Abstract
A method is desbribed for evaluating the flowability of cohesive powders on the basis of their yield loci. The measured yield loci are represented by the Warren Sqring equation that is defined by the three parameters K (the ratio of cohesion to tension), n (the shear index) and δ' (the slope of the straight line joining the end point of the yield locus to the origin). One of important properties of a cohesive powder is the unconfined yield stress f which is defined by the two parameters K and n. The unconfined yield stress varies with the major consolidating stress σ1 defined by the three parameters K, n and. δ′ The ratio σ1/f called the “failure function” by Jenike, however is not commonly a fixed value for the cohesive powder.
In this work the flowability of the cohesive powder is expressed by the ratio of the unconfined yield stress f to the corresponding bulk density γ. This flowability means the maximum height that the powder layer having an unit cross-sectional area is able to stand vertically under the force of gravity only. The value of f/γ varies with the major consolidating stress σ1, and an empirical equation of f/γ=aσ1b is confirmed for 13 kinds of cohesive powders used over the range of σ1 less than 100kPa. The two coefficients a and b are characteristics representing the index of flowability for the cohesive powder.
