Controlling tablets' individual weight is critical to tablet pressing. Two factors that are difficult to control, yet are
fundamental to a tablet's bioavailability and clinical effect, are tensile strength and disintegration rate. This article
will examine a recent development in tablet compression that enables consistent tablet production. The article will explain
how the development's flexibility can take into account the critical factors in tablet production and improve tablet quality.
Traditional tablet presses measure a tablet's weight either by measuring its variation in tablet height at precompression
or the force at main compression. These techniques, however, overlook one fundamental characteristic: tensile strength. A
tablet's tensile strength influences its bioavailability and clinical effect, which are crucial characteristics for any dosage
Compression to equal thickness
All rotary tablet presses on the market use the same compression principle. They compress a volume of powder, captured in
a die, between two rollers using an upper and lower punch.
By changing the distance between the rollers, one adjusts the force used to compress tablets. Once the distance between the
rollers is set, the compression force will stay the same, thus ensuring that tablets are compressed to an equal thickness
Several factors affect the precise amount of powder that is captured within each die. For example, granule size, size distribution,
and variation in punch length can have an effect. In addition, an excessively high rotational speed allows the powder insufficient
time to fall into the die. Variations in the amount of powder in each die result in tablets of different weights and densities,
and these tablets require different forces for compression to ET.
Figure 1 shows a simple force graph for different tablet weights. Tablet weight variations cause different forces to be applied
to the tablet because of the fixed distance between the rollers. Note that although the weight tolerance is symmetrical, the
forces are not. The force limits are clearly visible in the figure.
Khan and Rhodes showed that the magnitude of the compression force affects tablet hardness and disintegration time (1). An
increase in compression force produces an increase or decrease in disintegration time, depending on the formulation. These
results show that an inconsistency in the maximum compression force can result in inconsistent tablet properties, thus affecting
the efficacy of the dosage form.
Compression to equal force
Compression to equal force (EF) is a new concept that allows tablets to be compressed at the same peak compression force,
independent of tablet weight. This method relies on an air compensator. (see Figures 2 and 3).
The air compensator is installed at the precompression and main compression stations. The assembly is attached to a compression
roller mounted on a piston. The piston moves vertically in a cylinder filled with compressed air. The air pressure in the
cylinder is preset and kept constant by a pressure-regulating valve and an expansion chamber. Because the surface of the cylinder
and the air pressure are constant, the force is also constant.
If the air pressure in the cylinder is such that this maximum compression force is higher than the actual compression force,
the system compresses tablets to ET. On the other hand, the press can be set up so that the distance between the two rollers
and the air pressure in the air compensator make the compression roller move up at each compression.
In this arrangement, all tablets are compressed at the same peak force, which is determined by air pressure and the distance
between the two rollers (see Figure 4). The thickness of individual tablets might vary as their weight varies. Variance is
measured and kept within limits, however, to ensure that no problems arise during packaging. The tablet press is equipped
with a linear variable displacement transducer sensor that measures compression-roller movement accurately.
A heavy tablet causes the compression roller to move more, a light tablet causes less movement. When compressed to EF, variations
in tablet weight do not affect tensile strength. Therefore, tablets maintain a consistent clinical efficacy.
The compression roller's upward movement also has another important advantage: it increases the total compression time (i.e.,
the dwell time). The more the compression roller moves, the longer the dwell time.