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Sharkskin Extruder

This version was saved 13 years, 8 months ago View current version     Page history
Saved by Stephen M.
on August 2, 2010 at 4:03:29 pm
 

An Explanation of the Production of Sharkskin

When extruding non-newtonian polymers, temperature gradients cause surface ripples called "sharkskin."  This process has been recognized in the production of polymers since World War II.  The sharkskin instability is currently suppressed in the polymer production industry by carefully controlling the temperature of the extruded material or by addition of chemical additive.  Sharkskin could be used in drag reduction applications in the polymer industry.  In other areas of the industry, engineers are working hard at reducing or eliminating sharkskin formation so that manufacturing rates do not have to be sacrifice to allow for a sharkskin free product.  Either way, the production of sharkskin or prevention of it requires an intimate knowledge of the instability involved in order to manipulate it. 

 

Sharkskin is an occurrence that has been largely considered unwanted in the production of polymer extrusions.  As the polymer is forced out of the device the nature of the flow changes from no-slip to plug flow.  No-slip flow is the traditional fluid flow within a constrained container.  The sides of the fluid naturally adhere to the sides of the container, slowing them down with friction.  Meanwhile, the middle of the fluid is able to travel faster because its cohesive properties are weaker than the adhesive properties of the wall to the polymer.   When the fluid leaves the die it has a constant velocity across its cross section; the center of the fluid flow remains at the same velocity as the sides rapidly accelerate in the lack of friction.  This sudden acceleration at the fluid-die interface pulls the polymer away from the die; before the polymer lets go it stretches, gathering energy before snapping forward to catch up to the flow.  As the strained polymer relaxes and cools, it holds the form of a saw tooth

 

 

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