Grooved barrel sections

Since these benefits appeal to most extrusion processors, grooved barrel sections have become quite popular. Also, the use of grooved barrel sections allows processors to extrude materials that cannot be processed on conventional extruders, e.g., very high molecular weight poiyethyienes and powders. A grooved feed housing is shown in Fig. 7.18. 

Solids conveying rate versus barrel coefficient of friction 

When the coefficient of friction at the barrel is increased to a value about two or three times the coefficient of friction at the screw, the solids conveying rate increases substantially. At the same time, the slope of the curve is reduced. When 4 4, 

Despite the disadvantages, extruders with grooved barrel sections have found widespread acceptance in Europe. In many cases, the grooved barrel section has become a standard instead of an option. Somewhat surprisingly, the acceptance of grooved barrel sections in the U. S. has been quite slow. One reason may be that U. S. extruder 

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Extruders equipped with grooved barrel sections often have a specially designed feed throat section to accommodate this grooved section. A schematic feed throat section is shown in Fig. 3.12, where the effective length of the grooves may range from three to five diameters. 

The benefits of a grooved barrel section can be analyzed from the theory of drag induced solids conveying developed in Section 7.2.2. Figure 7.19 shows how the solids conveying rate varies with the coefficient of friction on the barrel 4, for a situation where the pressure gradient is zero. 

Metal-to-solid polymer wear occurs in the solids conveying zone of the extruder. The introduction of the grooved barrel extruder has significantly increased the interest and concern about wear in this portion of the extruder. Grooved barrel sections substantially increase the shear and normal stresses between the polymer solid bed and the metal surfaces. As a result, grooved barrel sections are much more susceptible to wear than smooth barrels. The first systematic study of wear in the solids conveying section of extruders was made by Fritz [1 Ij. He used a diamond-shaped specimen that protruded into the screw channel. 

The grooved barrel section has to be cooled well enough to avoid premature melting of the plastic in the grooves this reduces energy efficiency and adds to the complexity of the extruder. 

With a grooved barrel section, the motor load increases and with it, the torsional load on the extruder screw. 

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