Parallel twin-screw extruder

Counter-rotating twin screw extruders are used almost exclusively for the production of uPVC pipes, profiles and sheet because they treat material more gently than single screws. Parallel screw and conical twin screw extruders are compared. Cincinnati Milacron Austria introduced its Argos parallel twin screw extruders last year. These machines are intended to complement the company s Titan conical screw range. Coextrusion of foamed core pipes is also described. 

Figure 12.28 Comparison of intermeshing and nonintermeshing twin-screw extruders schematic summarizing the different types of parallel twin-screw extruders and applications.

Figure 12.29 Schematic of a parallel twin-screw extruder showing eight barrel sections and with major equipment components identified.

The relative comparison of processing parameters for the three basic types of parallel twin-screw extruders is displayed in Table 12.3 [55]. Definitions of the various processing parameters compared are given ... 

Parallel twin-screw extruders have more surface area compared to conical twin screws. However, the flight flanks in a conical have more surface area than in a parallel extruder and can transfer more heat to the material in the channel. The screw surface area is 40% larger compared to a parallel extruder. Due to the larger channel volumes, conical extruders generate less shear heat and more conductive heat compared to parallel twin-screw extruders, making them better for processing shear-sensitive materials such as PVC. 

The third type of compounding device is the extruder. Next to its use for fabrication (which will be dealt with in 11.4.2), it is applied as a mixer. In essence, it is a screw pump, in which the mass to be mixed is transported in a heated cylinder by a screw, or, with twin-screw extruders, by two parallel screws. During this transport melting and intensive mixing take place. At the end of the screw the blend is pressed through a number of openings and cooled down in most cases the strands thus obtained are on-line cut into granules. 

The twin-screw extruder may have parallel or conical screws, and these screws may rotate in the same direction (co-rotating) or in opposite directions (contra-rotating). Extruders with more than two screws are known as the multiple-screw extruder. These extruders are normally used when mixing and homogenization of the melt is very important, in particular where additives, fillers, and/or reinforcements are to be included in the plastic. 

Although there are very few twin-screw (TS) extruders in comparison to the many more single-screw extruders, they are used also to produce products such as window and custom profile systems. Their major use is in compounding applications. The popular common twin-screw extruders (in the family of multi-screw extruders) include tapered screws or parallel cylindrical screws with at least one feed port through a hopper, a discharge port to which a die is attached, and process controls such as temperature, pressure, screw rotation (rpm), melt output rate, etc.143... 

Many of these devices were developed in parallel with the evolution of twin-screw extruders (TSE) — in part to reduce the competitive advantage of TSE as far as their recognized capability for good mixing was concerned [Rau-wendaal, 1986 White, 1990]. 

Besides the single-screw extruder (SSE), there is the twin-screw extruder (TSE), that is, two parallel screws rotating within the same cylindrical housing [8]. 

Twin-screw extruder In the melting section of CORI extruders, virtually all the degradation mechanisms that can essentially be distinguished, such as quasi-steady drop breakup, folding, end pinching, and decomposition through capillary instabilities, take place in parallel Polente et al. 2001... 

Figure 6.22 TypicalarrangementofBerstorffgearmixingelementsassembledonaZE90A twin-screw extruder parallel gear rings (left-hand side) gear cut on a helix (right-hand side). Courtesy of BerstorfF.

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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