EXTRUSION

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EXTRUSION
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• 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.
• ExampleFilm,Pipe,Tube, Profile, Monofilament,
  Box Strapping etc.

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CLASSIFICATION OF EXTRUDERS

•  
• 1         Batch Type
• 1.1                 Ram Extruders
• 1.2                 Reciprocating screw
  extruders
• 2         Continuous Type
• 2.1                 Screwless Extruders
• 2.1.1            Disk Extruders
• 2.1.2            Drum Extruders
• 2.1.3            Other Extruders
• 2.2                 Screw Extruders
• 2.2.1            Single-Screw Extruders (SSE)
• 2.2.2            Twin-Srew Extruders (TSE)
• 2.2.3            Multi-Screw Extruders
•  

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SINGLE SCREW EXTRUDER
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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
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The Single-Screw Extruder consist of a screw
rotating in heated barrel or cylinder to which
the material is fed.

• Feed hopper
• Extruder Screw and Barrel
• Drive system (motor, gear box, transmission)
• Thrust Bearing
• Heating and Cooling Elements
• Screen Pack and Breaker plate
• Die
• Temperature and pressure controls.

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• 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 41
• L/D ratio - Length to nominal dia of screw
• - usually 20 to 221
• Important Specification
• Nominal dia of screw
• Output(kgs/hr)

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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.
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Screw Nomenclature
P is the screw pitch, distance between the
centre of a two adjacent flights. W is the
channel width L is the land width  ? is the
helix angle, defined as an angle between the
flight to the transverse plane of the screw
axis. D is the screw diameter, developed by
rotating the flight about the screw axis. R.D is
the root diameter Flight is the helical metal
thread of the screw. C is the channel depth o
radial distance form the bore of the barrel to
the root
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SCREW TYPES
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• 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
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Nylon screw
P A SCREW
Two stage screw/vented screw
TWO STAGE SCREW
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• Mixing elements
• Incorporated in the metering zone of screw
• Several designs
• Mainly to improve mixing, homogeneity

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THRUST BEARING

• The screw fits into a thrust bearing located
  behind the feed hopper.
• The function of the trust bearing is to absorb
  the thrust force acting on the screw inside the
  extruder barrel

Typical Thrust Bearing as used in Single-Screw
Extruders
Typical thrust bearing assembly Single Screw
Extruder

• The bearing prevents the screw from moving
  backward.
• Bearing life-time depends on the pressure and
  screw speed. For high speeds, oversized being is
  needed.
• For twin screw extruders several smaller bearings
  joined in one shaft is used.

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HEATING AND COOLING ELEMENTS
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• There are three methods of heating extruders
• Electric
• Fluid
• Steam Heating
• Electric Heating
• Induction Heaters
• Cast-in Heaters
• Band Heaters
• Mica Insulated
• Ceramic Insulated

• The electric heating is most commonly used due to
  
• Accuracy
• Reliability
• Easy to hook up.

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INDUCTION HEATERS

• AC Current passes through coil thus setting up a
  magnetic flux. Heat is generated from the
  resistance offered to the eddy current set up by
  the flux.
• The barrel is heated directly by its resistance
  to the induced current

Schematic Arrangement Showing an Induction Heater
in Section

• Advantages
• Accurate Control of Temperature.
• Good provision for cooling the barrel
• No possibility for hot or cool spots.

• Disadvantages
• Relatively high cost.

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CAST IN-HEATERS
The insulated heating elements are cast into
semi-circular or flat aluminium blocks, which are
machined to match the surface to be heated
Cast-In Resistance Heaters
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BAND HEATERS
They consist of Ni-chrome or other resistance
wires mica or ceramic insulated, then encased in
steel cover.
MICA INSULATED CERAMIC INSULATED
Flexible, supplied as a single piece. Rigid, supplied in 2 halves
Can withstand a load of 23-31 KW/m2 Can withstand higher heating load
Shorter service life Better services life
Less expensive More costly
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FLUID HEATING SYSTEM
The heating fluid, that is most commonly used for
extruders is oil. It may be heated by any
suitable means (mainly electrical). The heating
system consists of a heater a circulating pump, a
surge tank, and a heat transfer channel in the
extruder barrel.
STEAM HEATING

• The high specific heat and latent heat of
  vapourisation of water makes steam an excellent
  heat transfer medium. However, this system is not
  frequently used because of low maximum
  temperature that can be achieved, a need of
  working with high pressure piping, frequent leaks
  of steam that require shutting down of heating
  for repairs, and corrosion effects.

