Research Articles
The effect of punch tilting in evaluating powder densification in a rotary tablet machine

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ABSTRACT

The effect of punch tilting on the mechanism of punch penetration in the die of a rotary tablet machine during the compression cycle was evaluated by installing four displacement transducers on one station of a rotary machine. Two transducers were symmetrically positioned beside the upper punch in the upper turret, and the other two transducers were similarly placed beside the lower punch in the lower turret. Microcrystalline cellulose and dicalcium phosphate dihydrate were compressed at 5, 10, 15 and 20 kN using two different machine speeds, in order to quantify the effect of punch tilting in the evaluation of punch penetration. These compression data served to construct the powder bed reduction curves, from which it was possible to establish that punch tilting is directly proportional to the compression force used. Tilting is maximal at the beginning of the dwell time, disappears at half dwell time, and reaches a new maximum at the end of the dwell time. In latter case, tilting occurred in the direction opposite that of the first maximum. The impact of tilting in powder densification behaviour, evaluated through the construction of Heckel plots, depends on the compression force used in the analysis. Heckel plots are as distorted as the compression force is elevated. Consequently, the calculated Heckel parameters differ from the real values. Unless a very low compression force is used, a proper Heckel analysis can be performed in a rotary machine only if it is fitted with a device that includes the effect of punch tilting in the evaluation of punch penetration. © 2007 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:1277–1284, 2008

Section snippets

INTRODUCTION

Tablets are the most common pharmaceutical dosage form, but while the compression of powdered or granular material into a cohesive mass may seem simple, it is actually a complex and irreversible dynamic process.

Mechanically, the process consists of imposing a progressive strain on the powder, confining it to a certain final volume and porosity.

The dimensional constraints imposed by the punches and die are then removed and the compact is allowed to relax.

The compacted material exerts a certain

Punch Displacement Device

One of the eight stations of a Ronchi FA rotary tablet machine was equipped with four LVDT transducers. They were symmetrically installed beside the punches (one preceding and one succeeding) in either the upper or the lower turrets, according to the chart in Figure 1. Punches were equipped immediately below their head, with two symmetrical narrow flat steel arms that ended just above the transducer. The remaining seven stations were blinded.

Displacement Transducer Set Up

Transducer calibration and validation were performed

Powder Bed Reduction and Punch Tilting

Figure 2 shows a plot of bed reduction inside the die without any material in it.

As can be noted, there is a small difference between the preceding and succeeding curves and this, of course, depends on the signals retrieved from the preceding and succeeding transducers. As expected because of the mechanism of punch tilting, both signals are incorrect. Values of punch penetration from the preceding transducers are underestimated, whilst values from the succeeding transducers are overestimated.

CONCLUSION

Punch tilting always occurs in a rotary tablet machine during the compression cycle and should be taken into account together with the machine deformation. The correct values of punch penetration in the die depend on both phenomena.

Like machine deformation, punch tilting also depends in practice on the compression force used, regardless of the type of material under study. This means that if the device installed in the rotary machine for monitoring punch penetration fails to assess the tilting

REFERENCES (9)

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    The tableting machine was also instrumented with load cells for the force measurement and 4 LVDT trasducers (2 for the upper punches and 2 for the lower punches) for the punches displacement recording. More details on force and displacement devices set-up and calibration procedure were previously described (Cespi et al., 2008; Palmieri et al., 2005). Preliminary test performed without the materials at 50% UR and temperature of 25, 35 and 50 °C did not highlight any effect of the environmental conditions, inside the working area of the tableting machine, on the transducers and load cells transmission apparatus (Figure SF1 in supplementary data).

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