Commercial Twin Screw Extruders

Since twin screw extruders have become an important segment of the field of extrusion it is reasonable to discuss important aspects of commercial twin screw machinery. A good reference on capabilities of twin screw extruders is the book edited by David Todd [43]. In this book, some of the main suppliers of twin screw extruders describe their machinery and capabilities the companies are Leistritz, Werner Pfleiderer, Berstorff, APV (Baker Perkins), Japan Steel Works, Welding Engineers, Parrel, and Buss. The book describes both co- and counter-rotating twin screw extruders and internal mixers. Another good reference on twin screw extruders is the book by James White [44]. This book gives an excellent historical over- 

In 1957 W P introduced the first high speed, co-rotating, twin screw compounding extruder and has been a major player in the twin screw extruder industry ever since. W P offers a wide range of sizes from as small as 25 mm to as large as 380 mm. Data on the various W P extruders is shown in Table 10.3. 

Model Torque per screw [N-m] Motor power at N ,3, [kW] Max. screw speed [rpm] Screw diam. [mm] Flight depth [mm] Max. L/D ratio Screw shaft height [mm] Machine dimensions, iength, width, height LxWxH (mm) Totai weight [kg] 

WLE = World-Lab-Extruder MC = Mega Compounder SC = Super Compounder 

1975 Change from triple- to double-flighted screw geometry (3 generation) 60% increase in volume 

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FIGURE 14-32 A commercial twin screw extruder (Source Cincinnati Milacron). 

Although commercial twin-screw extruders can be as large as 300 mm size, capable of compounding up to 40 ton/hr, the acmal type and size of the equipment used depends on the type of the polymer blend and the production volume. Normally, for engineering polymer blends, twin-screw extmders of about D = 90 mm size (L/D = 30 to 40) and capable of compounding at 700 to 1000 kg/hr, are used. For blending PVC or elastomer blends other types of compounding equipment are used, e.g. Farrell continuous mixer (FCM), Buss co-kneader, or a batch mixer, such as Banbury. 

Fig. 3-5. Different types of commercial twin screw extruder mechanisms are used they differ widely in operating principles and functions.

Several commercial products of PVC/TPU blends are available. The BF Goodrich Chemical Group has a PVC/ TPU blend based on their Estane series TPUs. For example, their Estane 54620, a polyester-based TPU with a °ShA 85 hardness, shows excellent compatibility with flexible PVC. The blends are produced by mixing PVC, TPU, plasticizer, stabilizer, and lubricant in a twin-screw extruder. These polymeric blends show intermediate mechanical properties between PVC and TPU. 

The twin-screw extrusion (TSE) technology is utilized extensively in the plastics and food industries to complement continuous manufacturing of products. Ghebre-Sellassie et al. [37] and Keleb et al. [36] have cited the application of this technology in the continuous production of pharmaceutical products. Commercially available TSE provide the great Lexibility required for effective continuous mode of operation. The schematic of such a system is shown in Figure 23.6. The twin-screw extruder... 

Mours, Flecke and Kohlgrueber compared four commercially available twin screw extruder modeling programs [5] ... 

Several commercially available programs from different manufacturers are suitable in principle for the calculation of flows in twin screw extruders they include but are not limited to Cfx, Fluent, Polyflow (respectively Ansys) and Star-CD (CD-adapco). This list includes the programs tested and used by the author. The program manufacturers offer a number of introductory courses to instruct users in the effective use of the solver and postprocessing. These are recommended for all CFD newcomers. 

Reactive processing is limited to polymerization or chemical reactions of polymers in conventional singlescrew or twin-screw extruders, excluding processes in oscillatory kneaders, Banbury-type continuous mixers, or Diskpack equipment. Emphasis is placed on continuous processes that have been implemented commercially or that can serve as models for commercial purposes. 

The co-rotating twin-screw extruder is commercially available with both 2- and 3-lobe cross-section geometry... 

These new PTFE-PA materials can be processed without any problems on commercial processing equipment like twin-screw extruders and injectionmoulding machines. The specific adjustment of the melt viscosity could be realised through additives (lowering of MFI additive [10,18] increase of MFI acid-terminated oligoamides [9,10]). The new materials combine the good material properties of PTFE and polyamide with the processability of polyamide and are particularly well-suited for wear-resistant maintenance-free bearings. 

The first patent on the precursor of the TSE dates from 1869. However, the first commercial CORI for handling polymers was marketed 70 years later. Several intermeshing counter-rotating twin screw extruders (ICRR) were developed in the 1920-30 s. After the 2nd World War, the corotating, fully intermeshing TSE (CORI) started... 

A different approach was used by the Ferruzy Company, the main difference being the use of high boiling-point plasticizer instead of water for the destructuration of starch. In this technology, starch was plasticized together with polymers such as polyethylene-vinyl alcohol (EVOH), EAA, poly-e-caprolactone, with small amounts of moisture, in a twin-screw extruder [49], to produce an intimate mixture between starch and the synthetic polymer. The commercial trade name of this product family is Mater-Bi . 

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