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COOLING SYSTEMS
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BARREL COOLING

• Barrel Cooling is needed to prevent overheating
  that may cause degradation.
• For small extruders fans that blow air over or
  around the barrel are used
• Other cooling system used include
• Cooling channels inside the barrel wall
• Fins on the barrel or on the heaters to speedup
  heat transfer
• A water-fog spray over barrel.
• Continuous, controlled vaporization of liquid
  (Water)
• Copper tubing carrying cold water is sometimes
  used.

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HOPPER COOLING

• Water-cooling is used to cool the hopper throat
  to prevent bridging and to protect the rubber
  parts present in the screw support assembly.

SCREW COOLING

• The cooling may freeze a layer of plastic on the
  screw root, reducing the channel depth thus
  producing more shear at a cost of throughput.
• This may also reverse the required relationship
  between the friction coefficient (low friction
  coefficient on the screw, high on the barrel),
  further reducing the drag flow.
• Furthermore, there is a danger that the material
  staying a long time near the screw root will
  degrade, contaminating the product.
• It is important to remember that the conveying
  ability of the screw is controlled by the
  friction coefficient ratio f(barrel) / f(screw)
  ó it is important to maximize this ratio.
• Under normal circumstances the minimum value of
  the ratio that guarantees conveying is 1.4.

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SCREW COOLING

• Screw cooling may be recommended to prevent
  decomposition of heat sensitive materials
• However, it should be carried out using the
  cooling fluid at the temperature above the
  softening point of the principal polymeric
  component.

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BREAKER PLATE AND SCREEN PACK
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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.

Fig.3.7
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SCREEN PACK

• Wire mesh 40,60,80
• Arrests the unmelted particles and contamination
• Helps in developing back pressure

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DIE DESIGN
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The role of a die is to form the melt into a
desired linear product fibres, films, sheets,
profiles, rods,etc. The die is a channel, whose
profile changes from that of the extruder bore to
an orifice, which produces the required form.

• The dies can be classified using different
  criteria. For example, considering cross section
  of the extrudate one may recognize dies to
  produce
• Solid Cross-Sections
• Hollow Cross Sections
• Another classifications scheme is based on the
  die attachment to the extruder barrel
• Straight through dies
• Cross heat dies
• Offset dies

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SOLID CROSS - SECTIONS

• A Typical Die Design for extruding a solid rod is
  shown in fig.
• In the figure, DD is the diameter of die orifice,
  DB is the diameter of bore of extruder barrel, ?
  is the lead-in angle, and P is the die land.
• Because of the screen pack and breaker plate
  assembly, the pressure in the extruder (PE) is
  reduced by the pressure loss across the assembly
  (PL).
• Since the die outlet is at atmospheric pressure,
  the working pressure is the die pressure (PD)
  given by the difference PD PE PL.

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HOLLOW SECTIONS
Hollow products like pipe or tubes are produced
using the die design shown in Fig.

• The outer diameter of tube is determined by the
  diameter of the outer die ring orifice.
• The inner diameter is determined by the mandrel
  diameter
• To make the mandrel and outer die ring orifice
  concentric, centring screws are provided.
• The mandrel is held in position by a spider. In
  the centre of the spider a hole is drilled to
  supply air down the mandrel.
• To provide a smooth glossy extrudate, the die
  head is heated. A cold die may cause blockage of
  the die.

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STRAIGHT THROUGH DIES

• Those dies whose axes are arranged to be in line
  with the direction of supply of melt.
• Spider, Mandrel is needed for tubes
• Used for the extrusion of pipe, rod, profiles and
  sheet
• CROSSHEAD DIES
• Arranged with their axes at an angle of 908 (458
  and 308 are also used) to the melt feed.
• No need for spider assembly.
• Used for the production of insulated wires,
  cables
• OFFSET DIES
• Combination of both straight through die and
  off-set die.
• Used for the production of pipe.

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• EXTRUDATE SWELL
• Extrudate is contraction in the direction of
  extrusion and expansion in the cross-section
  while emerging from the die is called Extrudate
  Swell.
• The phenomenon (previously called die swell) is
  illustrated in fig.
• Numerically, the extrudate swell is defined as
  the ratio of the outer extrudate diameter (DE) to
  the other diameter of the die exit (DD), i.e., B
  DE / DD
• 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.

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• Extrudate Swell may be reduced by
• Decreasing the extrusion rate
• Increasing the melt temperature
• Increasing the die land
• Increasing the draw-down ratio.

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• Die entry effect and exit instabilities.

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• 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.

MELT FRACTURE
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• 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.

BAMBOOING at a Die
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EXTRUDER OUTPUT

• The simplest way to understand the operation of
  SSE is to mentally unwind the screw into a long,
  straight channel of decreasing depth.
• Now the barrel is visualized as a flat metal slab
  placed above the screw flights at the distance
  corresponding to the actual gap in the extruder,
  between the barrel and the screw flights.
• In this schematic, the screw rotation inside the
  barrel is equivalent to sliding the metal slab
  over stationery straight channel at an angle
  corresponding to the pitch angle of the screw.
• The movement of the slab engenders three types of
  flow
• Drag flow,
• pressure flow
• leakage flow.

