Mixing screws configurations

The screw configuration is normally either a ribbon-type helical screw or a series of paddles mounted on a common shaft. Materials of construction are selected based on the specific application and materials to be mixed. Typically, the screws are either steel or stainless steel, but other materials are available. 

Rios et al. [55] performed an experimental study with the above rhomboidal mixing section configurations using a 45 mm diameter single screw extruder. The mixing sections... 

Fig. 10.17 The counter- and co-rotating thin-flight, open C-shaped channels used in the three-dimensional FEM study of Katziguara et al. (32). The two-screw configurations are identical except for the sense of rotation. [Reprinted by permission from T. Katziguara, Y. Nagashima, Y. Nakano, and K. Funatsu, A Numerical Study of TSEs by 3-D Flow Analysis - Development of Analysis Technique and Evaluation of Mixing Performance for Full Flight Screws, Polym. Eng. Sci., 36, 2142 (1996).]...

If, in addition, the barrel of the trial machine is manufactured from a transparent material (Fig. 9.20), the mixing behavior in the machine can be observed first-hand. This provides valuable information about the mixing performance of a screw configuration. 

With the aid of the experimental techniques that have been briefly described here, the mixing performance of screws can be investigated with relatively little effort. Such investigations are useful, for instance, when developing new screw elements and optimizing screw configurations. With the proper equipment, it is also possible to investigate dispersion processes. 

Neutral mixing elements can be fitted at any point in the screw configuration. They are always completely filled and must be overrun by upstream elements. 

Liquid additives are mostly added downstream of the plastification section because they tend to lubricate the pellets or cause powders to agglomerate in the feed throat. If a significant amount of liquid is to be incorporated, it can be added at several locations. The most effective method for low viscosity liquid incorporation is to inject into a fully filled distributive mixing section. This requires a pressure injection valve and positive displacement pump. For small amounts of compatible liquid, non-pressurized injection into a low degree of fill area of the screw configuration may also be acceptable. 

As it seems that the dispersive mixing of the additives could be improved by increasing the melt shear rate, in the video conference the 3D simulation expert proposes a modification of the screw configuration as well as the screw speed. This is accepted by the team leader, a new MOREX simulation is done and the compounding extruder configuration is fixed. 

The twin-screw co-rotating extruder provides good compounding due to its unique features such as modular screw configuration, excellent mixing and independent feeding operation. 

Screw configuration selection with dispersive and distributive mixing. 

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