Main differences between cast and blown film with respect to process and film characteristics

1. Cast versus blown film
Ted Brink
polyamide-based

2. Page  2
Contents
• film requirements
• polyamide characteristics
• blown versus cast film technology
• conclusions
cast versus blown film

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Main applications PA-based films
cast versus blown film

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Food packaging - film requirements
• oxygen barrier
• moisture barrier
• grease and fat resistant
• shrink performance
• puncture resistance
• sealability
• printability
• transparency
cast versus blown film

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Main reasons to use polyamide
O2
mechanical strength barrier properties
cast versus blown film

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Polyamide properties
PA-type
Melting point
[°C]
H2O permeability
[cc/m2/atm/day]
O2 permeability
[cc/m2/atm/day]
PA66 255 8 12
PA6 220 15 12
PA6.12 215 5 45
PA6.66 195 16 14
PA11 190 4 120
PA12 180 5 190
cast versus blown film

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film production
cast versus blown film

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In spite of significant differences, both
technologies are used for similar applications
blown cast
cast versus blown film

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cast film
cast versus blown film

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1. Plasticising unit
2. Die
3. Casting station
4. Winder
5. Automation system
Cast film line
cast versus blown film

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Cast film basics
cast versus blown film

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Chill roll cooling
• efficient cooling
- temperature controlled water or oil
• chill roll surface structure
- high gloss or embossed
- influences quenching rate and film surface
cast versus blown film

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Chill roll temperature
• 20 – 40 °C for optimum thermoforming
• > 80 °C for optimum dimensional stability
• uniform temperature gradient across the roll
- prevent morphological differences
cast versus blown film

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blown film
cast versus blown film

15. Page  15 cast versus blown film

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Bubble cooling
• cooling medium: air
• cooling technologies
- external bubble cooling
- internal bubble cooling
• cooling influences:
- output
- film morphology
ambient air or
chilled air
cast versus blown film

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External bubble cooling
cooling from the outside
• commonly ambient air
cast versus blown film

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Internal bubble cooling (IBC)
IBC cools the bubble from the inside
• cool air injected (5 – 15 °C)
• warm air removed (± 75 °C)
• increased output
cast versus blown film

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cast ↔ blown
cast versus blown film

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Blown versus cast film – polymer viscosity
• blown film:
- requires melt strength → high viscosity
• cast film:
- less critical → medium to even low viscosity
cast versus blown film

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Blown versus cast film – cooling efficiency
• cooling medium blown film: air
- air not very efficient cooling medium
- outer bubble cooling
- inner bubble cooling
• cooling medium cast film: chill roll
- cooling by water or oil
- chill roll temperature between 25 and 125 °C
cooling efficiency
determines output
cast versus blown film

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Blown versus cast film – film morphology
• cooling rate determines morphology
- slow cooling → large crystals
- fast cooling → smaller crystals (crystals frozen-in)
- high quenching rate → film remains (almost) amorphous
cast versus blown film

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Blown versus cast film – film transparency
• transparency related to morphology
- slow cooling → large crystals → more haze
- fast cooling → crystals less time to grow → higher transparency
- very fast cooling → low crystallinity → highest transparency
films with low crystallinity
may show postcrystallization
cast versus blown film

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Blown versus cast film – film stiffness
• stiffness related to morphology
- slow cooling → higher crystallinity → higher stiffness
- fast cooling → low crystallinity → lower stiffness
films with low crystallinity
may show postcrystallization
cast versus blown film

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Blown versus cast film – thermoforming
• cast films perform better than blown films
- lower crystallinity → easier drawing at lower stress
film morphology
determines thermoforming
cast versus blown film

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Blown versus cast film – curling
• curling may occur in nonsymmetrical films
• nonsymmetrical films have different polymers
• different polymers have different crystallization rate
A
layer A and B liquid
B
layer B crystallizes
layer A follows
A
B
layer A crystallizes
layer B cannot follow
B
A
cause for curling
cast versus blown film

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Blown versus cast film – orientation
• difference in MD and TD stretching determines orientation
• cast film:
- fixed width
- uniaxial drawdown
• blown film:
- more balanced MD – TD orientation
- tools: BUR and DDR
cast versus blown film

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Blown versus cast film – process flexibility
• cast film:
- die has fixed width
- neck-in
- side trim
• blown film:
- adjustable bubble size
cast versus blown film

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Blown versus cast film – gauge uniformity
• cast film:
- ± 2 % independent on film thickness
• blown film:
- ± 10 % for thin films (< 20 µm)
- ± 5 % for thicker films (> 20 µm)
cast versus blown film

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Blown versus cast film – waste
• cast film:
- start-up ad shut-down waste
- change-over waste
- waste due to side trims
• blown film:
- start-up ad shut-down waste
- change-over waste
waste:
• cast film: 5 – 8 %
• blown film: < 5 %
cast versus blown film

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Blown versus cast film – costs
• cast film:
- floor space needed
- higher investment costs
• blown film:
- height needed
- lower investment cost
cast film requires higher
investment than blown film
cast versus blown film

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Blown versus cast film – PA6 versus PA6.66
• cast film:
- only PA6 is used
• blown film:
- PA6.66 used more than PA6
- PA6.66 more transparent than PA6
- PA6.66 shows less curling than PA6
- PA6 blended with PA6I/6T
cast film: PA6
blown film: PA6.66
cast versus blown film

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Cast versus blown film - overview
Polymer related:
• required viscosity
• morphology
• transparency
• stiffness
• thermoform performance
• curling
Machine related:
• cooling efficiency
• process flexibility
• gauge uniformity
• orientation
• waste
• costs
some grey areas
cast versus blown film

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Cast versus blown film – overview contd
Property Cast Blown
Cooling efficiency +++ +
Viscosity required Medium/low High
Transparency +++ +
Thermoforming performance +++ +
Curling in non-symmetrical films Less More
Film orientation More Less
Trim/scrap More Less
Gauge variation Good Medium
cast versus blown film

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Cast versus blown film - summary
• process:
- cast film: higher output
- cast film: better gauge control
- blown film: less floor space and investment
• films:
- morphology difference
 film crystallinity
 crystal size
- optical properties
- thermoforming
due to cooling rate
cast versus blown film

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More information and contact
Ted Brink
Email: ted.brink@extrusionist.com
Internet: www.extrusionist.com
Tel.: +31 651109899
Skype: ted.brink
cast versus blown film