The extruder throuhtput (Q) is given by the sum
of the drag flow, the pressure flow, and the leak
flow, i.e., Q QD - QP - QL Since both QP (the
pressure flow) and QL (the leak flow) will have
opposite signs to QD (the drag flow)
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Drag Flow

• Drag Flow takes place by virtue of adhesion of
  the melt to the slab (barrel wall).
• As shown in fig., the maximum melt velocity is at
  the barrel wall (the same velocity as that of the
  wall), linearly decreasing to zero (screw is
  stationery) across the screw channel depth.
• It is noteworthy that due to sliding of the slab
  at an angle, the polymer drag flow in the
  straight channel is helicoidal.

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Drag Flow
?2 2
QD D2 N h sin? cos? Where

• QD Drag flow (in3/min.)
• D Barrel diameter (in.)
• N Screw Speed (rpm)
• H Channel Depth (in.)
• ? Helix angle (17.8?)

V Pheripheral Speed of Boot Dia of Screw
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Pressure back-flow

• Pressure back-flow arises when a restriction,
  such as a die, valve or breaker plate and screen
  is attached to the end of an extruder, which
  gives rise to a pressure gradient in the channel.
  
• In the imaginary geometry, this is equivalent of
  blocking the end of the straight channel.
• The drag flow generates the maximum pressure at
  this end.
• However, if there is a pressure at the channel
  end and only atmospheric pressure at its entrance
  one must have a back flow through the rectangular
  screw channel.
• For melts with simple rheological properties the
  velocity profile is parabolic, as shown in
  fig.3.14 superposition of the drag and pressure
  flow profiles leads to net flow also shown in
  figure.

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Pressure Back - Flow
QP ? Dh3 ?P sin2 ? 12 ?L

• Where
• QP Pressure flow (in.3/sec)
• D Barrel diameter (in.)
• ?P Increase in Pressure (psi)
• h Channel Depth (in.)
• ? Helix angle (17.8?)
• ? Viscosity (lb sec/in2.)
• L Length metering section (in.)

V Pheripheral Speed of Boot Dia of Screw
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LEAKAGE FLOW

• The imaginary geometry of the pressure flow in
  SSE provides also a simple explanation of the
  leak flow.
• Imagine again the straight channel width a metal
  slab above the screw flights at the over flight
  gap distance.
• If pressure is generated near the channel end,
  the material will not only be pushed along the
  channel (as discussed above), but also across the
  over flight gap ò this is known as the leak glow.
  
• The over flight (a radial clearance between the
  lands and the barrel) is normally small, of the
  order of 0.13mm, thus the flow velocity is much
  smaller than for the pressure flow.

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LEAKAGE FLOW
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QL ? D2?3 tan? ?P SL ?

• Where
• QL Leakage flow (in.3/sec)
• D Screw diameter (in.)
• ?P Pressure drop (psi)
• ? Flight clearance (in.)
• ? Helix angle (17.8?)
• S Flight Width (in.)
• ? Viscosity (lb sec/in2.)
• L Length metering section (in.)

V Pheripheral Speed of Boot Dia of Screw
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Factors Affecting Extruders Output
S. NO FACTORS CHANGE OUTPUT COMMENT
1 MATERIAL
1.1 Shear Viscosity Increases Decreases
1.2 Elongational Viscosity Increases Decreases
1.3 Additives Increases Output can either increase (Lubricating oil) or decrease (Filler) depending on the type of additive.
2 FEED
2.1 Uniformity of Pellets Increases Increases Uninterrupted feeding is ensured
2.2 Sphericity of pellets Increases Increases Easier Feeding
3 SCREW
3.1 Diameter Increases Increases
3.2 Channel Depth Increases Increases
3.3 Helix Angle (upto 30?) Increases Increases
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4 BARREL
4.1 Grooved Increases Increases Grooved barrel in the feed section ensures higher compression
5 SCREEN PACK
5.1 No. and Size Increases Decreases
5.2 Back Pressure Increases Decreases
6 DIE
6.1 Cross Sectional Area Increases Increases
6.2 Land Length Increases Decreases
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• 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.

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Twin Screw Extruders
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• Twin Screw Extruder
• Two screws rotating inside a barrel.
• Intermeshing type are more popular.
• Different models/design available

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• 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.

