Closely Intermeshing Extruders

The iow speed extruder has ciosely intermeshing screw geometry where the flight profiie fits ciosely into the channel profile, i.e., a conjugated screw profile. A typical screw geometry of the closely intermeshing co-rotating (CICO) twin screw extruder is shown in Fig. 10.3. 

The conjugated screw profile shown in Fig. 10.3 appears to form a good seal between the two screws. However, a cross-section through the intermeshing region, shown in Fig. 10.4, reveals the presence of relatively large openings between the channels of the two screws. 

Therefore, the conveying characteristics of the CICO extruder are not as positive as those of a closely intermeshing counter-rotating extruder (CICT) see also Figs. 10.27 and 10.28. 

Cross-section through the intermeshing region of a CICO extruder 

The co-rotating twin screw extruder has a sliding type of intermeshing as shown in Fig. 10.5. 

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VII. MIXING EFFICIENCY AND MIXING DEFICIENCY IN CLOSELY INTERMESHING EXTRUDERS... 

For an isoviscous process material with Newtonian behavior, the filled length, expressed for self-wiping machines as a number of pitches and for a closely intermeshing extruder as a number of chambers follows from... 

For equal average residence times of fluid elements in the fully filled zone it is required that the movement of the channel remains constant. For closely intermeshing extruders this is easy to see because this is the time needed for a chamber to move from the beginning of the fully filled zone to the end of the screws however, this equation holds also for scale-up of other types of extruders. 

Co-Rotating Versus Counter-Rotating Closely Intermeshing Extruders... 

These two-phase polymers are generated in an optimized single-stage process using a corotating, close-intermeshing, twin-screw extruder. 

K. J. Gadzenveld and L. P. B. M. Janssen, Scale-up of Counter-rotating Closely Intermeshing Twin Screw Extruders without and with Reactions, Polym. Eng. Sci., 30, 1529-1536 (1990). 

Fig. 13.1. Different types of extruders, a) single screw b) co-kneader c) non-intermeshing, mixing mode d) nonintermeshing, transport mode e) counter-rotating, closely intermeshing f) co-rotating, closely intermeshing g) conical counter-rotating h) self-wiping, co-rotating.

Closely intermeshing co-rotating twin screw extruders occupy a dominant position among extruders and are used in a wide variety of applications. They are not only used extensively in the production, compounding, and processing of plastics, but also employed in large numbers in other branches of industry, e.g., the rubber and food processing sectors. 

The first closely intermeshing twin screw extruders were built by Bayer using their own design (Section 2.1) and featured a vertical arrangement of the screws. Figures 1.2 and 1.3 show such a screw arrangement built by Bayer for chemical reactions. 

Screw Elements for Co-rotating, Closely Intermeshing, Twin-Screw Extruders... 

From the time they were first developed, co-rotating, closely intermeshing twin-screw extruders have proven to be highly effective for the continuous treatment of viscous products and they have successfully replaced many discontinuous technologies. They will certainly continue to be successful in other markets in the future, facilitating new production processes. 

Fig, 5.25. Geometry cross section for closely intermeshing co-rotating twin screw extruders... 

To really control the mixture and dispersion of additives, or to do reactive extrusion, a twin-screw extruder is necessary. Twin-screw extruders are available in several different configurations, but for the work of compounding the most often used is a closely intermeshing co-rotating extruder, with a specially designed screw for the particular task. In order to accomplish this with the least amount of machining. 

The machines mainly used for compounding plastics, chemicals and food are co-rotating, closely intermeshing twin-screw extruders. Some 30 000 such extruders are in use worldwide, about 1/3 are ZSK from Coperion Werner Pfleiderer, Stuttgart. In the chemical industry increasingly more and more batch mixers are being replaced by continuous twin-screw knea-ders. 

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