12. Page 12
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
13. Page 13
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
20. Page 20
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
21. Page 21
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
22. Page 22
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
23. Page 23
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
24. Page 24
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
25. Page 25
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
26. Page 26
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
27. Page 27
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
28. Page 28
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
29. Page 29
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
30. Page 30
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
31. Page 31
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
32. Page 32
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
33. Page 33
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
34. Page 34
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
35. Page 35
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
36. Page 36
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