Screw pushing flight

Several solids conveying models were developed by Campbell and his students at Clarkson University [19, 20]. These models will be referred to as either the Clarkson University models or the Campbell models. They proposed that the movement of the screw flight was pushing the polymer bed as the screw turns rather than the frictional force at the barrel moving the polymer pellets down the screw. For these models, they assumed that the solid bed behaved more like an elastic fluid rather than a solid and removed the torque balance constraint. Campbell and Dontula [20] reasoned that because the solid polymer pellets more closely resemble an elastic particulate fluid, no torque balance in the bed would be necessary. They further assumed that the force normal to the pushing flight was due to a combination of the force due to the pressure in the channel and a force proportional to the frictional force exerted at the barrel by the solid bed. The Campbell-Dontula model was first published as ... 

Figure 6.6 Melting profiles for a 63.5 mm diameter extruder running an ABS resin at 60 rpm for screws with a 8.89 mm deep feed channel, 6 diameters of feed section, and a metering channel depth of 3.18 mm (C = 2.8) (a) 8 diameters of transition section for R = 0.00342, and (b) 4 diameters of transition section for R = 0.00684 [13, 14]. The pushing flights are on the right side of the section photographs...

Figure 11.14 Flight radii and tangent points in a screw channel. The pushing flight R, in this case is estimated at 0.5 the depth of the channel (H) while the radius R2 at the trailing side of the channel is estimated at 1.5 times the channel depth...

The modified Campbell-Dontula solids conveying model was based on screw rotation and a normal force at the pushing flight that was directly proportional to the frictional force between the solid bed and the barrel wall. The force balance on the slab is provided in Fig. A5.4... 

Thus, as in SSEs, the pressure rises linearly in the directions of the pushing flight and pushing screw, reaching a maximum at the comer between them. However, the absolute pressure cannot be determined from the model unless the chamber is partially empty, where the pressure can be assumed atmospheric. Otherwise, the leakage flow must be considered and the pressure profile along the screw determined. 

Once in the melt pool, the fluid particle settles at some position in the channel and commences the circulatory flow alternating between two positions. In the upper portion of the channel, it moves toward the pushing flight and down-channel relatively quickly, whereas in the lower portion of the channel, it moves toward the solid bed (which also slides down channel) or trailing flight (if melting is completed) and down channel relatively slowly. This continues until it leaves the screw channel. 

The partially filled counterrotating screws advance the free-flowing particulates against the region of the pushing flights by metal-particulate frictional forces. The... 

Figure 10.8. Cross-section view of extruder screw, showing the melt pool and the solid bed against the screw s pushing flight.

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