Extruder partially filled screw channel

It is critically important that the screw channel under the vent port is only partially filled with plastic this is true for single as well as twin screw extruders. Figure 67 shows a cross section of a partially filled screw channel. The vent opening has a tangential design this is beneficial in minimizing accumulation of plastic melt in the vent port. 

When devolatilization processes are conducted in screw extruders, the screw channels are only partially filled with the polymeric solution to be stripped of the volatile component (see Fig. 5) while the unoccupied portion of the screw channel serves to carry away the evaporated liquid. Because the barrel has a component of motion Vbz in the down channel direction, the solution is caused to flow from the extruder inlet to the outlet, which, in this case, is out of the plane of the paper. The crosschannel component of the barrel motion, Vtx, has two effects. First, it causes a circulation of the fluid in the nip and because of the continual... 

Fig. 6. Drag flow shape factors for partially filled channels in a single-screw extruder.

Fig. 14. Partially filled channel of intermeshing, corotating twin-screw extruder. Mass transfer occurs firom the films on the screw and the barrel and from the surface of the nip.

A low-viscosity feed stream also influences the working of the feed zone. In this partially filled zone, material with low viscosity is not likely to be dragged properly along the channels of the screw but tends to flow on the bottom of the extruder because gravity forces predominate rather than viscous forces. A dimensionless parameter that relates gravity and viscous forces is the Jeffreys number ... 

In starve feeding the material is metered into the extruder with a feeder. As a result, there is no accumulation of material at the feed opening. The first several turns of the screw are partially filled with material without any pressure development in this part of the extruder. The screw channel does not become completely filled until some distance from the feed opening at this point, the pressure will start building up in the extruder. In effect, starve feeding reduces the effective length of the extruder. 

A comparison of a Maillefer type barrier screw with a conventional screw in Section 7.5.1, showed for the barrier screw, a reduced output rate when used with PP and an increased output rate when used with LDPE. Following the discovery that the metering zone of a Maillefer type screw was running only partially filled when extruding medium density polyethylene (MDPE), Qui and co-workers [25] evaluated a grooved feed zone to ensure the metering channel was filled. 

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