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TWIN SCREW EXTRUDERS SHOWING THE TWO SCREW
ARRANGEMENTS
Co - Rotating
Counter - Rotating
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Comparison between Co-rotating and Counter
rotating
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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
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Blown Film Extrusion
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• 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

• In Blown Film Extrusion a tube of plastic
  material is extruded out of the die, while hot it
  is blown into a bubble, then cooled.
• The bubble is inflated by air pressure contained
  in between the die and the seating provided by
  the nip rolls.
• The bubble is flattened by a pair of collapsible
  frames before it passed through the nip rollers.

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• The film blowing operation can be accomplished
  theoretically in any of the following conditions
• Horizontal
• Vertically Upward
• Vertically Downward

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• Choice of any one or the other of the three
  methods is dictated by the plastic material and
  process limitations.
• The horizontal direction is very rarely used.
• The vertical upward blowing is preferred, e.g.
  for PE and PVC.
• The vertical downward blowing is used for the
  manufacture of high clarity PP film.
• This process requires water quenching of the
  bubble for fast cooling which is rendered
  convenient by this position.

BLOWN FILM TERMINOLOGY
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BLOWN FILM DIES

• Advantages
• Low initial Cost
• Adjustable die opening
• Will handle low flow materials
• Disadvantages
• Mandrel deflects with extrusion rate,
  necessitating die adjustment
• Die opening changes with pressure
• Non-uniform melt flow
• Cannot be rotated
• One weld line in film.

SIDE FEED DIE

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CENTRE FEED DIE
Advantages 1.Positive die opening 2.Can be
rotated 3.Will handle low flow resins Disadvantage
s 1.High initial cost 2.Very hard to clean 3.Two
or more weld lines in film
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SPIRAL FLOW DIE

• Advantages
• 1.No weld line in film
• 2. Positive die opening
• 3. Easy to clean
• 4. Can be rotated
• 5. Improved Film Optics
• Disadvantages
• High head pressure
• Will not handle low flow resins without
  modification

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DEFINITION OF TERMS

• Blow Ratio is the ratio between maximum diameter
  of the bubble (DBMmax) to that of die diameter
  (DD)
• BR DBMmax / DD
• Blow ratio indicates the maximum amount of
  stretching in the crosswise direction for a
  particular material. For Polyethylene BR 2 1.

• Lay Flat Width is the width of the flattened
  lay flat tubing
• LFW ( ? DD / 2 ) BR

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• Draw Down Ratio is a measure of the extent of
  thinning of the web without rupturing it. It is
  defined as a ratio of the die orifice gap to the
  film thickness at the nominal blow ratio BR 1,
  given by the product of the measured film
  thickness and blow ratio
• Draw Down Ratio
• Freeze Line Height is the height from the die
  face at which the melt freezes. The freeze line
  height affects the optical property of the film
  since molecular relaxation takes place at freeze
  point, thus
• Higher the freeze line height, poor will be the
  optical properties.
• Lower the freeze line height Brittleness of the
  film increases.

Die Gap

Film Thickness X Blow Ratio
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EXTRUSION BLOWN FILM

• Process Variables 
•         Melt Temperature 
•         Back pressure 
•         Internal Air pressure 
•         Efficient cooling 
•         Single lip/Dual lip cooling ring 
•         Take off speed 
•         Blow Up Ratio (BUR) 
• Typical Converter film 21
• Shrink film 41 
• Commercially viable combination
•   1.    Low LFW and Low Thickness
• 2.    High LFW and High Thickness

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BLOWN FILM EXTRUSION

•  
• MELT STRENGTH RELATED EFFECTS
• Higher melt strength Stiffer film allows more
  air to be blown against the bubble without
  causing bubble instability. Hence higher output.
• Recycled materials low melt strength. Hence low
  speed of operation and low output.
• Reduction in melt temperature will increase
  output. With Internal Bubble Cooling (IBC) the
  rate can increase 25-50.
• LDPE/LLDPE blends highest rates can occur when
  BUR is about 2.2 to 2.8. Low BUR not much
  surface area to cool hence low output rate.

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Cast Film line
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FLAT FILM - EXTRUSION
Chill-roll system for Flat-Film Extrusion Line
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.
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FLAT FILM DIES

• Basically two types of dies.
• T -type
• Coat Hanger / Fish Tail Type

Coat hanger dies ensures no stagnation of melt.
Hence preferred for Heat sensitive material like
PVC.
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COMPARISON BETWEEN BLOWN FILM CAST FILM

• BLOWN FILM
• Tougher than Cast Film
• More Stiffer
• Cheaper
• A High output Tubular film requires high tower
  Bigger building to accommodate
• Easily changeable film width by changing air
  pressure
• Less gloss clarity

• CAST FILM
• Less Toughness
• Less Stiffer
• More Costlier Process
• Requires less space.
• Not so easy
• Excellent gloss clarity

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TUBULAR-QUENCH

• It involves the downward extrusion of a tubular
  extrudate from an Annular die
• Followed by Quenching on watercovered converging
  boards
• Which causes rapid crystallization which enhances
  the optical property.
• The tube is inflated with air to give a film of
  required lay-flat width and thickness.
• It is widely recommended for PP film.

