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 6.2 Photograph of resin solidified in the transition section after a Maddock solidification experiment for an ABS resin. The pushing flight is on the left side of the photograph...

Maddock s and Street s famous solidification experiments revealed that melting in many situations takes place in a specific order. The experiments showed that after melting began, there was a continuous solid bed and a melt film over the inner barrel surface. Later the solid bed was completely surrounded by melt. Farther downstream, a melt pool developed between the pushing flight and the solid bed. The... 

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 11.13 Degradation at the pushing flight radius for LLDPE and a 152.4 mm diameter extruder with an 18 mm deep metering channel and a 5 mm flight radius. The flight radius was...

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... 

This is the velocity that is to be used in Equation A6.56. The film width at the pushing flight is calculated in a similar manner as the film thickness at the trailing flight surface. 

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. 

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