Shear Rate Coat-Hanger Die Using Casson Viscosity Model

Dastan Igali,Dichuan Zhang,Asma Perveen,Dongming Wei

doi:10.3390/pr8121524

Abstract

Coat-hanger die design aims for optimization of the die geometry of the body and the flow distribution manifold, such that through the exit at the die lip homogeneous distribution of the polymer melt is achieved. This paper proposes a novel methodology for deriving the design equations of the coat-hanger die geometry for some specific extrusion materials and provides fluid–solid interaction simulations for validations. The basis for the calculations is the Casson rheological model, analytic velocity profiles for the pseudoplastic flow through circular pipe and slit, and the constant shear rate coat-hanger die design methodology developed by Winter and Fritz. The geometry obtained was then evaluated using the fluid-structure interaction numerical simulation approach. The sensitivity of the outlet velocity uniformity and die body deformation due to the material and mass flow rate change were investigated using the finite element software, Ansys. It was found that the homogeneity of the outlet velocity is very sensitive to the extrusion materials. The structural analysis of the solid die body also resulted in higher deformations when using some other extrusion materials different from the initial design. Mass flow rate increase only resulted in large zones of stagnation, which occurred around the melt as it passes from the manifold to the slit region. Therefore, it is recommended to define the required range of mass flow rate to prevent the formation of stagnation zones.

Highlights

The coat-hanger extrusion die design is mainly focused on the geometry optimization of the flow channels capable of providing homogenous distribution of the polymer melt across the die width [1,2,3].There are a number of studies assessing the velocity uniformity of the polymer melt at the exit by means of the computational fluid dynamics (CFD) approach [4,5,6,7,8,9]
The proposed design approach is based on a flow model assuming power-law viscosity and uniform shear rate at the walls of the manifold and slit regions [11]
This paper aims to develop the procedure of deriving the design equations for the uniform commonly used power-law model in some application conditions

Summary

Introduction

The coat-hanger extrusion die design is mainly focused on the geometry optimization of the flow channels capable of providing homogenous distribution of the polymer melt across the die width [1,2,3].There are a number of studies assessing the velocity uniformity of the polymer melt at the exit by means of the computational fluid dynamics (CFD) approach [4,5,6,7,8,9]. Few studies carried out the fluid-structure interaction (FSI) method to evaluate the velocity distribution of the coat-hanger die with the manifold of the rectangular cross-section when the solid die body is subject to deflection [10]. The mostly used and adequate mathematical design method focusing on the distribution problem was proposed by Winter and Fritz [11] in 1986. Winter and Fritz [11] claimed that their design method is practically independent of flow rate and viscosity of the polymer melt being extruded. The proposed design approach is based on a flow model assuming power-law viscosity and uniform shear rate at the walls of the manifold and slit regions [11]

Objectives

Methods

Discussion

Conclusion