Poor flow materials

The compressibility index values reported in Table 2 were obtained by tapping powder until a plateau value was obtained. As can be seen from Fig. 4, 2000 to 3000 taps were sufficient to obtain plateau conditions in most cases when the material was relatively free flowing. For poor flowing materials, however, 7000 taps were required. 

Some of these limitations are overcome by utilizing a short screw feeder to deliver material into the inlet of a steep screw conveyor, see Fig. 2.9. Preferably this should be of the end delivery type in order to provide a direct infeed pressure. This feeder will give more accurate regulation of the feed rate, provide a reliable feed for poor flow materials, and restrict gross overloading of the incUned screw cross-section on restart from a full screw condition. However, once this approach is adopted, the inclined screw may as well be run at a faster speed to clear the incoming amount and the whole character of the application then changes. 

A bulk material which behaves in a fluid manner when very dilated, will flush out to a level surface of fill, as can be seen in Fig. 5.3, and display hydrostatic pressure conditions until settled. Poor flow materials, such as damp and cohesive powders, achieve steep filling repose angles. Therefore, appropriate allowances must be made in calculating holding volumes and fixing the location of level indicators. Coarse, firm, nonadhesive products tend to settle quickly, to form a stable bulk structure, when they come to rest after pouring. Such bulk materials attain consistent density conditions and repeatable angles of repose , virtually independent of the manner in which the slope of the pile is formed. 

Various techniques are used to promote good hopper flow with poorly flowing materials. A vibrating pad may be used at the base of the hopper to inhibit bridging. An auger... 

In contrast to the cylinder section, a hopper has changing cross-sectional area over its height. In most cases the hopper is converging, so its cross-sectional area is smaller at the bottom than at the top. However, some hoppers used to store poorly flowing materials such as wood chips have a diverging geometry. 

When a poorly flowing material is placed in a silo which was not designed to store and handle it, flow stoppages due to arching or ratholing are likely. Sometimes these obstructions will clear by themselves, but, more often, operators will have to resort to various (sometimes drastic) means to clear them. No matter which method is used, the resulting dynamic loads when an arch or rathole fails can collapse the silo. [26]... 

In order to produce an adequate tablet formulation, certain requirements, such as sufficient mechanical strength and desired drug release profile, must be met. At times this may be a difficult task for the formulator to achieve, due to poor flow and compactibility characteristics of the powdered drug. This is of particular importance when one only has a small amount of active material to work with and cannot afford to make use of trial-and-error methods. The study of the physics of tablet compaction through the use of instrumented tableting machines (ITMs) enables the formulator to systematically evaluate his formula and make any necessary changes. 

Compressibility is indirectly related to the relative flow rate, cohesiveness, and particle size of a powder. A compressible material will be less flowable, and powders with compressibility values greater than 20-21% have been found to exhibit poor flow properties [69]. 

If a material or mixture is compressible (but exhibits poor flow), one can use dry granulation. 

Incompatible Mixtures. Even at very low levels, many of the poly-ether additives led to incompatible mixtures. These blends were not successfully milled to a smooth sheet under any conditions tried. Instead, a mass of crumbs was obtained. These crumbs could be molded into a coherent mass, but the physical properties were poor. For example, addition of 8.75 parts of polybutene-1 oxide to Masterbatch B for CPVC alone gave a brittle, free-flowing material with these properties notched Izod impact strength, 0.7 lb/in notch, flow rate 452 g/10 min. This is a particularly interesting result, since PBO has the same chemical formula as PTHF but structurally is a substituted ethylene oxide polymer rather than a linear homopolymer. No further studies were made of such blends. 

Material flow and compressibility. A free flowing, highly compressible material such as microcrystalline cellulose may be used for drugs with poor flow or compressibility properties. 

Heat must also flow through the walls of a device. DEP utilises electric field inhomogeneities which means that the fields (and heat production) within the liquid tend to be quite localised. The external heat flows can occur over much wider areas so it is sometimes possible to use even poorly conducting materials such as ordinary glasses and quartz. In critical cases, silicon, which has a high thermal conductivity (150 W/m s °C), is used. 

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