Tablet Weight Control

The graph below shows the relationship between Force and Weight. To understand the graph, think about what will happen when continuously increasing the powder fill of consequential dies in a turret without changing the position of the compression rollers. In the beginning the compression force will increase slightly as only air is pushed out of the product. In the following dies, with increasing powder volume, the punches will bring the powder granules closer to each other. The powder will also rearrange in the die to adapt to the form of the punches. The force needed to reduce the powder to a constant height tends to increase but it only starts to really gain in magnitude when more powder is added and the particles bond into a solid.

In a real production environment the distance between the compression rollers is kept constant to have a uniform tablet thickness and hardness. The powder weight per die will vary even if the powder volume in the die is the same due to different particle arrangements in the die implying more or less voids. In addition individual tablet weights will vary according to the quality of powder flow, bottom punch length variations, powder scraping effect. Less powder weight in a die will result in less compression force for the same compression height versus higher compaction force for more powder weight. The assumption is that the powder density remains the same. Comparing actual compression force versus the nominal compression force will give an indication of the tablet weight. If the average compression force is deviating from the nominal the powder fill cam can be adjusted to bring the average compression force back to the nominal force.

The problem using Compression Force to trigger a weight correction and more to reject tablets is that the relation Force/Weight is expressed by the equation y=axⁿ. It is easy to understand why the equation is exponential. A lot more force is needed to reduce height of a consolidating product than it is to remove air out of the powder. The a and n values from the equation will not only change with different powders but also within different batches of the same powder due to e.g. the powder humidity, granule size, blending time...

This means that for every batch set-up, the operator needs to experimentally find the force limits corresponding to minimum and maximum tablet weight resulting in operator dependability and waste generation.

Force weight relationship

To overcome the cumbersome set-up of force controlled tablet weight, Courtoy developed and patented the displacement control. Instead of using the Force principle based on constant final compression height and variable force, the displacement principle uses constant force and variable pre-compression height. At final compression the tablets are compressed to a constant height.