Cast versus blown film

Ted Brink

polyamide-based

cast versus blown film

Contents

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

Main applications PA-based films

Food packaging - film requirements

• oxygen barrier• moisture barrier• grease and fat resistant• shrink performance• puncture resistance• sealability• printability• transparency

Main reasons to use polyamide

mechanical strength barrier propertiescast 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

film production

In spite of significant differences, both technologies are used for similar applications

blown cast

cast film

1. Plasticising unit 2. Die3. Casting station4. Winder5. Automation system

Cast film line

Cast film basics

Chill roll cooling

• efficient cooling- temperature controlled water or oil

• chill roll surface structure - high gloss or embossed- influences quenching rate and film surface

Chill roll temperature

• 20 – 40 °C for optimum thermoforming• > 80 °C for optimum dimensional stability• uniform temperature gradient across the roll

- prevent morphological differences

blown film

Bubble cooling

• cooling medium: air• cooling technologies

- external bubble cooling- internal bubble cooling

• cooling influences:- output- film morphology

ambient air orchilled air

External bubble coolingcooling from the outside• commonly ambient air

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 ↔ blown

Blown versus cast film – polymer viscosity

• blown film:- requires melt strength → high viscosity

• cast film:- less critical → medium to even low viscosity

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

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

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

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

Blown versus cast film – thermoforming

• cast films perform better than blown films- lower crystallinity → easier drawing at lower stress

film morphology determines thermoforming

Blown versus cast film – curling

• curling may occur in nonsymmetrical films• nonsymmetrical films have different polymers• different polymers have different crystallization rate

layer A and B liquid

layer B crystallizeslayer A follows

layer A crystallizeslayer B cannot follow

cause for curling

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

Blown versus cast film – process flexibility

• cast film:- die has fixed width- neck-inside trim

• blown film:- adjustable bubble size

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)

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 %

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

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

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 – 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

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

More information and contact

Ted BrinkEmail: ted.brink@extrusionist.comInternet: www.extrusionist.comTel.: +31 651109899Skype: ted.brink

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