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FigTubular Quench Process

• Tabular quench film process For PP

Cooling water
Collapsing Board
Water level
Nip Roll
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ORIENTATION OF FILMS

• Orientation of film by stretching it under heat
  is widely applied to films such as PP,PS, PA and
  PET to improve clarity, impact strength, and
  (particularly of PP) barrier properties.
• Basic PS film in its non-oriented form is brittle
  and has only a limited use as a dielectric in
  capacitors. When biaxially oriented, the film is
  tough and can be thermoformed into crystal clear
  tubs, trays and larger items such as cake covers.
• The largest application of orientation
  techniques, however, is in the manufacure of PP
  films and the various processes will be
  illustrated mainly with respect to this film.
• The main processes can be divided into linear
  and tubular.
• The principle of the linear type can be
  illustrated by considering the two-stage process
  shown in fig 3.27 (a) and (b).

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(a) Sequential Orientation Process using a Stemer
forward draw first.
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(b) Sequential Orientation Process using a
Stenter Sideways draw first.
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MULTI-LAYER FILM

• The process is carried out by Co-extrusion.
• It involves the extrusion of two or more layers
  of different or similar materials using Two or
  more extruders and Input combining adapter.

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POLYMER LAYER COMBINATON
CARRIER LAYER usually, LDPE, HDPE, LLDPE, PC,
PET, EVA etc BENDING LAYER Or TIE
LAYER These materials adhere to different types
of Polymers. Ex Ionomer Good adhesion to
LDPE, PA, EVA and LLDPE EVA Good adhesion to
LD, LLDPE, PA, PC, PET. Barrier Layer PA,
PET, PVDC(Best), EVOH
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Film Blowing, Mono- and Multi-layer and Double
Bubble Film Blowing

3-layer 3 extruders
3-layer 2 extruders
5-layer 3 extruders
5-layer 4 extruders
7-layer 4 extruders
7-layer 5 extruders
FILM BLOWING
There are two techniques of making film. One is
the cast film process. The other is the blown
film process. The difference in qualities is that
the blown film is more christalline than the cast
film
AXON manufacture machinery for both processes
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• Blown Film Co-Extrusion
• Barrier properties is the main reason to go for
  multi layer film

Five layer Co-Extrusion
Three layer Co-Extrusion
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MULTILAYER BLOWN FILM
Please check below a number of layer-designs
from 2 layer up to 7 layers. The glue-layers are
of great importance where two different polymers
are not compatible
3-Layer head for 2 extruders
-Layer head for 3 Extruders
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Multilayer Film Dies

• Two types of dies
• 1.Feed Block 2.Multimanifold
• In Feed Block the melt streams are brought
  together and flow out the die
• In Multimanifold the melt spreads independently
  and meet at the die exit
• Two types of process
• Oscillating Platform
• Oscillating Haul-Off

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MULTI LAYER FLAT FILM DIES
84
Three layer Blown film Die with internal bubble
cooling
85
5- Layer Blown film Die with Radial melt
distributor
86
COMPARISON BETWEEN FEED BLOCK AND MULTICHANNEL DIE SYSTEM COMPARISON BETWEEN FEED BLOCK AND MULTICHANNEL DIE SYSTEM COMPARISON BETWEEN FEED BLOCK AND MULTICHANNEL DIE SYSTEM
SELECTION CRITERIA FEED BLOCK MULTICHANNEL DIE
Investment cost Relative by low High.depending on number of layers
Number of layers  Nearly unlimited upto 9 layers possible Limited usually 2 or 3 layers
Handling Relatively easy no regulation of individual layers More expensive because individual layers have to be regulated
Thickness variation on individual layer 10 5
Permissible viscosity difference in components 12 to 13 Larger than 13
Flexibility Better easy variation number and position of layers by exchange of ports Low, number of layers are pre set.  
87
FigOscillating Platform
88
FigOscillating Haul-Off
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Advantages of Multi-layer Film

• Two to seven layers depending on the application.
• It possesses good barrier properties against gas
  and moisture.
• High tensile,impact and tear strength.
• Good stiffness,optical,carrier and printing
  properties.
• e.g. LDPE/HDPE/LDPE,LLDPE/LDPE..etc.

