Extrusion Processs 2011

8/1/2019 Extrusion Processs 2011

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EXTRUSION PROCESS

PROF. CHIRAG . P. PATEL

ASST. PROF. MECHANICAL ENGINEERING DEPT.,

U.V.PATEL COLLEGE OF ENGINEERING,

GANPAT UNIVERSITY,


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What is Extrusion:

Extrusion is the process of squeezing metal in aclosed cavity through a tool, known as a die

using either a mechanical or hydraulic press.


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Product of Extrusion

Railings for sliding doors

Tubing having various cross sections,

Structural and architectural shapes,

Door and window frames,

Fasteners and components fro automobiles,

Bicycles,

Heavy machinery,

Transportation equipment.


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DiesThe extrusion die is a steel disk with an

opening, the size and shape of the intendedcross-section of the final extruded product as

indicated below.


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Billet

The billet is the starting stock for the

extrusion operation.Extrusion billets may be a

solid or hollow form, commonly cylindrical,

and is the length charged into the extrusion

press container.


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Types of Extrusion:

Direct Extrusion (or) Forward ExtrusionBillet isplaced in a chamber and forced through a die opening bya hydraulically-driven ram or pressing stem.

-the die opening may be round, or it may have variousshape


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Indirect ExtrusionDiemoves towards thebillet.

Hydrostatic ExtrusionThe billet is smaller indiameter that the

chamber, which is filledwith a incompressiblefluid medium, and thepressure is transmittedto the billet by a ram.


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Impact Extrusion: Similar to indirect extrusion Punch descends rapidly on the blank, which is extruded

backward. Because of volume constancy, the thickness of

the tabular extruded section is a function of the clearancebetween punch and the die cavity.


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Production of collapsible tubes, such as for toothpaste.

Most nonferrous metals can be impact-extrusion in verticalpresses and at production rates as high as two part per second.

Lateral or Side Extrusion:


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(a) Flow pattern obtained at low friction, or in indirectextrusion,

(b) Pattern obtained with high friction at the billet-chamberinterfaces,(c) Pattern obtained at high friction, or with cooling of the

outer regions of the billet in the chamber. This type ofpattern, observed in metals whose strength increases rapidlywith decreasing temperature, leads to defect known as pipe,

or extrusion defect.

Metal flow in extruding with square dies.


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Hot Extrusion: Extrusion is carried out at elevated temperaturesfor

metals and alloys that do not have sufficient ductilityat room temperature, or in order to reduce the forcesrequired.


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Cold Extrusion Combination of operations, such as direct and indirect

extrusion and forging.

Advantages : Improved mechanical properties

Good control of dimensional tolerances

Improved surface finish

Elimination of the need for billet heating;


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Extrusion process:


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A machine which is successful in extrusion of plastic

materials is called EXTRUDER.In commercially extrusion there are three general types of

mechanisms are used.

1. RAM-CYLINDER TYPE EXTRUSION

2. WET EXTRUSION

3. SCREW EXTRUSION

Extrusion process is widely used in manufacturing &

continuous shapes such as rods, tubes, profiles, sheets,

filaments, films, pipes etc

Types of Extruders


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Single Screw Extruder.

Types of Screw Extruder


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Twin Screw Extrudera. Parallel configuration

b. Conical configuration


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SCREW EXTRUSIONThe rotating screw takes the material from a feeder which may be in the

form of free flowing, granules, chips, powder or cubes through a heated

barrel zones and compacts it against the breaker plate and otherrestriction so that pressurize built up. During this period, material comes

in contact with heated barrel wall and frictional heat is produced less

between screw and material and because effect of hot barrel and the heat

due to internal friction in the material cause thermoplastic to soften and

forced through the restriction to the extrusion die where it is given formof shape in continuous production.


