Powder mechanics in hopper flows

The onset of powder motion in a hopper is due to stress failure in powders. Hence, the study of a hopper flow is closely related to the understanding of stress distribution in a hopper. The cross-sectional averaged stress distribution of solids in a cylindrical column was first studied by Janssen (1895). Walker (1966) and Walters (1973) extended Janssen s analysis to conical hoppers. The local distributions of static stresses of powders can only be obtained by solving the equations of equilibrium. From stress analyses and suitable failure criteria, the rupture locations in granular materials can be predicted. As a result, the flowability of granular materials in a hopper depends on the internal stress distributions determined by the geometry of the hopper and the material properties of the solids. 

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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. 

Computational mechanics, and in particular the DE method, may be successfully used to analyze a wide range of operations involving powders and granular materials. Of particular relevance in a pharmaceutical context are applications related to flow of powders in hoppers (66), fluidized beds (67), and mixers (68,69). The method has been applied in the analysis of ball mills (70) and may be used to look more deeply into certain processes such as vibration-induced size segregation of granular materials (71) and packing (72). 

Figure 3.26 (a) Mechanical discharge with concentric pressures (b) air injection and air slides promote mass flow (c) pneumatic filhng of powders causes almost flat top surface (d) expanded flow hopper gives mass flow only in the bottom hopper. 

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