Processing Techniques

For
Plastic Films
(Part 1)

ABHAY MULAY
Product Application & Research Centre
Reliance Industries Limited
Mumbai
IPI,19.09.03
Index

¾ Flexible packaging Overview

¾ Blown film extrusion
∗ Upward blown
∗ Downward blown
¾ Cast Film extrusion
∗ Uniaxial orientation
∗ Biaxial orientation
¾ Calendering
Flexible Packaging Overview
Flexible packaging : Overview

¾ Flexible packaging is the largest sector in plastics

¾ One of the fastest growing sectors, CARG ~ 20 %

per capita consumption of plastics in packaging

2001-02 2005-06
2.2 kg 3.7 kg

¾ A growing demand due to:

∗ Growing population
∗ Urbanization & increasing “middle income” group
∗ Booming rural marketing
∗ Better convenience, need for small size packs
∗ Increase in “BRAND” awareness
∗ Entry of MNC’s
Flexible Packaging Sector : overview
Total Film Market (02-03) ~1. 2 MMT

10%

20%
45%

25%

LD/LLDPE PP HD/HMHDPE PVC

PE films: Largest share in flexible packaging

Polypropylene film : overview

PP Film Market (02-03) ~ 300KT

77% 3%

21%

TQPP CPP BOPP

...largest contribution from TQPP films

Processing Techniques
Processing Techniques - Film

¾ The most commonly used processing techniques for

the flexible packaging films are
− Blown film extrusion
∗ Upward blown
∗ Downward blown
− Cast Film extrusion
∗ Uniaxial orientation
∗ Biaxial orientation
− Calendering
Upward Blown Film
Blown Film Extrusion : Upward Blown

¾ Technique predominantly used for PE films

¾ Process :
∗ Resin is plasticized in an extruder & pumped
through the annular die
∗ Molten extrudate in form of tube is inflated by air
into a bubble of desired diameter
∗ Cooling of bubble using air ring
∗ Haul off through collapsing boards & nip rolls
∗ Slitting & winding
Blown Film Extrusion : Upward Blown
Blown Film Extrusion : Upward Blown

Machinery details :
¾ Extruders :
∗ Single screw extruders
∗ L/D 24 : 1 to 28 : 1
∗ Compression ratio : 2.5 to 3:1
¾ Dies
∗ Bottom fed spiral mandrel dies
∗ Single layer / multilayer (co-extrusion)
Blown Film Extrusion : Upward Blown
Machinery details :
¾ Corona treatment unit :
∗ Corona treatment is a high voltage discharge used to
increase surface energy
∗ Treatment is measured using mixture of Ethyl cellosolve &
formamide solution
∗ Surface tension requirements :
− For solvent based lamination : 44 dynes / cm
− For solvent based printing : 40 dynes / cm
− For water based inks & adhesives : 48 – 50 dynes / cm
¾ Winding : Center winding or surface winding
Key Processing variables

¾ Melt temperature :
∗ Higher the melt temperature better is melt
homogenization
∗ Typical melt temperatures
− LDPE : 185 – 195oC
− LLDPE : 205 – 215oC
− HDPE : 215- 225oC
− HMHDPE : 220 – 230oC
¾ Higher melt temperatures also gives better optical
properties & gloss
Key Processing variables

¾ Cooling air temperature :

∗ Lower temperatures ( higher cooling rate) leads to
− Higher crystallization rate
− Smaller crystal size
− Better opticals
− Better balance of properties for LDPE & LLDPE
∗ Internal bubble cooling (IBC) for higher outputs
Key Processing variables

¾ Die gap :
∗ LDPE & HDPE
− Broad MWD, Lower higher shear viscosity
− Narrow die gaps (0.8 – 1.0 mm) for better
properties
∗ LLDPE
− Narrow MWD, lower shear sensitivity
− Higher die pressures leads to ‘shark skin’ effect
− Use of wider die gaps (1.5 – 2.5 mm)
Key Processing variables