90
Sheet Extrusion
Sheet is produced by forcing molten thermoplastic
through a long horizontal slit die. The extruded
hot web passes around metal cooling rolls and is
then cut up or rolled up.
Material used HIPS is the most important sheet
material. HDPE, PVC, ABS are also used. Sheet
grades usually have high melt viscosity.
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Process Line

• The sheet leaving from the slit die is picked up
  by vertical stack of three rolls.
• The polishing rolls are usually chrome-plated and
  provided with temperature control by circulating
  oil. The polishing rolls imparts a good surface
  to the product without warpage. The temperature
  of the top rolls should be as high as possible
  without sticking, while the bottom roll should be
  just cool enough to prevent distortion.
• From the polishing rolls the sheet passes along a
  conveyor, which consist of free running rollers.
• The sheet is pulled by the pulling rolls are
  covered with elastomer. Their speed is adjusted
  to be slightly less than that of the polishing
  rolls to allow for shrinkage that takes place as
  the sheet cools.
• The sheet is cut into desired dimensions by means
  of razor blades (thin sheets), shear cutting
  device (Standard sheets), or circular saws (thick
  sheets).

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Sheet Extrusion Line
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SHEET EXTRUSION ON 3-ROLLER CALANDERS
94
Dies
Dies used in sheet extrusion are similar to that
of flat film dies. Various cross-section of a
flat sheeting die is shown in fig
Various Cross-Sections of Flat Sheeting Dies
Circular, Tear drop, Angular and Flat Teardrop
95

• 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

Sketch of Paper Coating for Extrusion Process
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• Dies
• Coat hanger die
• T type die

Manifold T-die (a) Die Body, (b) Manifold,
(c) Adjustable Lip and (d) Clamping Screw
Coat-Hanger Die (a) Die Body, (b) Manifold, (c)
Fixed Jaw, (d) Movable Jaw, (e) Choker Bar,
(f) Clamping Screw and (g) Jaw Adjusting Screw
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• 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.

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Wire Coating Extrusion Line
Diagram of a production line for the coating of
wire or cable with plastic. The conductor to be
covered unwinds at the left, is preheated, passes
into the crosshead die (center). The extruder is
behind the die, and feeds it with molten plastic,
which coats the conductor. The finished product
is cooled, tested and wound up at the right.
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• Die used
• Tubing die

USED MAINLY FOR PRIMARY INSULATION

• Pressure die

USED MAINLY FOR SECONDARY INSULATION
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Dies
T-TYPE DIE
COAT HANGER TYPE DIE ( Widely Used)
Coat Hanger type of Die is much more stream lined
than T-type Die
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• 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

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• DIES USED
• STRAIGHT THROUGH

OFF SET DIE
103

• 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
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• 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.

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• 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

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Orientation Systems
A
C
Liquid-bath method
Heated Point method
D
B
Air - Oven method
Cold Drawing
107

• 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.

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CORRUGATED PIPES

• The characteristics of corrugated plastic pipes
  depend on profile and material.
• Corrugated pipes have either parallel ring
  grooves or a continuous helical groove.
• The pipes design can be single walled or twin
  walled.
• Most common thermoplastics are PVC, PE,PP, PA and
  fluoropolymers.
• The most important advantages are
• Considerable raw material savings
• High pressure resistance with good flexibility.
• High impact strength
• Good hydraulic characteristics

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Process

• Essentially IIIr to extrusion of pipes except
  that the die and the calibration units are
  specially designed to produce corrugation on the
  pipes.
• The cylindrical part of the pipe die head extends
  into 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, it is
  cooled by contact with the mould blocks, and by
  that time it reaches the end of the chain, the
  tube must be sufficiently cooled to leave the
  rotating mould blocks in a stable form.

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a--Pipe die head with Insert b--Compressed air
inlet
c-- Shaping die d -- Sealing stopper
e -- Vacuum connection
111
In order to let the pressure or vacuum sizing
become effective, the molten tube must be brought
over a special extended outer die ring as close
as possible to the moulding chain inlet.
Otherwise the tube would be blown off in pressure
sizing and fail in vacuum sizing.The extended
position of the outer die ring cannot be heated
separately. So that it must be made of a material
of high thermal conductivity.
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Double walled corrugated pipe production

• The manufacturing process is the same as with
  standard corrugated pipes, but when the first
  tube has been formed, the second tube is laid
  smoothly on the inner surface of the still
  plastic corrugation and welded to the first with
  the aid of a sizing mandrel.

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• Double Walled Corrugated Pipe Production
• -- Die for coextrusion
• --Through flow guide
• --Mandrel extrusion for inner layer
• -- Shaping die

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Applications of Corrugated pipes are

• Conduits for cable protection, TV,Telephone,
  Glass fibre, power, control and computer lines,
  automobiles, machines and planes, protective
  pipes and conveyor pipes.
• Drain pipes for fields, streets, squares and
  houses.
• Pipes for vacuum cleaners, washing machines, dish
  washers, medical application drip irrigation,
  hoses for fields, hot houses and plantations.
• Protection pipes for district heating, domestic
  connection lines, structural and civil
  engineering.
• Large size pipes for sewage, waste water, control
  shafts and conveyor pipes.
• Corrugated pipes are produced with diameters
  form 3\5 to 2000mm.

