EXTRUSION PROCESSEXTRUSION PROCESS

PRESENTED

BY

ALOK KUMARALOK KUMAR

EXTRUSION

Continuous Process

In principle, the plastic raw material is plasticated by means of a screw plastication unit and

the molten material is continuously pumped out through a standard orifice (die) in order to take the shape and then the shape is set by cooling/sizing system.

Example:Film,Pipe,Tube, Profile, Monofilament, Box Strapping etc.

SINGLE SCREW EXTRUDER

Single Screw Extruder

Parts & its functions

Screw Pump : Combination of Screw & Barrel

Hopper : Funnel like device, mounted on Hopper throat. Holds a constant reserve of material.

Barrel : Cylindrical housing in which the screw rotates.

Hopper Throat : Circular opening at the feed end through which the material enters

the screw pump.

Drive System : AC/DC drives

Speed reduction gear box

Transmission system

Screw Support Assembly

Typical thrust bearing assembly – Single Screw Extruder

Definition of terms

Compression Ratio - Is the ratio between the channel depth is the feed zone to that of the metering

zone.

- Usually from 1.5 to 4:1

L/D ratio - Length to nominal dia of screw

- usually 20 to 22:1

Important Specification

Nominal dia of screw

Output(kgs/hr)

Zones of Extruder & its Functions:Feed Zone - Transport the material from hopper to

compression zone.

- Compacts, eliminates air gap

Compression Zone - Transport the material from compression to metering zone.

- Softens the material

Metering zone - Melts, Mixes, the material pressurizes and pumps the melt.

Breaker Plate & Screen Pack

Breaker plate: Perforated circular metallic disc of about 4-5 mm thick.

Functions

- Support for Screen pack

- Converts the Spiral flow of melt in to stream lined laminar flow

- Holds back contamination and unmelted particles.

Screen Pack

- Wire mesh 40,60,80

- Arrests the unmelted particles and contamination

- Helps in developing back pressure

Extruder Screws

General purpose screw

PVC screw

Nylon screw

Two stage screw/vented screw

Segmented screws is also available for special purpose

General purpose screw

PVC screw

Nylon screw

Two stage screw/vented screw

Mixing elements

Incorporated in the metering zone of screw

Several designs

Mainly to improve mixing, homogeneity

PIN TYPE

CAVITY TRANSFER TYPE

DULMAGE TYPE

SCREW COOLING

Helps in Extracting out the Frictional Heat

Also helps in developing back pressure

Barrel

High grade steel cylinders

Has to withstand up to 400 atm.

Iron Based Alloy Complex non-ferrous alloys

More hard & less corrosion less hard & more corrosion resistant resistant

These expensive materials are used as liners in steel barrels.

Barrels of Nitrided steel are also used.

They are Cheap, hard, less resistant to corrosion.

Screw material

Low carbon alloy steel

Flight tips are hardened by flame hardening to prevent wear or nitriding the entire screw.

Chrome plated screws for vinyl polymers

special nickel alloy steel for processing of saran.

Melt Fracture

It is a die-entry effect

In any converging flow there are tensile and shear forces

If tensile stresses become large and if they exceed the tensile strength of melt, the desirable smooth laminar flow is lost completely.

The extrudate emerging from die exit will be of irregular shape. This phenomena is called “Melt fracture”.

If die entrance is tapered

Dead spots are eliminated

Minimise development of tensile stresses and hence minimise distortion of stream lines.

Shark skin & Bambooing effect

Shark skin

Roughening of the surface of the extrudate

The melt as it proceeds along the die channel, has a velocity profile with maximum at the centre and zero at the wall.

As it leaves the die lips, the material at the wall has to accelerate to the velocity at which the extrudate is leaving the die.

This generates tensile stress and if the stress exceeds Tensile strength, the surface ruptures causing the visual defect - “shark skin”.

If the conditions causing shark skin becomes more intensive, eg. Pressure at the extruder becomes excessive or die temperature

drops, the extrudate “snaps back” -- “Bambooing effect”.

Die Swell

Also known as Extrudate swell.

When the melt emerges out of the die lips, there will be expansion in the direction perpendicular to flow and contraction in the direction parallel to flow.

Constrained molecules tends to relax at the die outlet. This leads to die swell.

This is nullified by higher take off speed.

