Pharmaceutical Manufacturing Handbook: Production and

1142 TABLET COMPRESSION
from a hopper. The fi ll volume is determined by the depth to which the lower punch
descends in the die. At this moment the particles of the powder or granulation
product fl ow with respect to each other, thus resulting in a close packing arrangement
and the physical characteristics of the material (particle size, particle size distribution,
density, shape, and individual particle surface properties) associated with
process parameters such as fl ow rate and compression rate, and the relationship
between the die cavity and the particle diameter will defi ne the number of potential
bonding points between the particles. The packing characteristic of the product to
be compressed is greatly affected by the shape of the particles. Since the product to
be compressed comprises components of different nature, the voidage of a closely
packed system is considerably changed.
When the upper punch goes down, its tip penetrates the die, confi ning the powder
or granulation product, letting the particle rearrangement stage to continue and
initiating the compression stage as the compression force is applied. As a result,
forces resulting from the compression force are transmitted through the interparticulate
points of contact created in the previous stage. The porosity of the powder
bed is gradually decreased, the particles are forced into intimate proximity to each
other, and stress is developed at the interparticulate points of contact. Once the
particles have formed contacts, they will deform plastically under the applied load.
Deformation of the particles will be characterized by elastic, plastic, fragmentation,
or a combination of these phenomena, which will depend on the rate and magnitude
of the external applied load, the duration of locally induced stress, and the physical
properties of the product under compression. When the particles are in suffi ciently
close proximity, they are bonded. Particles bond as a result of mechanical interlocking,
which is described as entanglement of the particles, phase transition at the points
of contact, and intermolecular forces, namely the van der Waals force, hydrogen
bonding, and ionic bonding.
After formation of the tablet by application of a compression force follows the
decompression stage, where the compression force is removed and the upper punch
leaves the die. Then, the formed tablet undergoes a sudden elastic expansion
followed by a viscoelastic recovery during ejection when the lower punch moves
upward.
6.6.5
EQUIPMENT FOR TABLET COMPRESSION
The equipment employed for tablet compression is generally categorized according
to the number of compression stations and dislocation mode. Therefore, eccentric
model presses have only one compression station (one die and one pair of punches,
upper and lower) while rotary models have multiple compression stations (each
station with one die and one pair of punches, upper and lower). The basic difference
between the two types of compression equipment is that for eccentric models the
compression force applied during compression is due to the upper punch whereas
for rotary models it is mainly applied by the lower punch.
A rotary tablet press machine (Figure 2 ) comprises a housing in which the compression
set and subsets (upper and lower roller assemblies) are mounted, the turret
head, the upper cams, the weight control assembly and the lower cams, the hopper,