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Cast versus blown film
Ted Brink
polyamide-based
Page 2 cast versus blown film
Contents
• film requirements• polyamide characteristics• blown versus cast film technology• conclusions
Page 3
Main applications PA-based films
cast versus blown film
Page 4 cast versus blown film
Food packaging - film requirements
• oxygen barrier• moisture barrier• grease and fat resistant• shrink performance• puncture resistance• sealability• printability• transparency
Page 5
Main reasons to use polyamide
O2
mechanical strength barrier propertiescast versus blown film
Page 6 cast versus blown film
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
Page 7
film production
cast versus blown film
Page 8
In spite of significant differences, both technologies are used for similar applications
blown cast
cast versus blown film
Page 9
cast film
cast versus blown film
Page 10
1. Plasticising unit 2. Die3. Casting station4. Winder5. Automation system
Cast film line
cast versus blown film
Page 11
Cast film basics
cast versus blown film
Page 12 cast versus blown film
Chill roll cooling
• efficient cooling- temperature controlled water or oil
• chill roll surface structure - high gloss or embossed- influences quenching rate and film surface
Page 13 cast versus blown film
Chill roll temperature
• 20 – 40 °C for optimum thermoforming• > 80 °C for optimum dimensional stability• uniform temperature gradient across the roll
- prevent morphological differences
Page 14
blown film
cast versus blown film
Page 15 cast versus blown film
Page 16 cast versus blown film
Bubble cooling
• cooling medium: air• cooling technologies
- external bubble cooling- internal bubble cooling
• cooling influences:- output- film morphology
ambient air orchilled air
Page 17
External bubble coolingcooling from the outside• commonly ambient air
cast versus blown film
Page 18
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
Page 19
cast ↔ blown
cast versus blown film
Page 20 cast versus blown film
Blown versus cast film – polymer viscosity
• blown film:- requires melt strength → high viscosity
• cast film:- less critical → medium to even low viscosity
Page 21 cast versus blown film
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 efficiencydetermines output
Page 22 cast versus blown film
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
Page 23 cast versus blown film
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 crystallinitymay show postcrystallization
Page 24 cast versus blown film
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 crystallinitymay show postcrystallization
Page 25 cast versus blown film
Blown versus cast film – thermoforming
• cast films perform better than blown films- lower crystallinity → easier drawing at lower stress
film morphology determines thermoforming
Page 26 cast versus blown film
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 crystallizeslayer A follows
AB
layer A crystallizeslayer B cannot follow
BA
cause for curling
Page 27 cast versus blown film
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
Page 28 cast versus blown film
Blown versus cast film – process flexibility
• cast film:- die has fixed width- neck-in- side trim
• blown film:- adjustable bubble size
Page 29 cast versus blown film
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)
Page 30 cast versus blown film
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 %
Page 31 cast versus blown film
Blown versus cast film – costs
• cast film:- floor space needed- higher investment costs
• blown film:- height needed- lower investment cost
cast film requires higherinvestment than blown film
Page 32 cast versus blown film
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: PA6blown film: PA6.66
Page 33 cast versus blown film
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
Page 34 cast versus blown film
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 MoreFilm orientation More LessTrim/scrap More LessGauge variation Good Medium
Page 35 cast versus blown film
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
Page 36
More information and contact
Ted BrinkEmail: [email protected]: www.extrusionist.comTel.: +31 651109899Skype: ted.brink
cast versus blown film