C-shaped chamber

Li and Manas-Zloczower (31) used the CFM commercial FIDAP FEM package to simulate the three-dimensional isothermal flow patterns and distributive mixing in three consecutive filled, closed C-shaped chambers of fully intermeshing, counterrotating extruders, having the dimensions of Leistritz 30.34 (30 denotes the centerline distance and 34 the barrel diameter in mm units). An equal pressure drop per C-shaped chamber was applied for the calculations. The melt was assumed to be Power Law above y() and Newtonian below it. The design, process, and material variables are given by the authors. 

The velocity in the down-channel direction, Fig. 10.15(a), indicates that most of the fluid undergoes circulatory flow, as expected. A circulatory flow is also generated in the plane perpendicular to the helical surface of the C-shaped chambers, Fig. 10.15(b). Velocity vectors indicating calender-gap leakage flows, Qc, are shown, without using them to obtain leakage flow rates. The interaction between the two circulatory flows eliminates the possibility of a stagnant layer at y/H = 2/3. 

Fig. 10.16 The evolution of the spatial distribution of 10,000 particles initially clustered in 10 randomly placed clusters in the C-shaped chamber. The fraction or number of pitches denotes the axial advance of the material in the chamber due to the counterrotation. [Reprinted by permission from T. Li and lea Manas-Zloczower, A Study of Distributive Mixing in Counterrotating TSEs, Int. Polym. Process., 10, 314 (1995).]...

Fig. 10.28 Flow diagram representation of the reaction model of Gadzenveld et al. (39). Each hox represents a C-shaped chamber, and the arrows represent the total leak flows from chamber to chamber.

The open area in the wedge part between the two screws is different. In counterrotating designs, there are closed C-shaped chambers with a very small volume such that no material transfer from one screw to the other takes place. In this type of design the distributive mixing efficient is greatly reduced. 

Material transfer from one screw to another, axially open extruders. Melt is conveyed by frictional forces, allowing for partially filled melting zone There is no transfer of material from one screw to another closed C-shape chambers are developed. Melt flow is plug type. Materials see a wide range of pressures Herrmann and Burkhardt 1981... 

The closely intermeshing twin-screw extruders, both counterrotating (e) and corotating (f), can best be modeled as series of C-shaped chambers. Due to the rotation of the screws these chambers transport the material from hopper to die, while... 

If the chambers of a closely intermeshing twin-screw extruder are fully filled with material, the maximal throughput of a zone, Q,/, can be written as the number of C-shaped chambers (Fig. 2.7) that is transported per unit of time, multiplied by the volume of one such chamber ... 

Both corotating and counterrotating twin-screw extruders can be modeled by means of a C-shaped chamber model. However, there are some... 

Also the volume of a C-shaped chamber can easily be determined by straightforward calculations. Therefore, we subtract the volume of the screw over the length of one pitch from the free volume of the barrel over the same length. The volume of the barrel follows from Fig. 2.13 ... 

A closer look at the two types of extruders shows that the closely intermeshing counterrotating twin-screw extruder has a better ability to build up pressures, even in an environment with low-viscosity liquids. This is due to the nearly closed C-shaped chambers, which provide a positive displacement action that enhances stability. Self-wiping extruders, on the other hand, are generally capable of much larger throughputs at comparable residence times. This leads to a preference of counterrotating extruders for... 

The pressure generation in the C-shaped chamber can be determined from ... 

The drag-induced pressure generation in the C-shaped chamber plus the pressure rise due to the diehead pressure should equal the pressure drop through the calender gap ... 

FIGURE 8.26 C-shaped chamber in a counterrotating closely intermeshing twin-screw extruder. 

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