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POST EXTRUSION FORMING
Inline postforming with extruder Embossing one
or both sides with shallow or deep patterns
116
INLINE FIXED / ROTATING RINGS USED TO TWIST
EXTRUDATE
117
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, SPRINGS, ETC., USING EXTRUDED ROUND,
SQUARE, HEXAGONAL, AND OTHER SHAPES
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TROUBLE SHOOTING
PROBLEM PROBLEM CAUSES (S) SOLUTION(S)
General Considerations General Considerations General Considerations General Considerations
Surging Resin bridging in hopper Resin bridging in hopper Eliminate bridging
Surging Incorrect melt temperature Incorrect melt temperature Correct melt temperature
Surging Improper screw design Improper screw design Check design
Surging Rear barrel temperature too low or too high Rear barrel temperature too low or too high Increase or decrease rear temperature
Surging Low back pressure Low back pressure Increase screen pack
Surging Improper metering length Improper metering length Use proper screw design
Gels (Contaminants that look like small specks or bubbles) Melt temperature too high Melt temperature too high Lower melt temperature
Gels (Contaminants that look like small specks or bubbles) Not enough progression in screw Not enough progression in screw use new screw
Gels (Contaminants that look like small specks or bubbles) Bad resin Bad resin Check resin quality
Melt fracture (Rough surface finish) Melt temperature too low Melt temperature too low Increase melt temperature
Melt fracture (Rough surface finish) Die gaps too narrow Die gaps too narrow Heat die lips
Melt fracture (Rough surface finish) Increase die gaps
Melt fracture (Rough surface finish) Use processing aids
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Bad colour Colour concentrate incompatible with resin Ensure melt index of concentrate base
Bubbles Wet material Dry thoroughly
Bubbles Overheating Decrease temperature check thermocouples
Bubbles Shallow metering section Use proper compression-ratio screw
Overheating Improper screw design Use lower-compression screw
Overheating Restriction to flow Check die for restrictions
Overheating Barrel temperature too low Increase temperature
Die lines Scratched die Refinish die surface
Die lines Contamination Clean head and die
Die lines Cold polymer Check for dead spots in head
Die lines adjust barrel and head temperature to prevent freezing
Flow lines Overheated material Decrease temperature
Flow lines Poor mixing Use correct screw design
Flow lines Contamination Clean system
Flow lines Improper temperature profile Adjust profile
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Blown Film Blown Film Blown Film
Wrinkles Dirty collapsing frame Clean frame
Wrinkles Too much web tension Adjust tension
Wrinkles Improperly designed air ring Use new air ring
Wrinkles Gauge variations See gauge variations
Wrinkles Insufficient cooling Use refrigerated air
Wrinkles Increase flow of Air
Wrinkles Reduces output
Wrinkles Misalignment between nip rolls and die Check alignment
Fold, creases Excessive stretching between nip and roller Reduce winding speed
Fold, creases Nip assembly drive not constant Adjust or replace drive
Blocking Inadequate cooling Use better cooling method
Blocking Excessive winding tension Adjust tension
Blocking Excessive pressure on nip rolls Adjust pressure
Blocking Bad resin Check resin
Port lines Melt temperature too low Increase melt temperature
Port lines Die too cold or too hot in relation to melt temperature Adjust die temperature
Splitting Excessive orientation in machine direction Increase Blow-up ratio
Splitting Degraded resin Reduce melt temperature
Splitting Poor resin choice Ensure resin is suitable
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Die lines Nick on die lip Chande die
Die lines Dirty die Clean die
Die lines Inadequate purging Increse purging time between resin changes
Gauge variations (machine direction) Surging Check temperature
Gauge variations (machine direction) Surging Check hopper for bridging
Gauge variations (machine direction) Inconsistent take-up speed Check take-up speeds
Gauge variations (transverse direction) Non-uniform die gap Adjust gap
Gauge variations (transverse direction) Non-uniform die gap Centre air ring on gap
Printing problems Insufficient treatment Use properly treated film
Printing problems Additives interfering with ink Use resins with no interfering additives
Printing problems Erratic treatment Reduce slip levels to about 600 ppm for water-based inks
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Sheet Sheet Sheet
Poor gauge uniformity Melt flow is not stable Use gear pump to stabilise flow
Viscosity not stable Poor mixing Use static mixer
Streaks Contaminated System Clean hopper
Streaks Contaminated System Check screw and die
Streaks Contaminated System Clean if necessary
Total discoloration Excessive regrind Check amount of regrind used
Discontinuous lines Too much moisture Increase resin drying
Use hot regrind
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Pipe and Tubing Pipe and Tubing Pipe and Tubing
Poor output Improper die or screw design Ensure die and screw are designed for desired output
ID blisters Insufficient vacuum Increase vacuum
ID blisters Excessive moisture Maintain normal percentage of moisture in compound.
ID blisters Gases entrapped Reduce temperature
ID blisters Water inside pipe Stop water access
ID burn streaks Mandrel heat too high Check mandrel heat
ID burn streaks Stock temperature too high Reduce temperature slowly
ID grooves Mandrel is coated with material Clean mandrel
ID wavy surface Screw clearance set improperly Adjust clearance
ID wavy surface Puller drive slipping Adjust or replace puller drive
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OD burn streaks Material hung up on die Clean die
OD burn streaks Temperature too high Reduce temperatures slowly
OD uneven circumference Too much air pressure on puller Reduce air pressure
OD uneven circumference Insufficient air pressure Check air pressure and all connections
OD discoloured Stabiliser level too low Check stabiliser level
OD pock marks Air bubbles adhering to pipe in flotation tank Install wiper in tank
OD pock marks Improper adjustment of spray rings that surround water tank. Readjust spray rings
OD oversized Air supply too high Adjust air supply
OD oversized Insufficient water supply Increase water supply
OD oversized Pipe hot when measured Allow pipe to cool before measuring
Wall too thick Mis-adjusted die bushing Adjust die bushing to achieve uniform thickness
Wall too thick Wrong die set-up Use correct set-up
125
QUESTIONS

