Particles Tracking: a Powerful Method to Determine RTD in a Complex Flat Die

The design of dies for flat film applications involves to take into account many constraints. Among these, one key issue is the thermal degradation of polymer melt during extrusion. As the thermal degradation depends on both temperature and time, in this article, the focus is done on the Residence Time Distribution (RTD).

Determining the residence time in an extrusion die is challenging: whereas the experimental measures require specific extrusion equipments (such as specific injection port), numerical method is an efficient alternative as it performs advanced analysis in a virtual way. The die flow channel presented here was designed by Cloeren Incorporated (Figure 1).

die RTD XimeX

In this die, a non-Newtonian polymer melt flow is computed by XimeX®, a 3D Computational Fluid Dynamics (CFD) combined with particles tracking method. The particles tracking consists in transporting mass-less particles with the previous computed velocity flow. As a result, XimeX® is able to gain an insight on the history of the fluid flow (Figure 2).

RTD and velocity for die

Initially, 25,000 particles are computed from the die entrance. Then they are tracked during 1 minute process. It can be seen that at the 60 seconds cut-off, about 94.3% of the initial amount of particles has come out of the die flow channel (Figure 2a).

The figure 2b shows RTD bar charts of the particle count at the die exit as a function of time. The calculated average residence time by particle tracking is 11.6 seconds. Most of the particles are exiting the flow channel in less than 20 seconds.
This RTD analysis is interesting as it can be correlated to the quality of the products (here, the thickness and defaults of the films).

As a conclusion, an advanced CFD tool featuring a powerful particles tracking method, XimeX®, was successfully used to determine the RTD of a non-Newtonian polymer melt flow in a complex flat die flow channel.
The particles tracking method provides also other tags about the mixing quantification such as the cumulated strain, cumulated heat, cumulated dissipated energy, erosion, average stretch…

*based on « Determination of Residence Time Distribution Patterns in Complex Flat Die Geometries with XimeX® » by O. Catherine (Cloeren Incorporated), M. Henri and C. David (Sciences Computers Consultants). Eurotec 2013, Lyon – France

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