusion
applications.
The output of an extruder is dependent on
size of extruder i.e. the ratio of L/D.
First estimate of out put is expressed as :
Output = A (L/2) x R x d

A=Cross sectional area of extruder
L=Lead Length
R=Revolution per minute
d=density of rubber
Generally these exists a liner relationship
between RPM & output
For feeding the cold rubber, it is
recommended that you use a feeding
conveyor with metal detector to remove
metal particles. This avoids damage to the
screw or barrel. In some plants, the
| EXTRUDERS |
Image of Hot Feed & Cold Feed Extruder Screws
14

15
sensitivity of the metal detector is
found to be calibrated to a low value
that it virtually renders the detector
useless.
Every CFE comes with a Temperature
Controller Unit (TCU) that controls the
barrel temperature so that the shape
and size of the extruded products are
uniform.
For feeding the cold rubber, it is recommended that you use a
feeding conveyor with metal detector to remove metal particles.
| EXTRUDERS |
Image from Web: Feeding conveyor with Metal Detector
Image from Web: A Set of Compact TCU’s
below Cold Feed Extruder.

The different Cold Feed Extruders (CFE)
and their uses are:
Plain Barrel Type Cold Feed Extruder:
These CFE’s as the name suggests have a
plain barrel and used in manufacturing of
hoses, blanks, fluorocarbon rubber, butyl
rubber, etc.
Pin Barrel Type Cold Feed Extruder:
These are equipped with metallic pins fixed
along the circumference in full length of
screw. These pins enhance the mixing and
dispersion of your rubber as it is kneaded
between the barrel and screw. And the
result is processed rubber with outstanding
homogeneity and extrudate quality. The
number of pins may be optimised by
process and if required pins from last zone
i.e. towards head may be blocked. These
pins are generally 8 to 10 in circumference.
For example, in 10 rows you will have 8x10
=80 pins protruding out of the barrel
towards the screw center.
The flexibility in Pin Type Cold
Feed Extruders endears to all
making it a ‘Universal Extruder’
for many rubber formulations.
| EXTRUDERS |
Image from Web: Pin Barrel Type Cold Feed Extruder
16

This flexibility in Pin Type Cold Feed
Extruders endears to all making it a
‘Universal extruder’ for many rubber
compound formulations involving varied
applications. Hard rubber compounds also
can be processed because of high extruder
torque.
Vent Type Cold Feed Extruder
Vent type or vacuum type extruders were
developed for production of non-porous
profiles and hoses. These CFE’s have a
custom-built screw, and a degassing barrel
with a vacuum pump attached to vent
bubbles out of extruded compounds.
Co-extrusion
Customer-specific customization and usage
complexity demands led to the introduction
of co-extrusion for manufacturing of
various profiles. And so you today have
Simplex, Multiplex (Duplex, Triplex,
Quadruplex, and Quintuplex) and Roller
Head technology. Multiplex lines of piggy-
back type of 2,3,4, and 5 layers have a
compact construction.
Multiplex lines of piggy-back
type have compact construction.
| EXTRUDERS |
Image Of Quadruplex.
Source: Canmade
18

Roller-Head-Extruder
Roller Head Technology involves a
combination of extruder with preform head
and two-roll calender. They offer twin
advantages of – high uniformity of the
material thickness over the entire sheet
width with absence of air traps even at
higher thicknesses (~20mm thick as against
conventional calender lines that give max
3mm thick sheets with or without air traps)
and excellent homogeneity of the material
produced.
Both these characteristics are important for
high-quality rubber products such as tire
components, V-belts, conveyor belts, tank
linings, cover sheets, blank sheets and
roofing sheets. For even thickness across
the entire sheet width of the roller head,
there are three options that can be used
alone or in combination with one another –
roll crowning, roll crossing or roll bending,
that will compensate for the elastic
deflection of rolls.
In today’s world you will see that usages of
these technologies are overlapped. For
example, in tire industry, you can notice
that
- Tread & Sidewall are extruded using
Simplex, Duplex, Triplex, Quadruplex
Lines
- Apex are extruded using through Simple
and Duplex.
- Inner Liner are extruded using Simple,
Duplex & Roller Head Technology.
| EXTRUDERS |
Image: KraussMafei Berstoff
20

