Hoppers gravitational flow

There are generally two types of gravitational flow in bins and hoppers [Fig. 4.7(a), 4.7(c)] mass flow and funnel flow. In mass flow, the whole mass of particulate solids... 

The most frequently used extruder is a plasticating extruder. Referring back to Figure 8.4, polymer pellets are fed to the extruder by means of a hopper (sometimes the pellets are metered in). The gravitational flow of solids in the hopper is rather complex and will not be covered here. The pellets are compressed in the channel of the screw and then dragged... 

In polymer processing practice, we need to ensure that the particulate gravitational mass flow rate of the hopper exceeds, over the complete operating range, the extruder open discharge rate (i.e., the rate without any die restriction). That is, hoppers must not be the production-rate limiting factor. Second, and more importantly, it is necessary for stable extrusion operations and extruded product quality that the flow be steady and free of instabilities of the particulate flow emerging from the hoppers. Finally, as we will see in Chapter 9, we need to know the pressure under the hopper in order to determine the pressure profile in a SSE. 

Flow in hoppers depends on their type (gravitational or forced), bulk density of the material, the shape and location of the feed inlet (direct or tangential feed, straight or with a chamfer on the down-going side), as well as on the type of screw and the barrel. 

This result is obtained by assuming the flow in the hopper to be one-dimensional and further assuming that at the orifice the convective acceleration in the upper converging flow is equal to the gravitational acceleration g, appropriate for the freely falling particles below the orifice level. 

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