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ADVANTAGE OF SCREW EXTRUSION

Continuous process

Uniform heating Proper mixing

Less residence time

Less pressure drop

Avoid leakage problem

Self cleaning

Higher production rate


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COMPARISIONS BETWEEN THE CHARACTERISTICS

OF TWIN SCREW AND SINGLE SCREW MACHINES.

TWIN SCREW M/C SINGLE SCREW M/C

1.For the twin screw extruder the

volumetric efficiency of the screw is

fairly high & is almost independent of

pressure.

2.Twin screw extruders are morecomplicated in construction.

3.Costly.

4.More difficult to maintain.

5.Twin screw extruders are used for

mixing and compounding for direct

extrusion from uncompounded powder

blends.

6.Twin screws do not generate as much

frictional heat.

1.For the Single screw extruder the

Volumetric Efficiencies are always low

are highly pressure dependent.

2.Single screw extruders are simple in

construction.3.Cheaper.

4.Easy to maintain

5.Single screw extruders are used for

compounded plastics material.

6. Single screw generates mach

frictional heat.


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CONSTRUCTIONAL DETAILS OF

EXTRUDER

MACHINE BASEThe machine base of an extruder is the structure on which the barrel,

reduction gear and other parts are supported.

Generally however a well matured casting is to be preferred because

a base made in this way is usually very rigid and free from spring.



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MACHINE BASEThe machine base of an extruder is the structure on which the barrel,

reduction gear and other parts are supported.Generally however a well matured casting is to be preferred because a

base made in this way is usually very rigid and free from spring.

DRIVE MOTOR AND REDUCITON GEAR

The range of screw speeds lies between 10 and 200

rev/min. for a single screw machine and between 5 and 50 rev/min. for a

multi screw machine. Therefore, since the majority of the extruders are

powered by electric motor with o/p speeds of 1000 rev/min. or more it isnecessary to build a high ratio reduction gear into the m/c drive.

Generally D.C. motor is used for this purpose because of its cost. But

nowadays A.C. motor is preferred due to its maintenance free long life

and simple drive mechanism


EXTRUDER


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THRUST BEARING

Ball bearings, Roller bearing, Taper Roller bearing etc, are

used for above purpose

Thrust bearing are designed to operate under specific load conditions.

DRIVE BETWEEN MOTOR AND REDUCITON GEAR

Belts, Direct coupling and sometimes chains are used to provide the

final drive between the motor and reduction gear

There are different types of belt drives. Flat belt, Vee belt and timer

belt. Generally flat belt and Vee belt drive are used for extruder. Veebelt are used to minimize the slippage. Timer belt gives slippage free

drive, but it cant be used in extruder because it is not available in big

size and if possible it will be very costly.


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SCREW

Depending on its design an extruder screw consists of a shallow cut

zone at the discharge end with the root dimension tapering down toa more deeply cut zone at the feed position and hence to parallel

plain shank where it picks up the drive by means of one or more

keyways or spines.

The commonest materials used for the manufacture of screws are alloy

steels, nit riding steels or stainless steels.

Screws for processing PVC material are hard chromium plated toprevent corrosion in only single screw extruders


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SCREW COOLING

Generally, there are two systems of screw cooling. Oil circulation in

screw requires oil pump for cooling the screw by force oil circulation

In sealed coolant system once the required quantity. i.e. one third of

volume inside the screw bore is filled up by coolant (distilled water). It

is plugged at the end.


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EXTRUDER BARREL

The barrel of an extruder is the parallel cylindrical chambers in which

the screw rotates & forwards the thermoplastic material. The barrel

manufactured from nitrides steel is equipped with heating & coolingzones.

D t il f diff t ti f b l t t d b l


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Details of different section of barrel are stated below :

[A] FEED SECTION

There are two basic requirements fro his section firstly the feed opening should be

designed to permit the feed material to flow freely from hopper or feed device, into

screws without the possibility of bridging.

[B] MAIN SECTION The main barrel section of the extruder is both pressure vessel & a heating chamber.