¾ Blow up ratio (BUR)

∗ Ratio of bubble diameter / die diameter
∗ To be optimized for balance of properties
∗ LDPE /LLDPE : 2.25 to 2.75
∗ HDPE / HMHDPE : 4 to 5
¾ Neck height :
∗ Lock-in position for branched polymers (LDPE /
LLDPE)
∗ 12 to 15 inch for linear polymers (HDPE/HMHDPE)
¾ Draw Down ratio : Die gap
Film gauge x BUR
Key Processing variables
Applications : PE Blown Films
Downward Blown Film
(TQPP)
Blow Film Extrusion : Downward Blown

¾ Upward blown extrusion is difficult to be adopted for

highly crystalline polymers like PP due to
∗ Slow crystallization rate
∗ Formation of bigger crystallites
∗ Leads to brittle & hazy film
¾ Downward blown extrusion process is conventionally
used for PP. The process is popularly known as
‘Tubular Quench’ Polypropylene (TQPP) process
TQPP Film

¾ Process :
∗ The resin is extruded & the molten material in form of
tube comes out of the circular die
∗ Hot bubble is initially cooled & stabilized by air ring
∗ Tubular film is then passed through water cooling / sizing
ring
∗ Water flows downward through sizing ring keeping
uniform & continuous contact with film
∗ Cooling water temperature depends upon film thickness
& is between 8 – 20oC
∗ Film is subsequently collapsed, passed through drying
unit & finally rolled up
TQPP Process
Performance of TQPP vs. Other films

¾ TQPP process is low capital intensive & economical

compared to Cast PP & BOPP

¾ Compared to BOPP

∗ Tear strength is higher

∗ Lower shrinkage due to low orientation

Performance of TQPP vs. Other films

¾ Compared to PE film
∗ Higher yield
∗ Better clarity
∗ Higher heat resistance
∗ Superior tensile properties
∗ WVTR & OTR better compared to LDPE/LLDPE & inferior
to HDPE
Effect of resin parameters

¾ Melt Flow Index (MFI)

∗ Higher MFI resin has poor melt strength & offers lower
bubble stability
∗ Higher MFI resin gives better opticals, however gives
poor mechanical properties
∗ Low MFI resin leads to difficulty in processing & poor
opticals
∗ Optimum MFI range : 8 – 12 g/10 min
Effect of resin parameters

¾ Low molecular weight fraction (LMWF)

∗ These are usually olegomers & atactic fraction
∗ Determined by % xylene solubles
∗ Higher levels of LMWF leads to die drooling, smoking
∗ Low levels leads to difficulty in processing
∗ Optimum levels of LMWF 2.5 – 3.5%
Effect of resin parameters
¾ Type of resin
∗ Homopolymer & Terpolymer PP grades are
commercially used
∗ Films made from Homo PP grades have
− Higher stiffness
− Higher SIT
∗ Films made from terpolymer PP grades have
− Higher softness
− Superior gloss
− Lower SIT
Effect of processing variables
¾ Desired film properties can be obtained by optimizing
process variables

∗ Tensile strength of the film can be increased by

− Higher cooling rate
− Lower quenching temperatures
− Lower gap between die & cooling ring
Effect of processing variables

¾ Clarity improved by
∗ Lower quench water temperature
∗ Higher flow of quench water
∗ Higher die temperatures

¾ Gloss improved by
∗ Raise the die & barrel temperature
∗ Lower quench water temperature
∗ Higher flow of quench water
Advances in raw materials
Resin for high throughputs
¾ significant improvement in throughputs

30 kg/hr 150 kg/hr

¾ over stretching leads to bubble breakage /punctures

¾ at higher shear rates,

∗ extensional viscosity decreases
∗ melts becomes thinner (stress thinning of PP)

