Gravity Induced Solids Conveying

Most feed hoppers have a cylindrical top section and a truncated conical section at the bottom see Fig. 7.4. 

Theoretical and experimental work on the transport of bulk material started in earnest around 1960. A collection of experimental and theoretical work was presented at a joint ASME-CSME Conference on Mechanics Applied to the Transport of Bulk Materials [1] and the U.S.-Japan seminars on Continuum Mechanical and Statistical Approaches in the Mechanics of Granular Materials [2]. Books on particulate solids have been written by Orr [3] and Brown and Richards [4]. Review articles have been published by Richards [5], Wieghardt [6], and Savage [7]. Some of the pioneering work on flow of bulk solids was done by Jenike [8, 9]. 

Various types of flow that can occur Funnel flow in feed hoppers 

Both in arching and in piping, the material is consolidated to the extent that it can support the material above it and form an exposed surface. Thus, both these flow problems are typical of cohesive (non-free-flowing) particulate solids. This is particularly true for materials with a high, unconfined yield strength see Section 6.1.2 and Fig. 6.4(b). 

In the analysis of flow of granular material, two types of flow can be distinguished. The first is slow frictional flow where the particles remain in continuous contact with each other the internal forces result from Coulomb friction between contacting particles. The second type of flow is much more rapid the particles are not in constant contact with their neighbors. The energy associated with the velocity fluctuations is comparable to that of the mean motion. In this type of flow, the internal forces arise because of the transfer of momentum during collisions between particles. The constitutive relations for this rapid flow are rate-dependent. This type of flow, therefore, is referred to as viscous flow (sometimes just rapid flow). Steady, 

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The unit can be fed polymer in the particulate solids form or as strips, as in the case of rubber extrusion. The solids (usually in pellet or powder form) in the hopper flow by gravity into the screw channel, where they are conveyed through the solids conveying section. They are compressed by a drag-induced mechanism in the transition section. In other words, melting is accomplished by heat transfer from the heated barrel surface and by mechanical shear heating. 

Once the particulate solids have reached the feed port of the extruder, the material will flow down until it is situated in the screw channel. At this point, the gravity induced flow mechanism will essentially cease. In most extruders, the screw and barrel are placed in a horizontal direction and the role of gravity becomes a very minor one. In fact, in most analyses of solids conveying in single screw extruders. 

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