Helmut Ridder

Pressure/Throughput Behavior of a Single-screw Plasticising Unit in Consideration of Wall Slippage

Abstract The mathematical models so far available for describing the throughput behavior of plasticising extruders were developed with the assumption of a wall-adhering melt. There are, however, a series of plastic melts, and also elastomers, polymer suspensions, ceramic materials and food products that display wall slippage during processing. A mathematical model has been developed for this material behavior, which describes the flow behavior for the unidimensional, Newtonian, isothermal case. Apart from the development of the analytical model, the flow behavior of wall-slipping polymer melts was also analysed with the aid of finite element calculations (FEM). A comparison of the results for the pressure/throughput behavior shows that the calculation results tally very well for the two methods. It is thus possible to develop a procedure which makes it possible to describe the phenomenon of wall-slippage for the non-Newtonian, multi-dimensional, non-isothermal case.

Influence of Non-Newtonian Behaviour on the Processing Characteristics of Wall-slipping Materials

Abstract So far it has been assumed that when describing the flow process in single-screw extruder, the melt will be wall-adhering. Using certain process conditions, however, some materials such as PVCs, high-molecular PEs, elastomers, polymer suspensions, ceramic materials and food products become wall-slipping [1 to 4]. During simulation of the flow process using the Finite Element Method (FEM), the boundary condition of “wall-adhering” was substituted for “wall-slipping”. The numeric results refer to one-dimensional, non-Newtonian, isothermal flow processes and have been evaluated accordingly. Since the wall-slipping properties are dependent on critical shear stress, the influence of non-Newtonian behaviour on wall shear stress is illustrated by investigating the barrel and screw root surface. Conclusions with regard to wall-slipping properties may be drawn from the resulting upper and lower limits. The effect of wall slippage on pressure/throughput behaviour is examined along with the various velocity profiles, resulting from different compression ratios, dimensionless critical shear stress, material-related constants and screw configuration.