Pharmaceutical Manufacturing Handbook: Production and

980 TABLET DESIGN
Another reason, to use a rotary press rather than an eccentric press for tablet compression
is the dwell time is usually shorter on a rotary press (see Figure 3 ).
All compression mixes have an optimum compressing speed. This is why tablet
press manufacturers install variable speed controls for the rotor or turret and the
powder feeding mechanism. Many compression mixes are speed sensitive and will
not produce satisfactory tablets at inappropriate speeds. The dwell time, where
maximum pressure is applied to the mix, is relative to the peripheral speed of the
turret and the diameter of the punch head fl at. Any air in the compression mix must
be expelled to avoid laminating or splitting of the tablet. If air is compressed within
the tablet, when the pressure applied by the punch is released, the compressed air
expands and breaks the tablet. Precompression or dies with a taper in the bore will
Eccentric press
Lower compression force
Upper compression force
Rotary press
FIGURE 2 Punch movements and compression profi les of upper and lower punches of
rotary and eccentric presses.
1.0
0.8
0.6
0.4
0.2
Consolidation time
Dwell time
Effective contact time
Maximum UP force
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Residence time
FIGURE 3 Dwell time as function of tableting machine.