Since the material is subjected to a gradually increasing pressure and suffers a

consider able temp. D-Rise during its passage through this part of the machine. Inorder to withstand the high internal pressures resulting from the process the parallel

section has the form of a thickwalled tube.

BARREL COOLING

During processing, it is possible for the screw to generate more heat inthe material than is required for the satisfactory working of the process

and it becomes necessary to remove this excess heat by some means.

A variety of cooling system is used for extruder barrels. Here more

popular methods being the use of number of fans

FEED HOPPER


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FEED HOPPER

DRIVE It consists of DC shunt motor with over load protection by electronic

current limitation.

GEAR BOX The gear boxes consists a helical gear pair especially for the drive of

feed hopper

[A] CONICAL HOPPER

The commonest from of feed hopper used on single screw machines is

the conical type of standard design. The material feeds into the screw

merely by its own weight and consequently, this type of hopper is

referred to a gravity feed

[B] CONTINOUS FEEDER

For the feeding of difficult materials such as PVC dry blends, and

some time in order to increase output or to obtain an acceptable output

from materials of low bulk density, special force fed hoppers have

been designed, knows as crammer feeders.

HEATING AND HEAT CONTROL


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HEATING AND HEAT CONTROL

One of the major requirements of an extrusion m/c for thermo plastic

is that it should be able to raise the temp, of the material passing

through it is at a controllable rate. Therefore, extruder must be

equipped with a heating system which can be accurately controlledever a wide temp, range.

Resistance band heaters, induction heaters, cartridge heaters etc are

used to heat barrel and die.

Here, on-off controller method with PID temp, controller temp, are

used for controlling electrical heating system


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BREAKER PLATE AND SCREEN PACK

Its function to convert turbulent flow into laminar flow & to increase

melt pressure for homogeneous melt it is made up of EN-8 material

& gas nit tided.

Screens of various mesh and wire sizes are usually used with the

breaker plate and are placed on the screw of the plate so that pressure

of the material maintains them in position.

CONNECTING HEAD

This is a transmission member between extruder barrel and die is

provided with heating zone.


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VACUUM SUPPLY

The water ring vacuum pump installed in the base frame generates the

vacuum required for the cylinder de-gassing. The fresh water supply

for the pump is provided via a solenoid valve and a flow restrictor. Aball type non return valve prevents water from the pump entering into

the screw cylinder.

ELECTRICAL EQUIPMENT

In the terminal housing there are the terminals for connecting the

drive, heaters and control to the main control cabinet.

The socket connection plate is situated at the front of the extruder base

frame and contains sockets for adaptor and die heaters andthermocouples.

In the operating panel there are the push buttons for ON-OFF

increase decrease etc. with indicating lights for the main drive motor,

vacuum pump & feeder.


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EXTRUSION DIES

The function of an extrusion die is to form molten material deliver by

the screw into a required cross section. The die is there for a channel

whose profile changes from that of the extruder bore to an orificewhich produces the required form

Extrusion dies may be attached to the extruder in three different ways

according to the requirements of the complete extrusion process.

1] STRAIGHT THROUCH DIE

2] CROSS HEAD DIE

1.SIDE FEED

2.CENTRE FEED

3] OFFSET DIE

Extrusion Force :


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Extrusion Force :F = Aok ln(Ao/Af)

Extrusion Ratio = Ao/AfAocross-sectional area of the billet

Af- cross-sectional area of extruded product

K- extrusion constant

Ao, Afbillet and extruded product areas


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CONCLUSION:

Extrusion often minimizes the need for secondarymachining,and as a result could result in financial

savings.

However extruded objects are not of the same

dimensional accuracy or surface finish as machined

parts.

Extrusion process is widely used in manufacturing &

continuous shapes such as rods, tubes, profiles,sheets, filaments, films, pipes etc.


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Self study topic in extrusion-

- defect in extrusion,- example of extrusion force calculation.

Extrusion Processs 2011

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