Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Driving mechanisms and their consequences

TIP Driving mechanisms and their consequences

- Mechanism for upstream instability The experimental observations carried out by CEMEF helped establish that the upstream instability was associated with the viscoelastic flow of the material within the entrance region of the die. The instability originated in the entry region of the die and propagated through the die to the final extrudate. In some cases a symmetric pulsing action was observed and in other cases as periodic asymmetric instability was seen. The experimental data provides a sound basis for comparison with numerical simulation and also provides commercial design rules in order to avoid this instability.

- Stick spurt instability Systematic experiments on the stick spurt instability were carried out at both CEMEF and LSP. Additional experiments were done by Repsol, Dow, LMPL, Dyneon, Pirelli. Both CEMEF and LSP developed experimental facilities to measure point wise velocities using LDV techniques. Both groups showed self consistent data relating to velocity profiles within the parallel section of a die. Below the stick spurt regime ‘power law’ velocity profiles were observed where either no slip or in some cases, partial slip was observed. In the stick spurt regime a substantial change in the velocity profile was observed. During the spurt part of the flow a near plug flow response was seen. These results unambiguously and for the first time show that stick spurt is related to wall slip. This result is of great significance in terms of an identified mechanism and is of relevance to essentially all partners.
-- The mechanism for stick spurt The experimental observations have shown that stick spurt is related to a slip condition at the parallel section of the die. Clearly compressibility is also involved and the form of the instability is caused by an interplay between compressibility and wall boundary conditions. The result highlights the region of the flow where attention needs to be given in order to avoid this instability.

- Sharkskin instability Systematic experiments were carried out by CAMB and LSP with additional work due by Dow, Dyreon, Argo and Repsol. CAMB established that side stream gas injection did not diminish sharkskin, however observation helped to confirm that the instability is related to a stress concentration at the exit of the die. CAMB showed that surface finish surface material and exit curvature influenced sharkskin. CAMB carried out experimental flow birefringence studies that indicated the instability originated from the local stress concentration at the exit. Both LSP and CAMB showed that Dyneon additivewas effective in eliminating sharkskin and LSP showed that sharkskin elimination could occur even without a velocity profile modification.
-- Mechanism for sharkskin From the experiments carried out it is clear that the sharkskin instability is associated with the local stress concentration at the exit of the die. Partial slip can reduce sharkskin but other factors also play a part in the mechanism. The addition of fluorocarbon additives was found to be the most effective way to eliminate sharkskin.

- Draw resonance Experiments were carried out by Dow and CEMEF using Rheotens apparatus. The critical draw ratio for a given draw length L was established for a range of different materials.
-- Mechanism for draw resonance It is generally accepted that the mechanism for downstream draw resonance is an interplay between the draw ratio, geometry and viscoelastic forces and the experimental results obtained supported this view.

- Additional experiments Systematic experiments on the time dependant viscoelastic flow of compressible polymer melts were carried out in CAMB in order that they can be compared with a numerical code developed at CEMEF/CAMB. The numerical capability that has been developed has the future potential to simulate and quantify, upstream, stick spurt , sharkskin and draw resonance instabilities. This code should provide a powerful tool for the future.

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Deparment of Chemical Engineering
Pembroke Street
CB2 3RA Cambridge
United Kingdom
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