Counterrotating extruders

Fig. 10.14 The location of four gaps in fully intermeshing, counterrotating extruders though which leak flows can take place. [Reprinted by permission from A. J. van der Goot, O. Poorter, and L. P. B. M. Janssen, Determination of the Degree of Fill in a Counterrotating TSE, Polym. Eng. Set, 38, 1193 (1998).]...

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 obvious difference between the co- and counterrotating extruder is that in the co-rotating geometry both screws rotate in the same direction, either clockwise or coimter clockwise. In the counter-rotating geometry, they rotate in opposite directions. In addition to geometry, this simple difference impacts the way the two screws interact. 

The combination of mixing and residence times also determines the response of the compatibilization reaction to variations in process parameters. Hettema (14) verified this by experiments for different systems consisting of polypropylene, high-density polyethylene, polystyrene, and PVC in corotating and counterrotating extruders, and he came to the following general conclusions ... 

An increase in screw speed has a positive effect on impact strength and elongation at break while it has no effect on Young s modulus. This tendency is the same for corotating and counterrotating extruders, but the influence of screw speed on impact strength is more pronounced in the corotating machines. 

Figure 13.15 Conversion as function of the throughput for BMA polymerization ( ) self-wiping corotating extruder (Z) = 40mm) and ( ) closely intermeshing counterrotating extruder (Z) = 50 mm).

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... 

With counterrotating extruders, the screws at any common point have the same helix angle but are oppositely pitched. Some intermeshing types feature conical rather than parallel, constant-diameter screws. The large diameter of such screws at the feed end promotes heat transfer and allows greater... 

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