¾ resin has to be designed for higher extensional viscosity

…resin has to be tailor made for high output lines

New grades of REPOL
New grades from RIL

¾ REPOL H 080 EY

¾ REPOL H 080EG
Repol H 080 EY
¾ PP homopolymer, with slip and antiblocking agents
MFI : 8.0 g/10min
¾ The grade is specially designed to provide:
∗ Good processability at high throughputs
∗ Good opticals
∗ Better mechanicals

¾ Typical applications :
packaging of textiles/food products, multi-layer TQPP
films
Repol H 080 EG
¾ PP homopolymer, without slip and antiblocking
agents
MFI : 8.0 g/10min
¾ The grade is designed to provide:
∗ Good processability on high through put lines
∗ Excellent clarity and gloss
∗ Good stiffness

Typical applications :
packaging of textiles / food / other commodities
New applications for TQPP films

• Soft TQ textile bags & Liners

• Processed food packaging films
• Lamination films
• Soft Blister
• Stationery products
• Perforated films
Applications : TQPP films
New Developments

¾ 2 layer co-extrusion TQPP plant developed by M/s. Rajoo

Engg.
¾ Co-extrusion of Homo PP & Random PP
¾ Inner layer of random PP in co-extruded film offers superior
sealing properties & low SIT
¾ Co-extruded film can be used for lamination application
PET/Adhesive/Co-ex PP film
Cast Film Extrusion
(Uniaxial Orientation)
Cast Film Extrusion

¾ Process
∗ Polymer is plasticized & homogenised in the
extruder
∗ Melt is passed through coat hanger die
∗ The extrudate comes out of a die as thin, wide
curtain of the film
∗ Molten film is quenched in water tank or onto a
chilled roll
∗ Finally film is corona treated, slit and rolled
Cast Film Extrusion
Cast Film Extrusion

¾ Advantages of Cast Film over blown film

∗ Higher output
∗ Better transparency, gloss due to quenching
∗ Better stiffness
∗ Better thickness control
∗ Better mechanical properties in MD due to
unidirectional orientation

¾ Advantages of blown film

∗ Lower machinery cost
∗ Width flexibility
∗ Balance of mechanical properties
Cast Film Extrusion

¾ Machinery details :
¾ Single screw extruders
∗ L/D : 28 to 32 : 1
∗ Compression ratio : 3:1 or more
¾ Typical outputs in grooved feed extruders

Screw dia (mm) Output (kg/hr)

75 140 - 160
90 220 - 240
120 350 - 400
150 500 - 550
Cast Film Extrusion

¾ Die : Coat hanger die

∗ uniform pressure drop due to triangular flow path
∗ Uniform material flow, uniform thickness
¾ For multilayer co-extruded film
− Feedblock die
− Multimanifold die
Cast Film Extrusion

¾ Chilled roll take-off

∗ Designed for maximum cooling efficiency
∗ Roll surface : Chrome plated, matt / mirror finished
∗ Roll width : 1200 mm to 3600 mm
∗ Roll diameter : 400 mm to 1200 mm
∗ Partially crystalline polymers : Cooling efficiency
determines crystalline structure & opticals
∗ Lay on aids : Force film against surface of film roll
ex. Air knifes & suction chambers
Cast Film Extrusion : Key variables

¾ Melt temperature :
∗ Increasing melt temperature gives slight reductions in
haze
∗ Caution :
− Increased volatilization of additives in raw material
− Odour issues
− Volatiles condensation on chill roll surface affects
opticals
∗ Optimum melt temperatures
− LLDPE : 215 - 225oC
− PP : 240 – 255oC
Cast Film Extrusion : Key variables

¾ Chill roll temperature :

∗ Increasing chill roll temperature
− Deteriorate film clarity
− Favours dimensional stability
− Increases slip properties
∗ Decreasing roll temperature
− Gives fine crystalline structure & excellent optical
properties
− Due to build in stress, additional shrinkage on
storage
∗ Optimum chill roll temperature : 15 – 25oC
Polymer selection for Cast film