Extruder Head and Die Design
The key to the extrusion is the extruder
head that transforms the shape of the
rubber while leaving the barrel to
produce a dimensionally accurate and
stable profile.
The purpose of extruder Head is to
equalise the pressure from the screw and
barrel andto transport the compound
smoothly at equal pressures and speed to
the die.
The purpose of Die is to give the compound
the desired shape. The extrudates shrink
along their length and increase in thickness
and width, the behavior being termed ‘die
swell’ which depends upon
(i) rheological characteristics compound
(ii) shape of head and extrudate
(iii) pressure in the head
(iv) the head and compound temperatures.
Generally die making and die design
depends on experience, however a good
control on compound properties gives
better extrusion.
It is necessary to get a swelling factor at
each point of profile for a particular
compound. This varies from centre of die to
extreme ends. Die swell is defined as ratio
of extrudate section & cross section of die.
| EXTRUDERS |
Image: Bharaj Make Cold Feed Extrusders equipped with Co-Extrusion Head
21

This depends on Polymer, Carbon type and
PHR, Viscosity, Lead angle (Bevel) of die etc.
Some of the die heads include:
1) Straight Head / Tube Head for hoses
(circular or square) and technical profiles.
2) Cross Heads for hoses and technical
profiles from one, two or three different
compounds.
3) Single Tread Heads for rubber profiles.
4) Multiple Tread Heads (Piggy-back) for
rubber profiles.
5) Slit Tube Heads for producing sheets
inexpensively.
6) Preform Heads for sheets and rubber
profiles.
7) Strainer Heads to strain raw rubber,
rubber compounds and reclaim rubber.
8) Pelletizer Heads to process rubber
materials ground into granules.
9) Pork chop Heads to produce granulated
raw rubber.
| EXTRUDERS |
Die swell depends on Polymer,
Carbon type and PHR, Lead angle
of die, etc.
Source: CGEC
24

Your main goal of extrusion is to get the highest output
at good quality of product within the safe temperature
limits of the products.
| EXTRUDERS |
Image: Triplex Extruder. Source: Troester
25

Gear Box
A gearbox is the single-most-expensive-
component on an extruder. Hence, you
should be more cautious while selecting.
First things first – Why do your single screw
extruders need gearboxes?
Extruder manufacturers prefer 1800/1500
RPM or 1200/1000 RPM motors (depending
on your country of use) because they are
economical, readily available and compact
in size to mount on your extruder base.
However, most rubber extruder screws
during production run in the speed range
of 4 rpm to 40 rpm.
Hence, the role of a gearbox or gear
reducer here is to reduce the drive motor’s
speed and, in turn, multiply the available
torque from the motor in order to produce
sufficient power to mix and push out your
rubber compound.
As an individual and independent
component, the key specification that
A gearbox is the single-most-
expensive component of an
extruder. Hence, you should be
more cautious while selecting.
| EXTRUDERS |
Image:Wikov
26

defines the capacity and durability of your
extruder gearbox is the power (HP/kW)
rating along with its service factor (SF).
Single screw extruder gearboxes are
normally rated for power (HP/kW) or
torque at a specific rpm based on common
calculations and standards. This uniform
standard allows you to compare gearboxes
from different manufacturers.
Your extruder manufacturers follow these
guidelines and select a model for different
service factors and applications. A key
question you should ask is whether, the SF
considered by your manufacturer is optimal
for your extrusion application or not.
Knowledge of AGMA (American Gear
Manufacturers Association)
recommendation will certainly help you to
discuss better with extruder manufacturers,
but experienced gearbox manufacturers
can guide you even better. I have witnessed
buyers in Asia been taken for a ride for their
ignorance and offered lower specification
gearboxes on their extruders to compete on
cost.
When you compare gearboxes, always
evaluate on calculated power. The formula
for calculated power of a gearbox is:
Calculated Power = Quoted Power X
Service Factor
Typically, single screw rubber extruder
gearbox has service factors of 1.5 or 1.75 for
optimal operating capacity. For example, a
6 inch pin type cold feed extruder gearbox
with a calculated rating of 367.5 kW would
have a quoted rating of 210 kW with a 1.75
service factor.
The overall rating of a gearbox is based on
the ratings of all its individual components.
This includes the gear teeth design, gear
hardness, shaft dimensions, bearing
selection and sizes, housing design
(thickness & rigidity), and thermal
considerations. All these considerations are
to ensure that your gearbox has sufficient
support and capacity to effectively transmit
the motor torque to the screw without
significant distortion or failure.
Within the gearbox, the most important
component (and most expensive) is it’s
thrust bearing. You evaluate a thrust
bearing based on its type and life (B-10 or
L-10 rating).
The B-10 Life (sometimes called L-10 Life)
of the thrust bearing is based on an
engineering calculation that estimates the
number of hours of operation at which 10
per cent of the bearings are likely to fail.
Additional rating adjustment factors are to
be applied to the basic B-10 life based on
application factors including how the
bearing is mounted.
For example, a thrust bearing that is
mounted between two radial bearings is
more likely to have precise thrust bearing
alignment, and will therefore have a higher
rating adjustment factor.
| EXTRUDERS |
30

