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Sharkskin Extruder

Page history last edited by Stephen M. 11 years, 1 month ago

In Fall '09 and Spring '10 I worked for Professor Rothstein in two semester-long independent studies researching the controls of extrusion.  I continued the work of other students' independent studies and graduate students' work as I sought to control the surface properties of extruded polymers using heat applied locally through a specialized die.

 

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

 

Research and Experimentation

Research in the rheological field of sharkskin instabilities has yielded the ability for researchers to control the formation of sharkskin explicitly enough to vary surface texture as a function of time and temperature; it can be controlled, but the state of the surface remains constant or slightly variable across the die.  A lack of research exists on varying the surface properties of an extruded polymer along the die slit.   I continued the project by creating a control system based on the Arduino microcontroller that could be programmed to certain outputs or to be closed-loop and autonomous.

 

The power control board attached to the Arduino microcontroller 

 

Current regulated from three independent power supplies is controlled by TIP 120 transistors that are isolated from the control circuit by means of optical isolators.  The temperature of each pin was regulated by pulse width modulation from the Arduino.

 

 

On the left, the compressed nitrogen feeding into the extruder.  On the right the three heater pins can be seen.

 

The experimental extruder consisted of two parts, the barrel and the die.  The barrel was fabricated from aluminum and accepts the coupling for the pressurized nitrogen at the top, and the die at the bottom.  The barrel was maintained at constant temperature of 125C using an Omega laboratory temperature controller and monitored using thermocouples inserted through the wall of the device.  The experimental die was machined from aluminum and holds three thermally isolated heater pins perpendicular to the surface of the extrudate.  Each of these pins was wrapped in Omega 30 gauge Ni-Chrome Resistance Heating Wire (60% nickel, 14% chromium) for an approximate resistance of 19Ω each.  The coils were powered separately by independent supplies providing a constant voltage to each for a maximum current of 0.5Amps per coil.  The temperature of the heater pin was controlled by the frequency and amplitude of pulsed current through the coil of Ni-Chrome wire.  For simplicity, the maximum amplitude of each pulse was set at a constant .5Amps.  The electrical current heating each of the pins was modulated as the output current of a TIP120 transistor toggled by input current from an Arduino microcontroller with the transistor collecting current from a power supply.  The current delivered to the coil was thus the amplified pulse-width modulated output of the microcontroller and could be controlled discretely.

 

Run at 400psi, with step sizes of 25 seconds, I was able to create an on/off patterned extruded surface that had a temperature of 115C at full gloss and 90C at fully developed sharkskin.  Using the interferometer I could measure points along the length of the sample to determine the local rms roughness.

Fully developed (steady state) sharkskin had an average rms roughness of 61.6.  A 50 second pulse yielded an average rms of 58.8 (95% of full sharkskin), a 25 second pulse yielded 51.7 (84% roughness).

At 400 psi, the extruder had a mass flow rate of 6.75mm3/s and an extrusion rate of 0.33mm/s.

 

Writing for a publication based on this project is currently underway.  

Most of the fabrication of the die was done in previous independent studies.  Information on one of those can be found here at my brother's site.

 

 

 

 

 

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