Self-Wiping Twin-Screw Extruders

In this section we consider the essential features of two of the most common types of twin-screw extruders self-wiping corotating (SWCOR) twin-screw extruders and closely intermeshing counterrotating (CICTR) twin-screw extruders. As discussed in Section 8.1.2, there are a vast number of types of twin-screw extruders, and it is not possible to discuss all of these here. Further discussion of the technology and theory for twin-screw extruders is given elsewhere (Rauwendaal, 1986 White, 1990). 

The constraint that the tip of one screw element wipe the flank of its mate in self-wiping, corotating twin-screw extruders leads to a unique relationship for the shape of the screw channel (Booy, 1978,1980). Figure 13 is an isometric view of this channel and Fig. 14 is a cross section of the channel in a plane that is perpendicular to the plane which defines the helix angle. Figure 14 shows the actual shape of the channel, which is described by the following expressions ... 

S. Bawiskar and J. L. White, Melting Model for Modular Self Wiping Co-rotating Twin Screw Extruders, Polym. Eng. Sci., 38, 727 (1998). 

C. D. Denson and B. K. Hwang, The Influence of the Axial Pressure Gradient on Flow Rate for Newtonian Liquids in a Self Wiping, Corotating Twin Screw Extruder, Polym. Eng. Sci., 20, 965 (1980). 

Self-wiping screw elements with the maximum possible number of threads give the largest free cross-section (see Fig. 2.5, Chapter 2). Thus, triple-flighted elements should be used in the intake zone of twin screw extruders with Dc/Dj < 1.3, and double-flighted elements in the case of D0/D, > 1.3. An intake zone length of 4 to 6 D is generally adequate for effective product feed. 

The enormous flexibility and variability of the system was possible thanks to the constant development of the process related components, such as the barrels and screw elements and new geometry variants being developed continuously. The initial aim of the design, to maintain the self-wiping function of the screw elements, is sometimes abandoned, in part or in whole, to permit innovative geometries. This enabled the co-rotating, twin-screw extruder to carry out the most diverse processes and handle the most diverse products with this single machine system. 

Other methods also exist for handling the reaction mixture. For instance, self-wiping, twin-screw extruder using an intermeshing has been proposed (271). The advantages claimed include higher saponification temperature (less catalyst), increased water solubility (271), and fewer impurities (272). 

Fig. 4 Self-wiping in the two and three lobed co-rotating twin-screw extruders. (Courtesy of Coperion Corporation.) (View this art in color at www.dekker.com.)...

Material transport in intermeshing, co-rotating twin-screw extruders is generally dependent on drag flow. The screws pick up the material as they rotate and, where the two screws meet, a complete transfer of the material from one screw to the other takes place, see Fig, 4. The tip of one screw wipes the flanks and roots of the other screw, resulting in a self-wiping action. As the material is transferred from one screw to the other, the direction of material flow is changed and new material surfaces are created with each screw revolution. 

Twin-screw extruder Dimensionless calculations of throughput and pressure gradients for flow of non-Newtonian fluids in screw and kneading disk elements of a modular intermeshing self-wiping corotating twin-screw extruder, as well as the temperature rise. Chen and White, 1992, 1993... 

Twin-screw extruders are important tools for the plastic industry. Their principal advantages (over SSE) are better feeding and more positive conveying, self-wiping of the screws, short residence times, a narrow residence time distribution spectrum, better mixing, larger heat transfer area, an easier scale-up, and large output. 

The concept of a self-wiping co-rotating twin-screw extruder dates to the beginning of the twentieth century (Figure 1.14) [77, 78]. However, an intermeshing co-rotating t vin-screw extruder was not commercialized until 1939. This was initiated by Roberto Colombo [79] and Lavorazione Materie Plastische (LMP) in Turin, Italy. 

Figure 1.14 Wunsche 1901 self-wiping co-rotating twin-screw extruder.

Generally, continuous mixers are superior to batch mixers. The compounds are more uniform have superior mechanical properties and much shorter residence times [135, 136]. The major continuous mixers used commercially in 2009 for compounding and blending are modular co-rotating self-wiping twin-screw extruders. 

Because the working of self-wiping twin-screw extruders is mainly based on drag in an open channel, the equations derived for single-screw extrusion can be used. Combination of drag flow and pressure flow in a single chaimel leads to an equation for the throughput per channel ... 

Here fp and f s are correction factors for the channel geometry, and Hq is the maximum channel depth. In case of rectangular screw chaimels the correction factors can be calculated analytically [Eqs. (2.6) and (2.7)] for the complex geometry of self-wiping extruders they can be approximated by Eq. (2.32) or they can be calculated numerically (6). The number of parallel channels in a screw with m thread starts equals 2w — 1. This leads to a throughput for a self-wiping twin-screw extruder of (6,7)... 

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