¾ LDPE / LLDPE / Homo PP / Co PP

¾ MFI : 3 – 10 g/10 min
¾ Broad MWD resins gives lower neck-in
¾ Higher the MFI, higher is clarity
¾ Random PP with 1.4 to 3% ethylene gives softer films
with low haze
¾ Random PP also offer low SIT
Applications – Cast Film

¾ Key applications of co-extruded cast film

Structure Special feature Application

LD + LLD / * One side cling Cling film, masking
LD + LLD + HD / * Good clarity film
LD + LLD + PIB
PP / PP filled / * Low shrinkage Fast food trays
PP * Smooth outer layer
RCPP / PP * Sealability Industrial packaging
/RCPP
RCPP / TL / * Good sealability Packaging of meat,
Nylon / TL / PP * Transparency Sausage & Cheese
* Thermoforming property
Applications – Cast Film
Cast Film Extrusion
(Biaxial Orientation)
Cast Film Extrusion : Biaxial
orientation
¾ Process :
∗ Biaxial orientation is a process in which continuous cast
film is heated to bring it to a stretchable temperature
∗ Subsequently it is stretched in machine & then in
transverse direction
∗ Commonly used polymers are PP & PET. The films are
termed as BOPP & BOPET films respectively
BOPP Film Extrusion

¾ Machinery Details :
∗ Extruders : 150 - 200 mm dia screws with L/D 26 to 30 : 1
∗ Typical outputs : 1000 – 2000Kg/hr
∗ Dies : 600 mm to 2000 mm wide, coat hanger type
∗ Casting & Quenching : Molten polymer extruded from the
die is placed against chilled roll or multiple chill rolls
BOPP Film Extrusion
¾ Machinery Details :
∗ Casting & Quenching :
− Popular method is casting onto a chilled roll partially
submerged in water bath
− Chill roll & water temperature : 20 – 25oC
− Chill roll diameter : 1000 – 2000 mm
− Cooled web proceeds for MDO
∗ Orientation
− Biaxial orientation of the film is done by tenter frame
process
BOPP Film Extrusion

¾ Machine direction orientation (MDO)

∗ Cast film is heated by passing over heated rolls (145 –
150oC)
∗ When film reaches necessary stretching temperatures, it
is passed over series of stretching rolls
∗ Stretch ratio is kept between 4 : 1 to 6 : 1
∗ Subsequently film is passed over annealing rolls to
prevent MD shrinkage
BOPP Film Extrusion

¾ Transverse direction orientation (TDO)

∗ From MDO, film is guided onto the chain & is preheated
at 160oC
∗ As film leaves preheat section, chain diverges rapidly
stretching film in ratio of ~ 8 : 1
∗ Film is passed in annealing oven at 155oC to reduce TDO
shrinkage
¾ Winding : Edges of the film are trimmed off. The film is slit
and wound on separate winders.
BOPP Film : Double Bubble process

¾ Equipment is smaller compared to tenter frame process

¾ Difficult to produce thin film
¾ Output in terms of quantity & width is very low
¾ Extruders : 50 to 75 mm
¾ Dies : Circular dies of 150 – 200 mm dia
¾ Quenching :
∗ Extruded tube from the die is directly taken into
quenching bath @ 20 – 25oC, collapsed between nip rolls
∗ Film passes to set of 2nd nip rolls through drying unit
BOPP Film : Double Bubble process
¾ Preheating : Tube in flattened state is heated to 150 – 160oC
using IR heaters
¾ Orientation :
∗ Heated tube is inflated to 5 – 6 times original diameter
(TDO)
∗ With set of nip rollers, film is stretched in machine
direction at ratio of 5 – 6:1
∗ Cooling rings cools the inflated bubble before being
collapsed
BOPP Film : Double Bubble process
Effect of Processing variables