Factors Influencing Extruder Output

Factor Direction of change Output

Screw speed Increase Increase

Channel depth Increase Increase

Helix angleIncrease(max. upto

300)Increase

Back Pressure Decrease Increase

Length of Metering sectio

Increase Increase

Viscosity Increase Decrease

Twin Screw Extruder

Two screws rotating inside a barrel.

Intermeshing type are more popular.

Different models/design available

Basic Types

Co-rotating

Counter-rotating

Mainly used for preparation of master batches/colour concentrates

Co-rotating Twin screw - used for compounding all thermoplastics except PVC.

Counter rotating - preferred for PVC.

Comparison between Co-rotating and Counter rotating

Sl.No. Co-rotating Counter rotating

1Both screws either turn clock-

wise or counter clock-wise.One screw turns clockwise and other

counter clockwise.

2 More complete self wiping Less self wiping

3Less likelihood of material

stagnationMore likelihood of material stagnation

4 Better mixing Less than co-rating

5 Total shear is moreLess shear compared to co-rotating

and single screw extruder

6 Mostly for compounding of TP Mainly for PVC compounding

7 Positive pumping Positive pumping

8Less power consumption than

single screwLess power consumption than

Single screw.

APPLICATIONS OF EXTRUSION

1. Film: Blown film, Cast film, Co-extruded films, BOF.

Material Used: PP,PVC, LDPE, HDPE, PET, Nylon etc.

2. Pipe/tube

Material: HDPE, LDPE, LLDPE, PVC etc.

3. Sheet

Material: HDPE, ABS, HIPS, PC etc.

4. Monofilament

Material: PP, Nylon etc.

5. Extrusion Coating/Lamination

Coated Playing Cards, Wrapping and LDPE laminated Woven sacks

Material: LD,PP,HDPE

6. Box Strapping

Material: PP, HDPE etc.

7. Tape/Woven Sack

Material: PP, HDPE

8. Wire Coating/Covering

Primary/Secondary insulation

Material: LDPE, PVC (Primary insulation) Nylon (secondary insulation)

9. Profiles ( Door and window )

Material: PVC

Blown film extrusion

Upward blown film - LD,HD,PVC, Nylon etc.

Downward blown film - PP (Mainly to get clarity)

Process outline

Melt emerging from extruder is inflated by air pressure (3 to 4 kgs/cm2)

Bubble is properly stabilized and cooled

Wound on the winder

AUTOMATIC SCREEN CHANGER

SIDE FEED DIE

CENTRE FEED DIE

SPIRAL FLOW DIE

BLOWN FILM DIES

TQ process

Tabular quench film process

For PP

FLAT FILM - EXTRUSION

The Melt emerging out of the die lips strikes the chrome plated chill roll where it solidifies.

Subsequently the film is pulled through nip rolls .

Trimming blades trims-off the thicker edges.

Then the film is wound on the winder.

Blown Film Co-Extrusion • Barrier properties is the main reason to go for multi

layer film

Three layer Co-Extrusion Five layer Co-Extrusion

Three layer Blown film Die with internal bubble cooling

5- Layer Blown film Die with

Radial melt distributor

SL.NO

MATERIAL COMBINATION

SPECIAL PROPERTIES

MOST IMPORTANT FIELDS OF

APPLICATION

1. LDPE/LDPE Pinhole-free ( multicolored) Milk film and carrier bags

2. LDPE/EVA Good weldability, sterilizableHeavy duty bags, medical

articles

3. HDPE/LDPE Good strengthBakery goods food stuffs,

tomato concentrate

4. LLDPE/EVA Good surface adhesion Stretch film

5. Ionomers/PA Gas and aroma-tightMeat, sausage, fish and

cheese

6. Ionomers/EVA Grease proof Coconut, biscuit

7. LDPE/HDPE/LDPEWeldability on both sides reduced

curling tendencylike 3

8. LDPE/TL/PA Gas water and aroma tightReadymade meals, sausage,

fish, cheese foamed PS granulate

9.LDPE/TL/PA/TL/LDPE or LLDPE/TL/PA/TL/LLDP

No curling tendency improved barrier properties, as PA protected against moisture absorption improved layer adhesion weldalble on both sides

Like 8

Extrusion coating

The plastic is coated over a substrate like paper, by extruding through a slot die downward between two rolls.

