Twin Screw Extruders Applications

Twin screw extrusion of polymers is most often used for 

Compounding is the highest volume use for twin screw extruders. By compounding, we mean the incorporation of one or more additives into a polymer matrix for the purpose of tailoring material properties to Anal product use. There are numerous additives used for a wide range of property enhancements, including stabilizers, lubricants, color concentrates, fillers, and reinforcements. 

Reactive extrusion takes place when the twin screw system is designed to incorporate one or more chemical reactions during the process, such as grafting or even polymerization. Grafting is the process of attaching pendant molecules onto the backbone of a polymer chain. For example, a small concentration of maleic anhydride can be incorporated into polypropylene to make the 

FIGURE 14 37 Schematic diagram illustrating the changes in viscosity and type of mixing in a twin screw extruder. 

FIGURE 14-38 Schematic diagrams of different dispersions course (leftt) and fine (right). 

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A twin-screw extruder is generally preferred for producing rubber-toughened, glass-filled PET compounds for injection moulding applications. The PET and impact modifier are added at the throat while the glass reinforcement is added downstream. The size of the rubber domains will depend on the amount of energy and the capability of the equipment used for dispersion. 

Extrusion. The main applications of this method include the production of film, sheet, pipe, and tubing. PB is usually extruded by using the same equipment (single- cu twin-screw extruders) as that used foi PP and HDPE, at melt and die temperatures of 170- 190°C. PMP is processed on extruders with a high length-to-diameter ratio at temperatures of 240-300°C. 

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... 

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 two examples, deliberately chosen for their simplicity, show that computational fluid dynamics facilitate a more in-depth examination of the local flow behavior of twin screw extruders. Local peaks in the mechanical and thermal stresses can be easily identified. By changing the geometry, stresses can be reduced and the quality of the polymer can thereby be optimized. Another application focus is the rapid determination of the dimensionless axis intercepts for the pressure build-up A, and A2 and for the power requirement B, and B2. The significance of these parameters has already been discussed in detail in the two previous chapters. 

For a majority of applications, the modular design has been accepted for the twin screw extruder. This allows optimum adjustment of wear protection in the respective process zone and material selection for the corresponding process zones to ensure a maximum life span. 

The first twin-screw compounder (ZSK) went into production at Werner Pfleiderer in 1957, marking the beginning of a success story for this type of machine. The first major applications were in the chemical industry. Today, the machine is predominantly used in the plastics industry, e. g., in extrusion and compounding. These screw machines are therefore also known as extruders and the twin-screw is known as the twin-screw extruder. 

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... 

Beecher, E.D. Carr, M.E. Grille, J.G. Starch compounding on co-rotating twin screw extruder starch as an encapsulation medium for a controlled release application. SPE Antec Proceedings, Boston, MA, May 7-11,1995 2037-2041. 

The most common way for three-dimensional simulations is the application of the FEA. The 3D analysis of flow processes in co-rotating twin-screw extruders is very time-consuming, because for transient geometrical adjustment in the intermeshing zone a new volume mesh must be generated for each new screw position (see top of Fig. 5.26). 

Kohlgrilber K. Co-Rotating Twin-Screw Extruders Eunda-mentals, Technology, and Applications. Munich Hanser Publishers 2007. 

Co-Rotating Twin Screw Extruder (CTSE). This system is commonly used for compounding and profile extrusion in high output applications. CTSE with its high shear is, however, harder to control than the counter rotating system (see below). 

A continuous hydrolysis reactor utilizing a twin-screw extruder has been designed [47] that can be heated to a temperature of 300°C and has a provision for injection of water into the extruder at a point where the scrap is almost in the pulp state. Polyurethane scrap in powder form is fed into the extruder and residence time is adjusted to 5—30 min. Separation of the two components, polyether and diamine, in the product may be effected by fractional distillation, by extraction with a suitable solvent, or by chemical means. The PU foams made from these recycled products can be used in several applications, one example being protection boards for construction sites. Hydrolytic recycling has not, however, found much application, since virgin raw materials are cheaper than the regenerated products. 

Leister, D., Geilen, T., and Geissler, T. Twin-screw extruders for pharmaceutical hot-melt extrusion Technology, techniques and practices. In Douroumis, D. (ed.), Hot-Melt Extrusion Pharmaceutical Applications, 1st edn. Wiley, Chichester, U.K., 2012, pp. 23-42. 

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