Volution Flow Theory

 

Powder flowability is defined as the ease with which a powder will flow for a specified set of conditions. Powder is generally defined as a collection of individual solid particles surrounded by gas phases. This includes granular materials, bulk solids, pelletized materials, etc. An accepted method for quantifying powder flowability is the Mohr-Coulomb Model. The Mohr-Coulomb Model is a limit state or "Go/ No Go" model and can be used to accurately predict flow behavior. This model quantifies powder flowability with two measurable parameters, Cohesion and Angle of Internal Friction, and two derived parameters, Unconfined Yield Strength and Major Consolidation Stress.

Cohesion is a measure of particle to particle bonding strength. This bonding strength results from various inter-particle forces generated by electrical charges, van der Waals forces, moisture, etc. The Angle of Internal Friction is a measure of the force required to cause particles to move or slide on each other. Internal friction is influenced by many parameters including particle surface friction, particle shape, hardness, particle size, etc. distribution, etc. Cohesion and Angle of Internal friction are determined by measuring a powder's yield locus. The Yield Locus is a graph of the shear force require to cause a powder to yield or fail relative to compressive load. Cohesion is the intercept of the yield locus and the angle of internal friction is the slope.

Yield Locus

Shear Stress versus Normal Stress

The Unconfined Yield Strength is the shear stress needed to fail or fracture a consolidated powder mass to initialize flow. The force used to consolidate the powder mass is called the Major Consolidation Stress. In other words, the unconfined yield strength is a measure of the strength of a powder mass when the powder experiences major consolidation stress. The Unconfined Yield Strength is calculated using the below formula:

 

A Flow Function Plot can be generated by plotting a powder's Unconfined Yield Strength versus Major Consolidation Stress. The flow function plot is a quantitative measure of the flowability of the powder. The inverse of the slope of the flow function plot can be used as a flow index. Generally, the closer a powder's flow function is to the x-axis, the more easily the powder will flow. The Volution is used to measure a powder's cohesion and angle of internal friction at various loads to generate its flow function and thus quantify its flow behavior.