Producing solid dose tablets is the bread and butter of the pharmaceu- tical industry. Two-thirds of all pre- scriptions are dispensed in solid dose
form; fully half of these are compressed tablets.
The reason solid dose compressed tables are the
most popular form is two-fold: cost and speed.
The cost to produce these tablets is minimal
and tablets can be produced at speeds of up to
50 milliseconds a tablet. This ensures maximum
production which equates to maximum profit. To
ensure maximum tablet production is maintained,
a bulk density test is an important metric. The
bulk density of a production batch needs to be
closely monitored as any change can cause a loss
in production. To further leverage a bulk density
measurement, two of the more common gages for
flowability used in the pharmaceutical industry
are the Hausner Ratio and Carr Index. Fill and tapped bulk
density results are used to compute these values.
A common method to compute a tapped bulk density is
to manually tap a cylinder of material a certain number of
times and calculate the change in volume. Due to the human
element involved and interpretation of the change in volume,
this measurement can be somewhat subjective and produce
an erroneous bulk density reading. If a Hausner Ratio or Carr
Index is used to gage powder flow, this incorrect calculation
of bulk density will cause these values to be erroneous.
Thus, in solid dose tablet manufacturing, a change in bulk
density can quickly bring the production run to a screeching
halt. A scientific, accurate test for bulk density is impera-tive…and does exist. We will look at an accurate method for
defining fill and final bulk density and, subsequently, a more
accurate way to compute a Hausner Ratio and Carr index.
THE IMPORTANCE OF BULK DENSITY
Bulk density is by far one of the most important tests in
the bulk solids industry and certainly in tablet manufacturing. It is used to compute fill bulk densities and tapped
bulk density (or final compaction). It can also be used
to gage a product’s flowability by calculating the Hausner
Ratio or Carr Index using this single value as a correlation.
A Hausner Ratio greater than 1.25 is considered to be an indication of poor flowability. A Carr index greater than 25 is
considered to be an indication of poor flowability and below
15 is an indication of good flowability.
In the manufacturing process for solid dose
tablets, an accurate bulk density calculation is
critical. This is due to the fact that tablet making equipment is calibrated to exert the correct
amount of force on a powder after it has flowed
into a die. If the bulk density of the material
changes due to environmental conditions such
as increased temperature or humidity, this
change may cause bad product to be produced.
If the bulk density increases, the tablet making
equipment, which was calibrated to produce a
certain force, may not have sufficient strength
to compress the powder properly to produce
a tablet; one possible consequence is that it
will split or cap. Conversely, if the bulk density
changes to the low side, too much force will be
exerted during production and the tablet being
formed will be crushed. At high production vol-
umes, this can be a lot of lost revenue in a big hurry.
The old tried, but hardly true, method for checking bulk
density is accomplished by performing the tapped bulk density test. A cylinder of a known volume is filled with material;
this volume divided by the weight of the powder sample is
the fill bulk density. The cylinder is then tapped, usually manually, for 100 or 150 taps or until the operator determines the
level of powder will no longer compact. The reduced volume
of powder in the cylinder divided by the weight of the powder sample is used to calculate a tapped bulk density.
The problem with a manually tapped bulk density test
is the potential subjectivity involved in operator judgment
and the possible lack of a uniform tap force during the test.
The operator stops tapping when it appears that the level
of powder in the cylinder is no longer changing. Repeating
the test a number of times may help to confirm that the
measured value is correct, but operators pressed for time
sometimes go with the result from a single test.
What is needed is a scientific bulk density test method that
delivers repeatable results without direct operator involvement. Shear cell technology provides the solution through
the use of an annular shear cell and a wall friction lid.
SHEAR CELL TECHNOLOGY: BULK DENSITY,
COMPRESSIBILITY INDEX & COMPRESSIBILITY
Shear cells have been around for decades and are accepted in the bulk solids industry for providing an accu-
■ By Vinnie Hebert, Product Manager - Powder Flow Tester, Brookfield Engineering Laboratories
Improvements to Carr Index and Hausner Ratio calculations
Bulk Density Testing
Figure 1: Brookfield Powder Flow
Tester with Shear Cell