cast versus blown film

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Page 1: Cast versus blown film

Cast versus blown film

Ted Brink

polyamide-based

Page 2: Cast versus blown film

Page 2 cast versus blown film

Contents

• film requirements• polyamide characteristics• blown versus cast film technology• conclusions

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Page 3

Main applications PA-based films

cast versus blown film

Page 4: 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

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Main reasons to use polyamide

O2

mechanical strength barrier propertiescast versus blown film

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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

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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. Die3. Casting station4. Winder5. Automation system

Cast film line

cast versus blown film

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Cast film basics

cast versus blown film

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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

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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

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blown film

cast versus blown film

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Page 15 cast versus blown film

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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

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External bubble coolingcooling 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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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

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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

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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

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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

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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

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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

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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

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

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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

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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)

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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 %

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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

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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

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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

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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

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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

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More information and contact

Ted BrinkEmail: [email protected]: www.extrusionist.comTel.: +31 651109899Skype: ted.brink

cast versus blown film