This document compares cast and blown film production technologies for polyamide films. Cast film uses a chill roll for efficient cooling, resulting in faster cooling rates that produce films with lower crystallinity, higher transparency, and better thermoforming performance compared to blown film. Blown film uses air cooling, which is less efficient than the chill roll, leading to slower cooling rates and more crystallinity. However, blown film requires less floor space and investment. Both technologies can produce similar applications, but cast film is generally better for properties requiring fast cooling while blown film has lower costs.

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