Tablet excentric press

Fig. 7 Density distribution in a compact prepared on an excentric tablet press.

Fig. 8 shows the output from force and displacement transducers fitted to both punches of an excentric tablet press. The upper punch describes an approximately sinusoidal path as it descends to penetrate the die (point A) and then rises after the compression event has taken place, leaving the die at point B. The lower punch remains motionless during the compression event and then rises to eject the tablet from the die (point C). 

Fig. 8 Force and displacement data from an instrumented excentric tablet press.

There are two types of tablet press. The excentric press has one die and one pair of punches. The rotary press has a larger number of dies which are fitted, with their corresponding punches, into a rotating turret. 

It should be noted that this discussion assumes that only one punch is actively applying the force to the powder mass while the other is stationary and passive. This is true in the case of an excentric press, but with a rotary tablet press, both punches move and hence both exert forces on the powder bed. The force distribution so obtained is thus different from that shown in Fig. 7, and results in two low density zones near the faces of the tablet and a high density zone in approximately the centre of the powder mass. 

Transducer output from a rotary tablet press differs in two aspects. Firstly, the lower punch plays an active role in the compression event and moves upwards as the upper punch moves downwards. The second difference is small but important. Because of the sinusoidal movement of the upper punch in an excentric press, the punch speed is only zero at the instant when it reverses direction (point E). Punches on a rotary press have a flat area on the punch head. As the punches pass under or over the pressure rolls, the flat area dictates that there is no punch movement. The period during which this occurs is called the dwell time, and though it only lasts a fraction of a second, it can have a major effect on the consolidation process. ... 

It has long been known that for some formulations, changing the speed of operation of the tablet press or changing the type of press can have a profound effect on tablet quality. Such circumstances may arise, for example, when changing from a slow excentric press in formulation development to a high speed rotary in production. 

The use of compressed pills, as they were then known, increased rapidly. It is likely that the term tablet for this dosage form was first used in the United States in the 1870s. Power-driven presses replaced Brockedon s hammer, and by 1874 there existed both rotary and excentric presses, which in their mode of operation were fundamentally similar to those in use at the present time. The tablet lent itself to mass manufacture by mechanical means, in contrast to the slower labour-intensive production of older solid dosage forms such as the pill. It is impractical for individual pharmacists to produce small quantities... 

Table 4 Tablet tensile strengths (MPa) produced by a compression pressure of 80 MPa on an excentric press...

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