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Soft-serve ice cream - perhaps an odd topic, coming on the heels of a recent record setting blizzard here on the US East Coast.  But if you have kids, you’re doubtless aware that there isn’t much of a seasonal restriction for ice cream demand.  And, what’s better than a plain vanilla swirl?  A vanilla-chocolate swirl of course – a more flavorful and visually appealing choice.  These concoctions are actually examples of two different kinds of extrusion (and soft-serve ice cream is wonderfully complex in terms of its rheology, but I digress).  The vanilla swirl is a simple extrusion while the vanilla-chocolate swirl is a co-extrusion – a process that combines two materials to create a unique product.  Today, we’ll focus on our latest capability in Computational Rheology: Co-extrusion. 

In STAR-CCM+ v10.06, we introduced the ability to model “single material” extrusion processes to create products like foam rubber door seals, plastic tubes, frames and even foods (pasta, for instance).  With STAR-CCM+ v11.02, you can combine two or more materials enabling the analysis of more complex products targeted to very specific applications.  For example, multi-luminal medical tubing has very narrow tolerances and needs to meet the highest quality standards.  Consequently, it’s critical to understand how design changes to the die profile or variations in feedstock properties can impact end product quality. 

To illustrate, let’s consider the notional tri-luminal tubing example shown below.  This product is created by the simultaneous extrusion of three similar materials.  In an effort to increase the overall structural rigidity of the extrudate, we’ve created a candidate design where the outer annular thickness is increased by 1 mm midway between the supports for each section.  The analysis shows exactly where, and by how much (using the Boundary Displacement Magnitude), the part goes out of tolerance.

Another scenario where this tri-luminal extrudate could fall out of tolerance is if the upstream processing conditions between each of the three feed streams varies.  The analysis below explores the sensitivity of the end product by introducing slight changes in material behavior to two of the three feed streams.  We can easily see the dramatic impact of these small differences which once again cause our notional extrudate to fall out of tolerance.  With the trend towards tighter and tighter tolerances coupled with an increasing range of choices for better performing polymer materials, the benefit of rapidly making informed decisions from simulation efforts becomes very obvious. 

 

A key to your success in solving extrusion/co-extrusion problems is our Directed Mesh capability.  Line Kinematic Conditions and numerical stabilizations that deliver rapid convergence and accurate results for these strongly coupled problems are designed to work with directed meshes.  And, having the ability to consistently generate high quality meshes lies at the heart of design optimizations and sensitivity analyses.  Referring back to our notional tri-luminal case, it is possible to not only refine the die exit geometry to achieve the desired extrudate cross section, but, also to understand how modifications to your feed stream delivery systems upstream will impact your end product quality.  

If you’ve already simulated an extrusion process in STAR-CCM+ v10.06, setting up a co-extrusion is a straightforward extension of that existing workflow.  Capabilities specific to co-extrusion are in the ability to specify different materials for each of the extrusion feed streams.  It is possible, for example, to extrude a viscoelastic material with a non-Newtonian (or Newtonian) material – there are no restrictions in this regard.  This allows you to fully explore the behavior of new materials, by being able to predict how well your complex extrudate is going to maintain its desired form during production.  Within the die itself, thin-walled baffle interfaces let you apply either no-slip or partial slip conditions uniquely at either side of the interface.  And within the extruded section itself, it’s possible to independently specify the morphing behavior at all of the internal, immiscible free surfaces.  In the exploded view of our notional tri-luminal extrudate, we can see how critical it is to apply the appropriate boundary conditions on the correct parts of this assembly.

 

We believe that the co-extrusion capability in STAR-CCM+ v11.02 will significantly broaden the scope of designs that you can review.  And we’ve given a lot of consideration to your productivity by keeping the integration within STAR-CCM+ as consistent and straightforward as possible.  Finally, thanks to the many of you who have already expressed interest in our Computational Rheology offering – as your experience with this capability grows, we welcome your feedback.  

Now that my driveway is finally clear, it’s time to get my vanilla-chocolate soft-serve reward!

Matthew Godo's picture
Matthew Godo
STAR-CCM+ Product Manager
Stephen Ferguson's picture
Stephen Ferguson
Marketing Director
James Clement's picture
James Clement
STAR-CCM+ Product Manager
Dr Mesh's picture
Dr Mesh
Meshing Guru
Joel Davison's picture
Joel Davison
Lead Product Manager, STAR-CCM+
Ravindra Aglave's picture
Ravindra Aglave
Director - Chemical Processing
Karin Frojd's picture
Karin Frojd
Sabine Goodwin's picture
Sabine Goodwin
Director, Product Marketing