Grooved Bore Solids Conveying

Grooved bore extruders have several disadvantages [42]. These include high levels of wear in the feed casing and especially on the edges of the grooves. The wear 

Barrel Diameter, mm Feed Channel Depth, mm Grove Width, mm Maximum Number of Grooves 

Grooved bore extruders are common on European extruders up to barrel diameters of about 120 mm and on some applications up to 150 mm diameter [44]. For a 60 mm diameter machine, the maximum feed channel depth for a PP resin application is about 5 mm and 6 mm for HDPE resin applications channel depths are not much larger than this for larger diameter machines. Ingen Housz and Meijer [44] 

Franzkoch and Menges [49] stated that the tip velocity of the flight should be In the range of 0.2 to 1.0 m/s for optimal grooved barrel performance. Designs should not be made with tip speeds higher than 1.0 m/s. For a 100 mm diameter extruder, a tip speed of 1.0 m/s corresponds to a screw speed of 190 rpm. 

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This chapter provides a description of the solids conveying process and the theoretical models in the literature. The literature models will be presented before the experimental solids conveying data because only recently has experimental data become available for this process section. That is, the early theoretical models were developed without actual solids conveying data. Data will be presented regarding the temperature and forces that are associated with solids conveying of different polymers. Next, a comparison of the models with the experimental data will be provided. Both smooth bore and grooved barrel feed sections will be presented. The field experiences of the authors, however, are dominated by smooth bore extruders. 

Spalding et al. recommend using a flight flank radius in the feed section of about 1/4 of the channel depth. However, the basis for this recommendation is not entirely clear because the radii tested were all larger (0.54 H and 1.71 H) than 1/4 of the channel depth (0.25 H). This recommendation may be appropriate for smooth bore extruders where the pressure development in solids conveying is modest. This recommendation most likely is not appropriate for grooved feed extruders because the pressure development in these extruders is usually substantial. 

If the actual solids conveying performance is as sensitive to the coefficient of friction as the theory indicates, small changes in the actual coefficient of friction can have a substantial effect on the entire extrusion process. This is particularly true for low values of the coefficient of friction against the barrel. The high sensitivity to the coefficient of friction will tend to result in unstable extruder performance. At high values of the barrel coefficient of friction the extruder performance will be less affected by changes in friction and the extruder will be more stable under these conditions. This explains why grooved feed extruders tend to be more stable than smooth bore extruders. 

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