High-speed single screw extruders

Very high speed single screw extruders have been commercially available since about 2005-2010. This is one of the most significant developments in single screw extrusion over the past several decades. These relatively small extruders (50-75 mm) that run at screw speeds as high as 1,000 to 1,500 rpm and achieve output rate about an order of magnitude above the rate of conventional extruders. 

Since approx. 1995, several extruder manufacturers have worked on developing high-speed single screw extruders (HS-SSE). Most of these developments have taken place at German extruder manufacturers such as Battenfeld, Reifenhauser, Kuhne, and Esde. Currently, HS-SSEs are commercially available and have been in use for... 

Over the past five to ten years very high speed single screw extruders have been developed. These extruders are now commercially available and they are used by dozens of companies around the world. These machines run at speeds up to 1500 rpm they achieve outputs that are about an order of magnitude above those of conventional extruders. This high speed single screw extruder technology is one of the most significant developments. Therefore, this topic has been added to the new edition in Chapter 2. 

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. 

For the mechanical blending in an extruder, particles of PBT and EPR were dried in a vacuum oven at, respectively, 120°C and at RT for 3 h. The components were first mixed in a high-speed blender, then extruded in a single-screw machine and pelletized. During the extrusion, a small amount of DCP was added to crosslink EPR. DCP did not affect the properties of the blend, although it eliminated the delamination. The surface of the sample was coarse. 

It is important to note that the back flow increases in proportion to the third power of the depth of the thread. For this reason, screws with deep cut channels are not the best choice for thermoplastics (but are okay with thermosets). If the speed of single screw extruders is increased, especially in the processing of high molecular weight viscous melts, the extrudate obtained may be rough, unattractive in appearance, and unsalable. Such results also can occur with slower-running machines using relatively deep cut screws in conjunction with extrusion dies of low resistance to flow. 

The analytical predictions compare well to numerical predictions. This indicates that the analytical equations can be useful in the analysis of melt temperature development in single screw extruders. The results indicate that the melt temperatures can become fully developed if the heat flux through the barrel is substantial and if the screw speed is not too high. When the screw speed is high and the consistency index large it is not likely that the melt temperatures will be fully developed at the discharge end this is particularly true for large diameter extruders. 

The barrel has to be cooled if the internal heat generation in the plastic raises the barrel temperatures above the set point. This is likely to occur when extruding high viscosity plastics and when running at high screw speeds. Cooling on single screw extruders is usually done with air. Blowers are placed imder the extruder barrel and temperature zones are partitioned, so that one blower cools only one temperature zone (see Fig. 11). 

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity. Extrusion pressures are lower than those encountered in injection molding. They are typically 500 to 5000 psi (3.5 to 35 MPa). In extreme cases, extrusion pressures may rise as high as 10,000 psi (69 MPa). Variants on the single screw include the barrier or melt extraction screw and the vented screw (Chapter 3). 

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