¾ BOPP film properties depends on different processing

variables
¾ Tensile strength & modulus can be improved by
∗ Lowering the temperature of chill roll & quench water
∗ Increasing stretch roll temperature
∗ Increasing the line speed
∗ Increasing the stretch ratio
¾ Lower shrinkage achieved by
∗ Using higher stretching temperatures
∗ Using higher annealing temperature
Effect of Processing variables

¾ Haze can be reduced by

∗ Lowering temperature of chill roll & quench water
∗ Increasing the temperature of stretching & annealing
¾ Gloss can be improved by
∗ Using higher melt temperature
∗ Using chill roll, MD roll & die having clean surfaces
Applications : BOPP Film
Calendering
Calendering

¾ Process of forming film/sheet by passing mass of molten

material through successive pairs of counter rotating rolls
¾ Process is commonly used for PVC, Rubber, & Non-woven
¾ A typical PVC calender consist of 4 to 5 metal rolls arranged
in various configuration
¾ Each roll has control for temperature, speed, roll gap
¾ For processing of PVC, 3 basic roll arrangements are used
inclined Z, ‘L’ & inverted ‘L’
Calendering

Roll Configurations

Calendering line
Calendering : Roll configuration

¾ L type calender can have 4 or 5 rolls of ~ 2 ft. dia & ~ 4 ft.

length

¾ ‘L’ type is commonly used for rigid PVC

¾ Inverted ‘L’ type is commonly used for flexible PVC

¾ Relative separatory forces for ‘Z’ type calender are more

compared to ‘L’ type calender. This leads to higher gauge
variation in transverse direction
PVC Calendering

¾ Process involves following steps :

∗ Blending
∗ Fluxing
∗ Calender feeding
∗ Calendering
∗ Take-Off
PVC Calendering

¾ Blending : Resin, Plasticisers, Modifiers, Fillers,

Stabilizers, etc. are mixed together in ribbon blender.
High intensity mixer gives better distribution compared to
ribbon blenders
¾ Fluxing : The output of the blender, typically well
dispersed, dry free flowing powder is fed into one of the
fluxing equipment like
∗ Banbury / Intermix batch mixers
∗ Farrel continuous mixer
∗ Ko-Kneaders / Twin screw extruders
∗ Planetary extruders ( Rigid PVC)
PVC Calendering
¾ Calendering feeding :
∗ The output for fluxing is fed to either two roll mill or
extruder strainer. Function of extruder strainer / two
roll mill is
− To maintain stock temperature
− To act as a accumulator
− Degassing
− Straining of foreign particles
PVC Calendering

¾ Calendering
∗ Output from two roll mill or extruder strainer is fed to
calender unit by a conveyor system
∗ Compound passes through metal detector system
∗ Conveyor feeds the PVC compound into 1st set of
calender rolls. The sheet is formed as the material
passes through the rolls
∗ Each nip in calender stack reform the sheets & makes it
wider, thinner & refinished
∗ For easy & faster release, last calender roll is kept at
lower temperature & higher speed compared to
preceding roll
PVC Calendering

¾ Calendering take off :

∗ Hot, unsupported sheet from the last calender rolls is
removed using driven rolls called ‘stripper rolls’
∗ Rolls are normally teflon coated to prevent sticking
∗ Subsequently sheet passes over embossing rolls,
chilling rolls & winding station
PVC Resin characteristics

¾ Resin for flexible PVC

∗ Suspension PVC with relative viscosity of 2 to 2.80
(K value 60 to 77)
∗ High MW resins are used when higher physical
properties are required or sheet is subjected to
further heat (eg. Embossing)
∗ High MW resin have better UV resistance
PVC Resin characteristics

¾ Resins for Rigid PVC

∗ Rigid PVC compounds are formulated with lower MW
resins
∗ Relative viscosity 1.85 to 2.0 (k value 55 to 60)
∗ Resin can be homopolymer or copolymer of VC & VA
Applications :PVC Calendering

R A IN W E A R
Thank You !