Extrusion, Feed Screw MODELING PROGRAM
MODELING OF SINGLE AND TWIN SCREW EXTRUSION
MODELING OF SINGLE AND TWIN SCREW EXTRUSIONWith the development of Tadmor's melting model [4] based on Maddock's experiments at Union Carbide, it became possible to simulate the process of single screw extrusion. The extruder is subdivided into three sections for solids conveying, melting and melt pumping. The solid bed is usually modeled by applying a force and torque balance. In the melting zone, melting occurs in a thin film between the solid bed and the heated barrel. A melt pool forms in front of the rear flight. The melt channel is unwound and the flow is assumed to be two-dimensional. These are the basic ideas which led to the development of commercially available software packages EXTRUD [5], SSD [33], REX [34], CHEMEXTRUD [35], and EXTRUCAD [36]. The most recent version of EXTRUCAD has capabilities for simulation of extruders having standard single flighted screws, multistage screws with venting, barrier screws, and screws with mixing elements [37]. Some typical results are shown in Figure 3. The weakest link in EXTRUCAD as well as all other commercially available packages and extrusion models is perhaps the solids conveying zone. Transport of a packed bed of solid pellets, powders or flakes is difficult to describe mathematically. The friction coefficients between solid bed and barrel and between solid bed and screw are difficult to measure experimentally. Pressure, temperature, speed of rotation, material of construction and surface machining have considerable influence on these coefficients. The whole approach of balancing forces and torques which necessitates the introduction of the friction coefficients is just a convenient empiricism. There is need of better understanding of the contact problem between a solid bed of packed particles and screw and barrel surface.
Twin screw extruders come in a great variety of configurations. There are differences in screw placement, shape of the flights, and direction of rotation that makes extruders as different from each other as they are, as a group, different from single-screw extruders [38]. There are co-rotating, counter-rotating, intermeshing, non-intermeshing, conjugated, and non-conjugated machines. Strictly speaking, twin screw extrusion is a 3-D, unsteady state process. As such, the problem is extremely difficult. Various simplifications have been introduced by various groups, i.e. 3-D steady flow (by Manas-Zloczower at Case Western Reserve University, Gogos and co-workers at Stevens Institute of Technology, Kiani at Werner-Pfleiderer, Hrymak at McMaster University, and others). Simpler models have been developed by White at University of Akron and Potente at the University of Paderborn in Germany. For a comprehensive review of the various modeling approaches up to about 1990 the reader is referred to White's book [39]. AKRO-CO-TWIN SCREW [40] is a commercially available package for simulation of co-rotating screw extruders. Bang and White [41] have recently proposed a model for modular tangential counter-rotating twin screw extruders, based on the lubrication approximation.
The complexity of twin screw extruder geometries, screw configurations and the variety of the operational characteristics will pose a computational challenge for many years to come. There is no unique and recommended methodology to follow. There is certainly a need for new ideas.
MODELING OF SINGLE AND TWIN SCREW EXTRUSION - 2003-09-04
