Pharmaceutical Manufacturing Handbook: Production and

However, for mechanical compaction simulators the movement of the punches
is mechanically determined and, compared to hydraulic compaction simulators, not
freely programmable. Thus, for basic material characterization and early formulation
development, hydraulical compaction simulators can be advantageous.
6.4.7
INSTRUMENTATION OF TABLETING MACHINES 1067
INSTRUMENTATION OF TABLETING MACHINES
To describe the tableting process more precisely, tableting machines have been
instrumented since the middle of the last century. Measured values are force, displacement,
and temperature and they are always measured with dependence on
time. Thus time is another variable.
6.4.7.1 Force Measurement
The fi rst instrumentation of a tableting machine for measurement of upper punch
force was performed by Brake [60] . Thus it was for the fi rst time possible to visualize
the compression process with regard to force development which results from
the material stresses during tablet formation. Only shortly after that, similar measurements
were published by another research group [61, 62] .
Besides upper punch force, lower punch force, die wall force [63 – 65] , ejection
force [66] , and tablet scraper force can be measured. Die wall force measurement
will be discussed separately.
For measurement strain gages are mostly used. These strain gages consist of
constantan. They are applied in eccentric tableting machines at the upper or lower
punch holder and in rotary tableting machines at the machine frame or the compression
roll pin. Alternatively piezoelectric crystals can be used which have to be placed
inside the punches [40] .
The sensitivity of force measurement is dependent on the distance between the
force transducer and where the force occurs. Thus for force measurement, instrumentation
of the punches is more advantageous than instrumentation of the machine
frame. However, since punch and die sets have to be exchanged between different
runs of the machines, instrumentation of the punch holder, the machine frame, or
the roller pin is most widely spread.
The most often measured force is the upper punch force. For the eccentric
machine it is the force which controls densifi cation; for rotary tableting machines
upper and lower punch forces have ideally the same values. Schmidt et al. [67]
measured force with a single punch of a rotary tableting machine. Ejection force is
visible as a small lower punch signal which occurs shortly after the end of one compaction
cycle. It is measured by lower punch instrumentation but needs more resolution.
A review of force measurement is given by Bauer - Brandl [68] .
Die Wall Force Measurement During compression of the powder the forces are
evolving not only at the punches but also at the die wall [63 – 65] . Therefore die wall
force measurement complements upper and lower force measurement. Since the
compression process occurs axially, these radially evolving forces are smaller than
the forces at the punches. Measurement of die wall force allows, for example, for
indication on die wall friction, tablet capping, and lamination. Instrumentation for