The transport phenomena underlying the extrusion of non‐Newtonian fluids in single‐screw extruders is investigated numerically and experimentally. The viscosity of the investigated fluids is a strong function of the temperature and, for the non‐Newtonian case, of the shear rate. Therefore, the governing equations of motion are coupled to the energy equation through the viscosity. The velocity in the down channel direction of the screw extruder is a result of both shear and pressure driven transport. The pressure acts in a direction opposite to that of the drag flow, and comparatively high pressures arise at the die in typical extruders. When a narrow die is used in the screw extruder, the pressure gradient in the down‐channel direction becomes so large that the down‐channel velocity near the screw root becomes negative in terms of the coordinate system fixed to the screw. The conventional marching schemes fail to simulate the fluid flow when the down‐channel velocity becomes negative, since the downstream conditions are not known. Two different numerical schemes used to simulate the fluid flow in a single‐screw extruder for this circumstance, which often arises when dies with high flow resistance are used and which is termed as pressure back flow in the literature, have been discussed. One scheme is based on including the down‐channel thermal diffusion, making the problem elliptic, and the other scheme uses a different coordinate system. Both formulations are found to yield results that are fairly close. Experiments were also carried out to measure the pressures at three different locations in a single‐screw extruder. The computed results were found to be in good agreement with the experimental results. The pressures at the die obtained numerically by treating the flow as isothermal are found to be lower than those obtained when the flow is treated as nonisothermal, indicating the strong influence of thermal transport in this problem.

No abstract

The fluid flow and heat transfer in polymer extrusion in a twin‐screw extruder was studied numerically by using the finite volume method. In the mathematical model, the coordinate system is fixed to the screw so that it is held stationary and the barrel is moved to simplify the complicated geometry. The screw channel of a twin‐screw extruder is approximated as two regions: translation and intermeshing. The flow in the translation region is similar to that in a shallow single screw extruder and is treated by the numerical methods given in the literature. In the nip or intermeshing region, strong mixing effects are expected, along with the diffusion of energy and momentum. The full governing equations are solved in this region to determine the velocity components in all the three coordinate directions. The energy equation is coupled with the equations of motion through viscosity, since the viscosity of the polymeric, non‐Newtonian, fluids considered here is dependent upon the shear rate and temperature. There is no clear physical demarcation between the nip region and the translation region. Therefore, a domain matching was employed at an arbitrary location that was varied numerically to ensure that the results were independent of this location. The variation of pressure and bulk temperature along the helical channel of the twin‐screw extruder is obtained, along with the shear rate. An experimental investigation of the velocity profiles in the translation region of a self‐wiping twin‐screw extruder, which is often used in practical applications, was carried out using a Laser Doppler Anemometer. The numerically predicted velocity profiles are compared with those from the experiments, yielding fairly close agreement.

In a plasticating screw extruder, a polymer melt forms in the melting zone of the extruder. Pressurization of the molten polymer takes place in the melting and the metering sections so that the melt can flow through the restricted passage of the die and assume a desired shape. In a melt fed extruder, the throughput is governed by the pressure rise over the entire length of the extruder. The pressure developed in the screw channel may also be employed in rapid filling of molds, such as those in injection molding. When the geometry of the screw, the barrel temperature, and the die are selected, a unique set of operating parameters arise for a particular flow rate or screw speed. In the present study, numerical and analytical methods are used to calculate the transport in the extruder and the pressure drop in the die. An iterative numerical method based on solving the equations of motion and energy in the screw channel and a correction scheme to couple the die with the screw channel is discussed. The numerical algorithm is capable of handling an arbitrary variation of the viscosity of the polymeric fluid with the shear rate and temperature. The results obtained by simulating the fluid flow in the screw channel are compared with available numerical and experimental results in the literature, indicating good agreement. The performance characteristics of the extruder, for chosen thermal boundary conditions and screw geometry, are presented for different die geometries and different fluids. The important considerations that arise in the numerical simulation of the extrusion process are also discussed.

The analysis of fluid flow in a single screw extruder channel is presented by treating the screw as stationary and rotating the barrel in the opposite direction. It is shown that the calculated shear rates at the barrel and at the screw, during extrusion of a Newtonian fluid under isothermal conditions, do not depend on the choice of formulation, i.e., barrel moving or screw moving. It is also shown that they are independent of choice of coordinate system.