Coat-hanger Die Research Articles

Optimization of Non-Newtonian Flow through a Coat-Hanger Die Using the Adjoint Method

The use of coat-hanger dies is prevalent in the plastic film and sheet extrusion industry. The product quality and the power of the extrusion machine depend on the uniformities of the fluid velocity at the exit and the pressure drop. Die manufacturers face the challenge of producing coat-hanger dies that can extrude materials uniformly and with a minimal pressure drop. Previous studies have analyzed the die outlet’s flow homogeneity and pressure drop using various numerical simulations. However, the combination of the scheme programming language together with the Adjoint Method of Optimization has yet to be attempted. The adjoint optimization method has been demonstrated to be beneficial in addressing issues related to shape optimization problems and it may also be beneficial in optimizing the design of dies used in polymer melt extrusion. In this study, the proposed innovations involve incorporating both the Scheme programming language and Adjoint solver to examine and optimize the coat hanger’s flow homogeneity and pressure drop. Before optimization, the outlet velocity was almost 10 times higher at the die center than at the edges but after optimization, it became more uniform. The proposed optimized coat-hanger die geometry results in more uniform melt flow as demonstrated by the velocity contour plot and the outlet velocity graph in the die slit area, reducing the deviation value from 0.097 to 0.015. Additionally, the mass flux variance across the die outlet decreased by 71.6% from 0.015069 kg m−2 s−1 to 0.004281 kg m−2 s−1. Therefore, using this method reduces the amount of time wasted on trial and error or other optimization techniques that may be limited by design constraints.

A Multi-Rheology Design Method of Sheeting Polymer Extrusion Dies Based on Flow Network and the Winter-Fritz Design Equation.

In the polymer sheet processing industry, the primary objective when designing a coat-hanger die is to achieve a uniform velocity distribution at the exit of the extrusion die outlet. This velocity distribution depends on the internal flow channels of the die, rheological parameters and extrusion process conditions. As a result, coat-hanger dies are often designed for each polymer based on its individual rheological data and other conditions. A multi-rheology method based on a flow network model and the Winter–Fritz equation is proposed and implemented for the calculation, design and optimization of flat sheeting polymer extrusion dies. This method provides a fast and accurate algorithm to obtain die design geometries with constant wall-shear rates and optimal outlet velocity distributions. The geometric design when complemented and validated with fluid flow simulations could be applied for multi-rheological fluid models such as the power-law, Carreau–Yasuda and Cross. This method is applied to sheet dies with both circular- and rectangular-shaped manifolds for several rheological fluids. The designed geometrical parameters are obtained, and the associated fluid simulations are performed to demonstrate its favorable applicability without being limited to only the power-law rheology. The two such designed dies exhibit 32.9 and 21.5 percent improvement in flow uniformity compared to the previous methods for dies with circular and rectangular manifolds, respectively.

Flow behavior of polypropylenes with different molecular structures in a coat-hanger die

An experimental investigation of the flow behavior of three polypropylene melts with different molecular structures during extrusion through a coat-hanger die is presented. Two linear and one long-chain branched material, rheologically characterized in shear and elongation, were investigated. Using laser–Doppler velocimeter measurements of the velocity profiles across the gap height were performed at five various locations along the die. The uniformity of the velocity distribution along the die has been assessed using the maximum velocities v0 of the corresponding velocity profiles across the gap. The velocity distribution along the die changes with throughput and temperature. Regarding the rheological properties, it was found that the power-law index of the viscosity as a function of shear rate has a decisive influence on the uniformity of flow but that the pronounced strain hardening in elongation typical of the long-chain branched polypropylene is not reflected by the velocity distribution along the die. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Fluid flow in coat-hanger die of melt blowing process: comparison of numerical simulations and experimental measurements

The coat-hanger die is an important part in the melt blowing process, the performance of which is characterized by the flow uniformity at its outlet. The primary objective of this study is to validate the finite element method applied in the simulation of fluid flow in the coat-hanger die, so as to search for the optimal flow channel geometry based on simulation and optimization algorithms. The fluid flow in the coat-hanger die is simulated through three-dimensional finite element method in this paper. Simulation results are experimentally verified qualitatively and quantitatively using particle image velocimetry. Good agreements are obtained in the flow patterns and velocities calculated at the die outlet, which suggests that the numerical methodology can reliably predict the details of fluid flow in the coat-hanger die.

Computer Aided Design of Coat-Hanger Die in Polymer Film Processing

With the aid of computer simulations, coat-hanger die was designed for more uniform residence time distribution. Analysis of velocity distribution and flow path line indicate that stagnation areas exist in the coat-hanger die, which is caused by the geometry abrupt change between the circular section manifold and the rectangular section slot gap. The stagnation areas affect the melt residence time directly. After changing the circular section manifolds to tear-drop section manifolds, the stagnation areas decrease from 8.21 mm2 to 5.40 mm2. Compared with the melt residence time in circular section manifolds, both the maximum residence time and the residence time dispersion were decreased.

Three-Dimensional Simulation of Polymer Melt Flow in a Coat-Hanger Die.

The power-law fluid of polymer melts in a coat-hanger die is modded three-dimensionally and simulated using finite element method. In order to get an optimum coat-hanger die, design equation devioped perviously based on one-dimensional flow model in manifold and slot is employed. From the calculated flow rate distributions, the optimum process condition for maximum flow rate uniformity can be determind. The optimum conditions were mainly dependent on the manifold angle and power-law index of polymer melt. The effects of disturbances of power-law index, die width, slot thickness, and manifold angle from the optimum values are investigated thoroughly. Results show that any departure of process conditions from optimum value decreases flow rate uniformity. The effect of land length is also analyzed. Manifold angle is a primary factor governing uniformity of residence time distribution. Two-dimensional analysis is also conducted and its results are compared with those of three-dimensional to show that application of two-dimensinal model using lubrication approximation in the region of manifold is subject to critical error.

Extrusion of Rubber Compounds and Highly Filled Thermoplastics through Coathanger Dies

Abstract The isothermal flow of plastic fluids in coathanger dies is studied. A general three-dimensional finite element code is developed for the flow analysis purpose. The pressure distribution, the velocity distribution and the transverse flow rate distribution are calculated. The yield value of filled compounds gives rise to a mobile plug in the flow channel. The development of plug zone along the flow direction is also shown by the finite element analysis. The effect of the die geometry on the transverse flow rate distribution is critically discussed. It is found that the cross sections of the flow channel in a coathanger die has the shape of a dog bone or a dumbell which make a nearly even transverse flow rate distribution possible.

Design of Sheet Dies for Minimum Residence Time Distribution: A Review

Abstract The various die designs that have been proposed for the manufacture of sheets and flat films from polymeric materials are reviewed, along with the corresponding flow analysis and derived design equations. Successively, the T-die, the linearly tapered coat-hanger die, the curvilinearly tapered coat-hanger die (with manifold of circular or rectangular cross-section), and the fishtail die are examined. In each case, care is taken to outline the assumptions used to derive the equations, and to state the advantages and disadvantages of each design. The limits of these mentioned assumptions are explained and illustrated. Particular attention is paid to the aspects of the design which affect the processing of sensitive or reactive materials (i.e., residence time distribution and wall shear stress).