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’