• Which type of die is preferred for sheet
  extrusion.
• What is TDO and MDO?
• What is TQ PP?
• Tubular quench film process is used for
• LDPE b. NYLON c. LLDPE d. PP
• In extrusion process the function of screen pack
  is
• To filter contamination. b. Arrest unmelted
  particles.
• c. Developing backpressure. d. All the above.
• 6. The relationship between MFI and viscosity is
  
• Directly proportional b. Equal
• c. Inversely proportional d. None of the above
• 7. State the effect of grooved barrel on output
  of an extruder
• 8. What is barrel?
• 9. Mention any one material for making corrosion
  resistance barrel.
• 10. What is helix angle?

126

• State the helix angle for PVC screw.
• Name the blown film dies.
• How the thrust bearing is rated?
• State true or false
• Co-rotating twin-screw extruder is preferred for
  PVC compounding
• What is the other name for two stage screw?
• What is the other name for die swell?
• PP blown film is produced by _____________
  process (mention specific name.)
• State the compression ratio for nylon screw.
• The effect of backpressure in extrusion is to
• a. Improve mixing b. Reduce mixing
• c. Increase Viscosity d. None of the
  above
• Classify extruders.
• Name the different extruder screws.
• State any two merits of Twin-screw extruder
• Name the different co extrusion dies
• State the different types of Twin screw extruder.
• Why PP blown films are always produced by
  downward extrusion process.

127

• The cut of an extruder ________ (increase /
  decrease) with increase in Back Pressure.
• How an extruder is specified?
• Name the coextrusion blown film dies.
• Define LD ratio.
• Define Compression ratio.
• The mixing elements are incorporated in
• a. Feed zone b. Compression Zone
• c. Metering Zone d. None of these
• What is melt fracture?
• What is shark skin?
• What is Bambooing?
• Define Blow Ratio?
• Define Blow Up Ratio?
• What do you mean by FLH?
• Name the different wire coating dies?
• State the Compression Ratio for Rigid PVC Screw.
• State the Compression Ratio for Nylon Screw.
• Nominal dia of screw Root dia 2
• What is Barrier screw ?

128

1. State the purpose of hopper cooling.
2. State the purpose of screw cooling.
3. State the functions of breaker plate.
4. State the functions of breaker plate.
5. State the equations to find out output of an
   extruder.
6. What is draw down ratio?
7. State any two merits of blown film over cast
   film.
8. State the various orientations systems used in
   monofilament extrusion.
9. What is fish eye? Suggest remedies.
10. Name any two applications for corrugated pipes.

129
REFERENCE

1. Extrusion of Plastics Fisher
2. Extrusion of Plastics Allan Griffth.
3. Plastics Extrusion Technology Friedhelm Hensen
4. Plastic Materials and Processing A. Brent
   Strong
5. Tools and Manufacturing Engineers Handbook (T
   meh)
6. Polymer Processing D H Morton Jones
7. Plastics Processing Data Handbook (Second
   Edition) Dominick Rosato
8. Plastics Technology Handbook Manaschanda, Salil
   K. Roy
9. Principles of Plastics Extrusion - Brydson and
   Peacock
10. Handbook of Plastics Materials and Technology
    Irvin Rubin.

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