Extrusion Die Design and Simulation

[Pages:58]Simulation and Design of Extrusion Dies

Kirubel Bogale

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Degree Thesis

Plastics Technology

2011

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DEGREE THESIS Arcada

Degree Programme:

Plastics Engineering

Identification number: Author: Title:

9609 Kirubel Bogale Extrusion Die Design and Simulation

Supervisor (Arcada):

Mathew Vihtonen

Commissioned by:

Abstract:

The aim of this engineering thesis was to design and simulate a die for a rectangular profile. The project consisted of studying melt flow characteristics of plastics and how the die structure should be set in order to get the desired shape. After the die has been designed on 3D Solid edge design software it was simulated on COMSL flow simulation software. On the simulation software it was easy to study how the melt flows and in which part of the die is the shear stress and strain strong and weak.

The drag flow and pressure flow were then extracted from the screw parameters which lead into the calculation of the operation point and the operating pressure for the die. It was also possible to calculate the mass flow rate and the velocity drop. And finally die optimization of die profile for rectangular shape was done.

Keywords:

Number of pages: Language: Date of acceptance:

Extrusion, Comsol, Solid edge, flow rate, output, density operating point, Die design, Viscosity

English

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TABLE OF CONTENTS

1 INTRODUCTION................................................................................................... 10

1.1 Background ...................................................................................................... 10 1.2 Objective .......................................................................................................... 11

2 Literature Survey..................................................................................................... 12

2.1 Extrusion .......................................................................................................... 12 2.2 Types of Extrusion........................................................................................... 13 2.3 Extrusion Process and Operation ..................................................................... 13

2.3.1 Single screw.............................................................................................. 14 2.3.2 Mechanism of Flow .................................................................................. 14 2.3.3 Die Design ................................................................................................ 16 2.3.4 Pressure Gauges........................................................................................ 17 2.3.5 Heating Elements...................................................................................... 18 2.3.6 Cost........................................................................................................... 19 2.3.7 Die land .................................................................................................... 20 2.4 Polymeric Melt Behavior................................................................................. 21 2.4.1 Rheology................................................................................................... 21 2.4.2 Viscoelastic Fluid Behavior...................................................................... 22 2.4.3 Non-Newtonian Fluid Behaviour ............................................................. 23 2.4.4 Melt Behavior ........................................................................................... 24 2.4.5 Density...................................................................................................... 25 2.4.6 Melt Density ............................................................................................. 25 2.4.7 Bulk Density............................................................................................. 26 2.5 Balance Equations............................................................................................ 26 2.5.1 Momentum Balance Equations................................................................. 26 2.5.2 Energy Balance Equations........................................................................ 27 2.5.3 Mass Balance Equations ........................................................................... 27 2.6 Flow Analysis .................................................................................................. 28 2.6.1 Drag Flow ................................................................................................. 28 2.6.2 Pressure Flow ........................................................................................... 28 2.6.3 Shear Rate................................................................................................. 29

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2.6.4 Shear Strain .............................................................................................. 29 2.6.5 Shear Stress .............................................................................................. 29 2.7 Flow Models .................................................................................................... 30 2.7.1 Newtonian and non-Newtonian Fluids ..................................................... 30 2.7.2 Newtonian Fluids...................................................................................... 30 2.7.3 Non-Newtonian Fluids ............................................................................. 31 2.7.4 Power Law Model .................................................................................... 32 2.7.5 Drag and Pressure flows ........................................................................... 33 2.7.6 Flow analysis ............................................................................................ 34 3 Method .................................................................................................................... 36 3.1 Die Modeling ................................................................................................... 36 3.2 Extruder line calculation .................................................................................. 39 3.3 Die characteristic for rectangular channel ....................................................... 41 3.4 Subdomain settings .......................................................................................... 42 4 Results ..................................................................................................................... 45 4.1 Post processing ................................................................................................ 45 5 Conclusion............................................................................................................... 53 6 References ............................................................................................................... 54 Appendix ........................................................................................................................ 56

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Figures

Figure 1 Definition and Principle of Extrusion. ............................................................ 12 Figure 2 Extrusion Process and Operation.. ................................................................... 13 Figure 3. Flow coefficient as a function of channel geometry ....................................... 16 Figure 4. Standard mica band heaters............................................................................. 18 Figure 5. Effect of die orifice shape on a square extrudate ............................................ 20 Figure 6. Rheological models ......................................................................................... 21 Figure 7. Newtonian Fluids ............................................................................................ 30 Figure 8. Viscosity - Shear rate curve for some common plastics ................................. 33 Figure 9. Solid Edge modeling of the Die ...................................................................... 36 Figure 10. Metric Draft of the Die.................................................................................. 37 Figure11. Die detailed drawing ...................................................................................... 38 Figure 12. Die inlet area ................................................................................................. 43 Figure 13. Die outlet area ............................................................................................... 43 Figure 14 Velocity curve at 0.41MPa............................................................................. 45 Figure 15 Pressure curve ................................................................................................ 46 Figure 16. Pressure distribution in the die at 0.41MPa .................................................. 48 Figure 17. Shear rate curve at 0.41MPa ......................................................................... 49 Figure 18. Dynamic Viscosity Curve ............................................................................. 50 Figure 19. Uneven stress distribution at the corners ...................................................... 51 Figure 20. Optimized die geometry................................................................................ 52

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Tables

Table 1. Density and melting points of some common plastics ..................................... 25 Table 2. Power law parameters for some common plastics ........................................... 32 Table 3. Extruder operating points ................................................................................. 42 Table 4. Subdomain settings for LDPE .......................................................................... 42

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Nomenclature

P = pressure (N) Qd = Drag flow (m3/s) Qp = Pressure flow (Pa) N = Screw revolution (rpm) H = Channel depth of the screw (m)

= Helix angle of screw

L = length of the screw (m) D = diameter of the screw (m) R = die radius (m) Ld = length of the die (m)

= viscosity (Pa.s)

= Shear rate (S-1) = Shear stress n = Power law index Oop = Operating point Pop = Pressure at the operating point (Pa) = Melt density (kg/m3)

= Mass flow rate

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