Extrusion of non‐newtonian fluids in a single‐screw extruder with pressure back flow

Abstract

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.

Transport processes and feasible operating domain in a twin‐screw polymer extruder

“…Therefore, if a coordinate system is chosen with the axes in the direction of the screw axis and along the tangential direction, then one can use the energy equation to march along the screw axis instead of worrying about the back flow. This approach has been verified and shown to be consistent with the down-channel formulation in single screw extruder (12). The geometry and the coordinate system are shown in Fig.…”

“…The distributions of velocity, pressure and temperature in the translation regon are very similar to that in a single-screw extruder, which has been studied extensively in the literature: see, for instance, the work of Karwe and Jaluria (8) and of Chiruvella et al (9,12). Therefore, the more interesting results in a twinscrew extruder are related to the nip region.…”

“…However, the velocity distribution in the channel can be obtained by the axial formulation as shown in FXg. 12. It is observed that the down-channel velocity is negative in some part of the channel depth, i.e.…”

“…It has been observed that if the throughput is too small for a specific screw rotational speed, bacMow occurs in the down-channel direction, and a marching procedure for the translation region will not work because of the limitation of the algorithm itself, as discussed by Chiruvella et al (12). However, in an actual screw extruder, the fluid does not flow towards the hopper.…”

Flow/pressure characteristics and modeling of deformation processes of single‐screw extruders

“…However, theoretical and experimental results are in qualitative agreement, there is no sufficient accuracy of quantitative correspondence by use of inadequate rheological model. Investigations that include fluids with complex rheological properties and higher pressure gradients are currently under progress [12, 13].…”