Substrate is fed between the molten plastic and the roll and is joined with the plastic by pressure between rolls without the use of an adhesive.

Material used LDPE & PVC

PP, HDPE, Ionomer etc. are also used.

Equipment compresses of

Pre treatment unit

Coating unit

Take off & winding

Dies

Coat hanger die

‘T’ type die

Tube/Pipe Extrusion

Wall thickness & flexibility/Rigidity differentiates between tube/pipe

Pipes are produced by horizontally extruding molten polymer through an annuler opening into several sizing, cooling devices that stabilizes the final dimension.

Comprises of

Extruder

Die

Sizing device

Cooling bath

Cater puller

Cutter or winder

Dies used

Straight through

Off set die

Sizing Equipment - Methods

Vacuum Trough

Widely used

With the help of vacuum, Pipe is stabilized and sized to retain the shape

Sizing Sleeve

Methods fixes the outside pipe diameter as it hardens by contact with a water cooled metal sleeve.

FLOATING PLUG SYSTEMS- USED FOR RIGID PIPES OF MEDIUM AND LARGE SIZES

TO PREVENT LOSS OF AIR PRESSURE FLOATING PLUG SYSTEM IS USED

Extended Mandrel

Method uses a water cooled extended mandrel

Provides additional internal cooling and internal support

Sizing plate

Method involves pulling the pipe through a series of brass plates

Mainly for small dia pipes/tubes.

Sheet Extrusion

Process out line

Plasticated melt in pumped out through the sheeting die by screw pump

Molten sheet emerging from die lips is feed through three roll calendar, cooling rolls and subsequently pulled by Nip rolls.

Cut the sheet by shear cutter

Stacking/ Inspection

Dies

T-TYPE

DIE

COAT HANGER

TYPE DIE ( Widely Used)

Coat Hanger type of Die is much more stream lined than T-type Die

Wire Coating/Cable Covering

Unit comprises of

Un wind unit (For conductor)

Pre treatment unit

Wire coating unit

Steps Involved Wire/conductor is unwound & straightened by Tension Control Unit. Pre treated to promote adhesion of molten plastic Then passed through the Cross head die of the coating unit Coated wire is then cooled by passing through cooling

trough Wound on the winder

Cooling Trough

Take off/ wind up.

Die used

Tubing die

Pressure die

USED MAINLY FOR SECONDARY INSULATION

USED MAINLY FOR PRIMARY INSULATION

Extrusion of Mono filaments

Mono filaments are wise like polymer strands of dia 0.09 to 1.52 mm. Usually they have circular cross-section. The polymer melt from extruder is pumped out through a multi-hole die, quenched, stretched/oriented and annealed to get the filament of enhanced properties.

The production process comprises of Extrusion Filament forming Stretching (orientation) Annealing Winding

Orientation Systems

A. Heated liquid bath method

B. Heated oven method

C. Heated point method

D. Heated roll method

Depending upon material & configuration system is selected

A

B

C

D

Extrusion -- Box- Strappings

The process sketch is similar to Monofilament line except the die – A slotted die is used in place of multi-hole monofilament die

Plastic Strappings, made of PP/HDPE replace iron because of their flexibility.

Process outline

Plasticated melt from an extruder is pumped out through a slot die

Quenched in water bath

Bath temperature - 800C for PP

- 900C for PA-6

Passed through a orientation system and stretched to about 8 times in order to improve tensile properties.

Annealed in an annealing chamber to relieve the stresses

Wound on winder.

Special Extrusion Process

Corrugated Pipes

Similar to extrusion of pipes except the die and the calibration units are specially designed to produce corrugation on the pipe.

The cylindrical part of the pipe die head extends in to the closed area of a revolving mould block chain.

The plastic tube is pressed against the profiled, revolving mould block halves by internal air pressure or by vacuum calibration.

As it passes through the forming machine its cools and solidifies.

a) Internal Pressure System b) Vacuum System

POST EXTRUSION FORMING

Inline post forming with extruder embossing one or both sides with shallow or deep patterns

INLINE VACUUM/PRESSURE FORMER FOR PLASTIC SHEEET WITH MATCHED, WATER COOLED FORMING MOULDS ON CONTINOUS CONVEYOR SYSTEM

AN INLINE COIL FORMER CAN PRODUCE TELEPHONE CORDS

THANK‘U’