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described - the powder must be tested again with a disk
having a larger hole.
Physicomathematical Interpretation
It is simple to demonstrate the direct relation between the
radius of the hole and the internal-friction coefficient -i.e.,
the viscosity - of the powder. Using K as the internal-
friction coefficient, r as the radius in centimeters of the
smallest hole that allows the powder to flow freely, and d
as the nontapped bulk density of the powder in grams per
milliliter, we can easily obtain
K s 490
Here, K is expressed in dynes per square centimeter, or
poises; and 490 is equal to one-half the acceleration of
gravity, or Y2
The weight of the cylinder of powder that is compelled to
fall must be greater than the friction on the side surface of
the cylinder itself:
d . g 2 2
where
h =
height of cylinder of powder
volume of cylinder of powder
9 8 0 cm/sq set (acceleration of
gravity)
=
side surface area of powder cylinder
coefficient of friction per square
centimeter
Figure 2: When loading is completed, 30 set should be
r d
r
h K
Simplified, the equation reads:
It can also be said that a powder having viscosity K and
nontapped bulk density d is sure to fall freely if
For example, a powder with a density of 0.5 g/ml passing
through a hole with a diameter not smaller than 24 mm
(i.e., r = 1.2 cm) has a viscosity (or shearing strength) of K
The force initiating flow for powders having good flow-
ability is the weight of the powder cylinder. Such powders
flow from the top until the plane is inclined in such a way
as to stop the flow.
Test Acceptability Limits
The limits that ensure good fractionation or granulation
of the powder - that is to say, that ensure a low average
weight - depend on machine type, working conditions,
and powder composition. Our experience, however, allows
make the following generalizations:
With Zanasi (Fratelli Zanasi SpA, Bologna, Italy)
and MG 2 (MG2 Macchine Automatiche SpA, Bo-
logna, Italy) capsulating machines, a good range is
within disk-hole diameter limits of lo-24 mm.
With tableting machines, limits of the disk-hole size
are 50%-120% of the diameter of the punch used.
Figure 3: A veryflowable
powder
small-diameter holes, leaving a cavity shaped like an
upside-down, truncated cone.
Teledyne Hanson Research Document 99-380-001 Rev. 06-19
r
r
d
will slowlyjlow through the
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