Compuplast International Inc. 2010 ANALYSIS OF EXTRUSION PROBLEMS WITH FLOW SIMULATION Vijay Kudchadkar Compuplast International Inc.
Objectives Demonstrate the application of CAE/CFD/Simulation towards extrusion Show how simulation can be used to troubleshoot an extrusion process Explain certain observations or phenomena with the aid of simulation. All simulations performed using the Compuplast Virtual Extrusion Laboratory (VEL) Suite of CAE Tools for Extrusion
Problem: Particles in Film Sample In this system, periodic showers of particles were observed in the film sample.
Photo of Extruder Screw The screw used to produce the film had a noticeable discoloration in the feed and compression region.
Comprehensive Tests with the Macro Glass Window Extruder Song, Perdikoulias and Planeta, ANTEC 2000
Comprehensive Tests with the Glass Window Extruder LDPE 50 RPM @ W2 & W3, melt @ ~ 17D
Solid Bed Break-Up Improper screw design results in solid bed break-up and there is no mixer to compensate
Inhomogeneous Melt Flow Direction Gray areas represent material that has not been properly mixed or homogenized.
Define: Solid Bed Ratio Solid Bed Melt Pool Flow Direction Channel Width Ratio of Solid bed width to Channel Width Indication of melting capability of the screw Solid Bed Ratio = SB Width Channel Width
Analysis of Solid Bed Ratio Simulation indicates that the Solid bed ratio does not reduce gradually as preferred but actually increases near center of screw.
Solution: Modify Process Conditions, Change Material or Screw Design to achieve better melting performance Good melting
Problem: Polymer Degradation Screen Changer Flat Die Manifold
Degradation on SCREW / BARREL / SCREENCHANGER 90 mm Diameter 250 kg/hr LDPE @ 190 C Shear Rate = 1.3/s Shear Stress = 5 kpa
The role of wall shear stress = Velocity = 0 = max Shear Stress High enough shear stress cleans the wall by scrubbing What is high enough? LDPE PVC 30 kpa 35 kpa HDPE LLDPE 30 kpa 30 kpa
Degradation SCREW / BARREL / SCREENCHANGER
Original Shape Shear stresses inside the flow domain
New Shape Shear stresses inside the flow domain
Degradation on SCREW / BARREL / SCREENCHANGER (Improved design)
Problem: Degradation and Damage in Flat Die Manifold The outer edges of the manifold exhibit degradation and even corrosion damage.
Simulation Results: Shear Stress Low shear stress at outer edge of manifold results in insufficient purge characteristics. Polymer exhibits long residence time and ultimately degrades. Some polymer degradation results in corrosive byproducts.
Polymer Degradation Solution Ensure adequate shear stress in all flow channels. Proper Channel Design Proper Operation Conditions
Problem: Periodic Lines in Tubular Film Bubble Lines appear as a visual defect in film produced on a spiral mandrel type die.
Potential Problem With some Spiral Mandrel type dies, a flow defect is sometimes observed that appears like a weld line in the product. It may also be observed in the product as a very localized thin section that repeats around the circumference of the die as many times as there are spirals.
The Spiral Mandrel Flow Field
Spiral Mandrel Flow Video
3D FEM - Pressure Drop
3D FEM Velocity
3D FEM Velocity (2 of 3 sections)
3D FEM - Shear Stress Low Shear Stress
3D Flow Analysis Particle Path Line Seeds Velocity contours for 2/3 of the die and path line seed locations
3D FEM - Particle Path
3D FEM - Particle Paths Particle Path From Next Spiral
Particle Path Lines Particle path lines showing "back flow"
Particle Paths Avoid Low Shear Stress
The Back-Flow Essentially Forms Weld Line
Problem: Lines in Flat Film
Poorly designed flat spiral distributor
Poorly designed flat spiral distributor
Poorly designed flat spiral distributor
Spiral Back-Flow Solution Design spiral distribution system to control the leakage flow from the spirals and ensure adequate velocity (and shear stress) along the entire channel.
Problem: Wave pattern in feed-block coextrusion Photograph of film sample exhibiting "wave" instability
Problem Definition
Problem Definition In a 3 layer coextrusion, the thin center layer exhibits an interfacial instability that is observed on the extruded film. Why? How can it be removed?
Feed-block system
Velocity profile on the original feedblock V max = 113 mm/s V max = 14.6 mm/s V max = 64.6 mm/s Central layer acceleration 64.6/14.6 = 4.4
Second material flow passage original design Area of high acceleration
Elongational viscosity dependence Log e LDPE HDPE, LLDPE 10 A material with increasing elongational viscosity Log does e not want to be accelerated. It generates a high resistance against the acceleration. This resistance influencesthe flow of neighbor materials and thus fluctuations are generated.
Deformation rate of the central layer 4.5 1/s 2.97 1/s
Velocity profile on modified feedblock V max = 82 mm/s Better balanced velocities V max = 26 mm/s V max = 64.6 mm/s Central layer acceleration 64.6/26 = 2.5
Second material flow passage modified design Lower acceleration
Deformation rate of the central layer 2.5 1/s 1.95 1/s
Explanation and Solution It was observed that the flow balancing in the lamella region helped to improved the production stability (interfacial instability disappeared) The flow balancing reduced the elongational rate of the center layer and the interface distortion was minimized.
Concluding Remarks Process simulation provides more information for correlation between process conditions and production problems. Simulation gives new insight into problems, which can lead to faster and more precise solutions.
THANK YOU!!! Questions? Vijay Kudchadkar www.compuplast.com vk@compuplast.com