Compounding single-screw extrusion

At times, low- or high-intensity blending alone can produce a suitable product for use by the fabricator. An example of this would be a polyvinyl chloride (PVC) blend used for several large-volume extrusion applications. More frequently, however, a compounding process is required to obtain the desired physical property. The five primary compounding processes used in the industry for controlling the above parameters are single-screw extrusion (SSE), twin-screw extrusion (TSE), continuous mixers, batch mixers, and kneaders. Table 18.1 summarizes key aspects of each process. 

Melt compounding is most commonly utilized for the preparation of PO/silica nanocomposites. POs and their blends, such as PP [326-337], PE [338-343], ethylene-propylene copolymer [344-354], ethylene-octene copolymer [347], thermoplastic POs [348-350], PP/ EPDM [351,352], and PP/liquid-crystalline polymer (LCP) [353-357] blends, have been used as the matrices in the preparation of PO/silica nanosystems and nanomaterials by twin-screw extrusion and injection molding or lab-scale single-screw extrusion and compression molding. 

Complex profiles require specialty manufacturing skills to build, maintain, and operate extrusion dies as well as cooling and sizing equipment to deliver the exact dimensions required. Cubed compound, where the PVC grains are already broken down, can be run faster on simple single-screw extruders on account of the typical low melt temperatures. 

Wire and Cable. PVC has been used in wire and cable applications since Wodd War II, when the U.S. Navy demanded lower combustibility materials in construction. These products are manufactured by cross-head extrusion, usually from pellet compounds on single-screw extruders. Some line speeds are 1524 m (5000 ft) per minute (60 mph). The compounds are optimized for the requirements, including low temperature flexibility, high use temperature, especially low combustibility, weatherability, and high resistance to cutthrough. 

Not all ceramic compounds are put through single-screw and twin-screw extruders. If the idea is to achieve very high extrusion pressures, minimal contamination, short series, etc., an intermittent piston extruder can be the machine of choice. 

In compounding, single- and twin-screw extrusion facilitate the removal of entrapped gas or air, associated moisture or reaction byproducts which can arise in a formulation. Therefore, the equipment requires a low pressure decompression zone to enable volatiles to be removed under vacuum. Optimisation of the compounding equipment enables control of volatiles, facilitates diffusive and convective material transport and accommodates design and operational variables of the machine [4]. 

Single-screw extruders are commonly used for the conversion of polymers into prodncts such as pipe, cables, film and so on. It is used for compounding with a modification of the screw, feed zone and mixing zone. The compounding operation can be synchronised with product extrusion in a single operation. 

In a single-screw extruder, a screw is rotated inside a cylindrical barrel. The feed may be in the form of a pre-blend or dry blend, pellets or in the form of viscous melt in melt extrusion. The extruder acts as a pumping and mixing device. The extruder performs the additional functions of conveying and melting and also performs compounding and devolatilisation functions. 

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