Blown film & bubble stability
Ted Brink
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WHATdetermines
bubble stability?Bubble stability in blown film
Page 3 Bubble stability in blown film
Overview of discussion topics
• blown film• bubble cooling• die construction• melt viscosity• melt strength• temperature influence
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frost line
below frost line:melt phase
above frost line:solid phase
the bubble is:• pulled up by nip rolls• supported by melt strength
Bubble stability in blown film
Page 5 Bubble stability in blown film
Bubble stability
Bubble stability is a combination of:• integrity of the molten film
- melt strength- melt viscosity- shear rate
• integrity of the solid film- thickness- material (stiffness)
melt phase
solid phase
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Integrity of the solid film
Film characteristics:• film thickness• materials used (stiffness, crystallization rate, Mw or
MFI)
Machine characteristics:• output• die gap• cooling efficiency• bubble height
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Polymer rheology
• high zero shear viscosity positive for bubble stability• for multilayer extrusion
- flow simulation is useful tool- most layers consist of more than one component
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Melt strength
• melt strength determines- bubble stability- stretching behaviour
• melt strength = resistance to drawing• melt strength is related to entanglements
- Mw- Mw/Mn- branching
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Melt strength
Melt strength is influenced by:• molecular weight• molecular weight distribution• chain entanglements• molecular branches
determined byresin supplier
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Melt viscosity
Melt viscosity is influenced by:• molecular weight• polydispersity MWD• polymer temperature• bubble cooling• shear rate
determined byresin supplier
determined byfilm producer
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Melt strength and viscosity are influenced by:• molecular weight• molecular weight distribution• chain entanglements• molecular branches• polymer temperature• bubble cooling• shear rate
determined byresin supplier
determined byfilm producer
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Extrusion temperature - 1
A low polymer temperature:• improves bubble stability• reduces thermal polymer breakdown
But:• increases shear
- risk of uncontrolled polymer temperature- polymer temperature influenced by output
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Extrusion temperature - 2
Common practice:• flat temperature profile on the extruder barrel
Why?• adequate for low shear 3-zone screws• operation simplicity• reduce chance for mistakes
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Example extrusion temperature control for PA6
1 2 3 4 Tpol230
250
270
290set actual
barrel zone
tem
pera
ture
[°C]
1 2 3 4230
250
270
290set actual
barrel zone
tem
pera
ture
[°C]
flat temperature profile reversed temperature profile
poor control of polymer temperature
good control of polymer temperature
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Bubble cooling
• external bubble cooling- chilled air
• internal bubble cooling (IBC)
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Pancake dies versus spiral dies
Pancake dies• temperature isolation of the layers
- nylon at 230 – 250 °C, EVOH at 175 – 205 °C• relative long residence timeSpiral dies• short residence times
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Measurement of melt strength
• Göttfert Rheotens
• oscillatory rheometer
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Summary
Improvement of bubble stability can be realised by:• high melt viscosity of the polymer in the bubble
- high Mw- branched polymer- low polymer temperature- efficient bubble cooling- lower frost line
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Contact
Ted BrinkEmail: [email protected]: www.extrusionist.comTel.: +31 651109899
Skype: ted.brink
Bubble stability in blown film