Assumption of Stationary Screw and Rotating Barrel

The theory developed up to this point is based on a model where the screw is stationary and the barrel rotates around the screw. It is assumed that the flow that results is the same as when the barrel is stationary and the screw rotates in the opposite direction. This assumption was considered valid for over fifty years until several workers challenged this assumption, first in the early 1990s [272-276], and then more recently [323]. Because of the importance of this issue we will critically analyze this assumption to determine to what extent the assumption is correct. Flow will be analyzed using the parallel plate assumption with either the barrel or the screw considered moving. Flow will also be analyzed without the parallel plate assumption, using a cyhndrical coordinate system, again considering both cases. This analysis is based on a study by Osswald et al. [281]. 

1 Parallel Plate Analysis with Moving Screw 

The parallel plate analysis with moving barrel and Newtonian fluid was already discussed in Section 7.4.1. If we take the barrel stationary and the screw moving at velocity v t = tiD N, the velocity profile can be determined from Eq. 7.196 using the 

The maximum tangential pressure gradient occurs when there is no net output from the extruder. The maximum pressure gradient can be determined from  

If it is assumed, again, that the leakage flow is negligible (V = 0), the pressure gradient can be determined to be  

---