KELACHANDRA MACHINES
(MFRS. OF RUBBER INDUSTRIAL MACHINERY)
Chingavanam - 686 531, Kottayam, Kerala. India.
Phone Off: +91- 481-2430325. Fax: +91- 481-2430596
e-mail: km1912@gmail.com

Here are 6 other key criteria of a gearbox
evaluation, which you should know
1. Gear Design, Hardness and its
Construction
Each of the individual gears that go into
your gearbox assembly are rated for power
or torque based on their strength and
durability ratings. The calculations would
be according to industry standard AGMA
rating systems. Factors include the gear
tooth pitch, center distance, material and
hardness.
2. Gear Shafts
The shafts must be designed to transmit the
full power and torque capacity of the gears.
The length and diameter of these shafts is
decisive and must match the ability to
transmit this torque without excessive
deflection, fatigue and failure. The diameter
of the input shaft must be adequate to
properly support sheaves (in the case of belt
driven models) or a coupling. The output
shafts must be properly designed to handle
the correct range of screw shanks that will
be inserted. Adequate access to the drive
keys is beneficial when they become worn
or damaged and need to be replaced.
3. Radial Bearings and Seals
The radial bearings support the rotational
forces of the gear shafts and must be
designed to handle the load forces and
speeds effectively. The dynamic load
capacity of these radial bearings must also
be considered when evaluating the design
and durability of the gearbox. Radial
bearings must also be properly lubricated
and sealed.
4. Gearbox Housing Design - Construction
Cast Iron is the cost-effective material of
choice for most manufacturers.
Traditionally, CI gearboxes are made in two
pieces, split either horizontally or vertically.
Newer designs have the gearbox housing
cast as one piece to reduce any potential
leakages.
5. Thrust Bearing
Thrust bearing isolates the backward forces
from the screw. The larger the screw and/or
the higher the back pressure, the greater
the backward thrust forces. There are three
basic types of thrust bearings – cylindrical,
spherical and tapered.
6. Serviceability
When you select a gearbox, you should give
prime importance to the availability to
affordable parts and service. It is best to
select rubber extruder suppliers who
purchase their gearboxes from proven and
reputable manufacturers that specialize
only in gearboxes for better serviceability.
An old gearbox manufactured around
through-hardened process and shaved gears
technology has shafts, bearings and
housings designed accordingly. Replacing
new hardened gears with a higher HP
capacity, does not automatically guarantee
the gearbox rating to increase, if you do not
replace the assembly with stronger shafts,
bearings and housing.
Gear manufacturing technology today has
| EXTRUDERS |
33

changed and consists of carburized and
ground gears. These gears are capable of
delivering much more power in its smaller
size. When old gear designs are
constructed using the new materials and
process, the power calculations yield much
higher gear tooth ratings. But if the rest of
the design is unchanged, and the same
bearings, shafts, and housings are used, the
total gearbox rating cannot simply be based
on the new higher gear rating alone.
The higher torque could never be applied to
the original sized input shaft without
causing bending or twisting. The bearings
and/or shafts would be overloaded with the
higher forces, and the housing would
probably not have sufficient strength to
resist significant distortion. Reputed
gearbox rebuilders will guide you well.
Automation
The different levels of automation of down-
stream line for extruders makes the
extrusion operations highly user-friendly
and efficient.
In the case of extruder heads, there are
different options depending on user
requirement - a simple, single-compound
single-component mechanically opening
extruder head to a sophisticated cost-
effective PLC controlled hydraulically
opening multi-compound multi-cavity head.
So, rubber extrusion is in itself a vast and
interesting subject.
| EXTRUDERS |
Image of Extrusion Line. Source: CGEC
RMW
35

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SERVICE TO THE NATION FOR FOUR DECADES
For all your enquiries please contact:
AMCL MACHINERY LIMITED
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Mumbai ofﬁce: 202, Ackruti Centre Point, MIDC Central Road, Andheri (East), Mumbai-400 093.
Contact person: Mr. S.H. Mehta
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