Correlation Between Compaction Force and Tablet Weight The correlation between compaction force and tablet weight on a multi-station

press will be valid only where the friction between the punches and their guides remains constant (i.e. the punches do not become tight); the overall length of the punches is constant; the tip geometry is constant; the die bores are uniform, and the pressure rolls are perfectly cylindrical and mounted centrally. The contribution of all these factors, with the exception of the friction between the punch and guide, to the total force measured on a stationary member of the machine, will be small. However, it cannot be ignored, and consequently there is no direct correlation between the compaction force and tablet weight for individual tablets.

The errors introduced by variation in the friction between the punches and their guides will, in most cases, consist in a gradual increase in measured compaction force for a given tablet weight. It is possible to monitor the frictional force for individual punches and to arrange for the machine to be stopped if it rises to a level which would invalidate the control settings being used. If the machine is kept clean and correctly lubricated the frictional force will remain constant and have no effect on any control system based on compaction force. In view of the fact that under correct operation conditions it will have no effect and even under adverse conditions it can be monitored and prevented from causing errors, it will be ignored in further consideration.

The effect of the other factors means that it is possible for slightly different levels of compaction force to result from tablets having identical weights, and conversely identical levels of compaction force can result from tablets of slightly different weights. This variation will be limited because the variables causing it have a fixed range, i.e. punch lengths and die bores are not changing. Once the machine is set up with a set of tooling, the range of variation is fixed. Similarly, the effect of punch tip geometry and the degree of eccentricity of the pressure roll will be limited for a particular machine and set of tooling. Consequently, the correlation between the mean compaction force and the mean tablet weight is valid for each individual granulation bulk density, machine, and set of tooling.

If the compaction force/tablet weight relationship is plotted for a number of individual tablets having a range of weights (all from the same material and made on

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Figure 8.3 Relationship between tablet weight and compaction force on a multi-station machine.

Each cross represents the compaction force and weight of an individual tablet

the same machine), a diagram similar to that shown in Figure 8.3 will result. The relationship between the mean tablet weight and the mean compaction force, over the range considered, can be obtained by plotting the best option through the points.

If the control system is to be used to reject individual tablets, where the relationship is not valid, the situation can be represented by Figure 8.4. This figure shows a plot of mean compaction force/mean tablet weight for a range of weights, slightly larger than the required control limits. The maximum acceptable tablet weight is indicated by the line ‘H’ and the minimum by the line ‘L’. To ensure that all tablets above and below these limits are rejected, it is necessary to reject every tablet which results from a compaction force lower than ‘A’, or higher than ‘B’. This means that

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Figure 8.4 Relationship between tablet weight and compaction force on a multi-station machine

where individual tablets are rejected. Each cross represents the compaction force and weight of an individual tablet

tablets represented by crosses, in the two shaded areas, which have acceptable weights, will be rejected. It has therefore been decided to refer to the tablets which are rejected by the force measuring system as SUSPECT, rather than reject tablets. The quantity of good tablets rejected will depend upon several factors. It is possible, depending upon the range between the limits, that with a good granulation and a machine and tooling in good condition, that few, if any, tablets will be rejected. In this case the fact that some may be acceptable can be ignored. However, the importance of maintaining the machine and tooling in good condition, if this type of reject system is in use, cannot be overemphasized. Figure 8.5 shows the effect a machine and/or tooling in poor condition can have on the quantity of acceptable tablets which would be rejected. The shaded areas in Figure 8.5a, which represents a

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Figure 8.5 (a) Machine tooling in poor condition, (b) Good tooling, well maintained machinery.

Each cross represents the compaction force and weight of an individual tablet

machine and tooling in poor condition, are much larger than those in Figure 8.5b, which represents good tooling in a well maintained machine. The amount of acceptable tablets rejected by a control system working under the conditions represented by Figure 8.5a, will, therefore, be much larger than that for Figure 8.5b. If the tablets being produced are of low value, the fact that a few good tablets are returned for rework will not be significant. However, if the tablets are of high value and the quantity of suspect tablets rejected is significant, equipment is available which is capable of weighing and sorting all the suspect tablets at high speed so that the good tablets can be recovered.

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NB. Figures 8.3, 8.4, and 8.5 are not taken from actual results but are diagram- matic, and the effects have been exaggerated to enable the situation to be more easily understood.

Tablet Sampling Device

This unit has been designed as a quality control or development tool. It enables individual tablets from particular stations to be sampled under normal operating conditions.

It uses a similar gate mechanism to that used in conjunction with the Sentinel or Micro PW. It can be used alone or in conjunction with the Sentinel. It cannot be used with the Micro PW as that unit already incorporates its own sampling system. Different parts must be supplied when it is to be used in conjunction with the Sentinel from those which are supplied when it is to be used alone.

The standard equipment consists of a control unit, a gate mechanism, a special take-off chute, and a pair of proximity detectors with a mounting bracket. The proximity detectors are used to identify the station which is to be sampled, and the mounting bracket will be specific to a particular type of machine. If they are to be fitted to an existing machine the stations must be numbered and a small hole drilled in the part of the turret containing the upper punch guides, between the last and the first station. On a new machine this operation would be carried out during manufacture. One detector counts the punches as they pass, and the other re-sets the count to one when it detects the hole after the last station. This part of the equipment is common to the options on the Sentinel for faulty station identification and individual tablet reject. If the sampling unit is being used in conjunction with a Sentinel with one of these options the signal can be shared, i.e. it is not necessary to fit two sets of detectors.

The signal from the detectors is fed to the control unit, and when the required station is detected the unit holds the information until that particular tablet reaches the take-off position. A signal is then sent to the gate mechanism to deflect the tablet into the sample section of the take-off chute.

The unit can be operated in various modes. It can be set to take sample tablets from one particular station. The station number is selected by a thumb wheel switch on the front of the unit. Alternatively, it can be set to sample sequentially from each station, or it can be set to take one sample from each station for one complete revolution of the turret.

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