Compressibility

In most cases, the bulk density of a material varies continuously as a function of the consolidating pressure acting on it. Therefore, it is not sufficient to describe a material simply in terms of its apparent density or tap density. Instead, this density-to-pressure relationship can be measured (Ref 7), and the results are often expressed as a straight line on a log-log plot (Fig. 18). In the bulk solids literature, this relationship is often called compressibility, although this term has another definition in the P/M industry.

Fig. 18 Bulk density as a function of consolidation pressure (metal powder). Increasing consolidation pressure increases the bulk density.

The following variables can affect the bulk density of a material:

• Moisture: Higher moisture content usually makes a material more compressible.

• Particle size and shape: Often, the finer the bulk solid, the more dense and compressible it is. The shape of the particles can affect how they fit together, as well as their tendency to shear while being compacted.

• Temperature: Some materials become more compressible as their temperature increases.

• Particle elasticity: Elastic materials tend to deform significantly when they are compressed.

Some of the uses of compressibility data are:

• Wall friction angle: Bulk density values at various points in a hopper are used to calculate the pressures acting perpendicular to the hopper wall. After running a wall friction test, the wall friction angle is determined for a variety of pressures and used to calculate limiting angles in a mass flow hopper.

• Feeder design: To calculate the loads that act on a feeder or gate, one must know the bulk density of the material at the hopper outlet. Knowing this density also helps in sizing a volumetric feeder and choosing its speed.

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