Controlling your weight

A new solution sets out to improve the process by which correct tablet weights are maintained in a tablet press. By Jutta Hartmann.

Ensuring the correct tablet weight is the top priority when compressing tablets for pharmaceutical application.

To ensure that the tablet weight is always within the prescribed limits, modern tablet presses apply a constant compression force control. This ensures that the powder dosing is adjusted to maintain the tablet target weight if needed. In addition, tablets that are outside the individual limits for compression force and hence weight, are identified and rejected.

However, compression force control can usually not determine a weight drift due to a variation in bulk density, as it can occur during the processing of a large volume batch, as well as temperature drifts of the tableting equipment and tablet tooling.

Here, an automatic check-weighing device or IPC is used to sample tablets from good production of the press at regular intervals, which are then checked off-line for weight and other parameters.

The traditional offline sample sequence is 20 tablets every 20 minutes, of which typically 10 tablets are evaluated for their weight. If a weight drift is detected, the IPC will give feedback to the tablet press, in order to trigger a weight adjustment.

This procedure raises two questions. What happens within the 20 minutes it takes to process a tablet sample? And is it really necessary to take tablet samples, which are later discarded?

A newly developed system is aiming to address both of those questions and in doing so, bring great improvements to the entire process.

CWC (continuous weight control) is a new system to measure the weight of the produced tablets in-line in the tablet chute, without destroying them.

During tablet production, selected tablets are fed into the CWC from the good production channel of the press. They are then checked for weight and (if found to be OK) released as good product.

After 10 weighing operations, the average weight of the evaluated 10 tablets is compared to the set weight specifications and corrective feedback is triggered if necessary.

The weight measurement can be performed on tablets from individual punches per revolution or from a random sequence of punches. The system is integrated into the machine control system and measuring results can be used directly for machine regulation/adjustment.

After the weighing process the tablet is returned into the good channel of the tablet chute.

In-line tablet weight measuring provides the highest possible precision and continuity of the tablet weight even when there are variations in the bulk density.

It minimises the product losses, which is especially important when expensive ingredients are processed. Instead of a statistical process control, the CWC provides a continuous monitoring and therefore a higher level of certainty. No operator is necessary for the weighing process, which means that potential user errors are minimised.

It is possible to check up to one tablet per each turret revolution, with an accuracy of 0.1mg.

With the traditional ‘20 tablets every 20 minutes’ set-up, if the tablet press runs with a production speed of 200,000 tablets per hour and the batch size is one million, 300 tablets are sampled. This means that 0.03% of the batch is being checked. During the 20 minutes it takes to conduct the conventional weight test, the operator gets no information on the current tablet weight development.

Weight control with CWC means that during the same batch 22,000 tablets are checked, which is about 2.22% of the whole batch. This is about 74 times more than the traditional sampling sequence.

Due to the higher sampling rate, the reaction time of the process control is significantly reduced compared to the traditional approach and the process security is also increased.

Its makers hope that this new system could replace the traditional off-line tablet sampling for weight control in the near future.

For more information at visit www.scientistlive.com/eurolab

Jutta Hartmann is with Romaco Killian in Germany. 

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