Question about rheological properties - die-swell

Hello! everyone
I have some question about die-swell.
We have two HDPE samples which were produced by different catalyst.
They have almost same M.I value and shear viscosity, but are different in die-swell.
I would like to know why they are differerent in die-swell and which factors effects on die-swell difference.
thank you.
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Die-swell is related to melt elasticity and elastic melt deformation and is affected by:molecolar weight, molecular weigth distribution, side chains & degree of branching (I'd check this via the Rheometer-Test), but also by die-geometry (typically, the longer the die the less is die-swell}. See ->
http://www.chemwide.com/techinfo/01manufact_theory/01blow/04condition.html http://www.rheolabo.jp/Thermoplastic%20.pdf
hyh wrote:

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Extrudate swell is an elastic response to the deformation of the polymer in the converging zone of the die. As the polymer passes through the converging die it is stretched and elongated. These deformations draw out the molecular structure and extrusion conditions may not provide sufficient time for the molecules to 'relax'. I expect that one of your resins is less linear with more entanglements.
Swell can be reduced or controlled by 1. Reducing the elongational strain-rate (i.e., make the converging section longer or reduce the extrusion rate) 2. Increase the length of the die land. (Shape maintained with more time to relax.) 3. Increase the pull force (More tension can reduce swell - increase draw ratio) 4. Warm the die land (Increase rate of relaxation) 5. Increase lubricant level (Promote controlled slip on die walls)
These are a few parameters that I recall off the top of my head. You have many interacting parameters to play with.
Good luck!
Ken.

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Ken wrote:

Extrudate (die) swell can be the result of memory of the entrance flow, but it is not required. If you stop the flow for a period longer than the memory of the fluid, and then start it again, you still have the swell. (Reference: A.S. Lodge, in "Rheology of Elastomers", P. Mason and N. Wookey (Eds.) 1958, pp 70-85).
John Aspen Research, - www.aspenresearch.com "Turning Questions into Answers"
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