Insufficient Weight Uniformity in Tableting

A common issue in tableting is the lack of uniformity in tablet mass. Although the goal of any tableting process is to produce tablets with consistent weight, technical limitations exist in reality: high-speed weighing systems capable of accurately weighing or dosing each individual tablet are (currently) not available. Therefore, dosing in practice is volume-based – meaning a defined amount of powder is filled into the die and then compressed. However, quality requirements refer to the weight. Differences in bulk density as well as incompletely filled dies inevitably lead to some variation.

The acceptable extent of such variation is defined by the applicable pharmacopoeias. If tolerances are exceeded, deviations in active ingredient content may occur – potentially affecting efficacy and safety. For this reason, rapid and uniform die filling is crucial. In many cases, granulation is performed prior to tableting to improve flow properties. The goal is to ensure the most reproducible dosing behavior possible.

For powders with a wide particle size distribution, segregation must additionally be avoided – for example, due to mechanical vibrations in the feeder unit. Larger particles may settle at the bottom and be dosed unevenly. Decoupling the feeder from the press can help. A common rule of thumb is: the largest particles should not exceed 20% of the die diameter. If necessary, a downstream screening or milling unit may be required.

The use of forced die filling can also be beneficial. While gravity filling relies on powder falling into the already opened die by gravity, in forced die filling, the lower punch initially remains at the level of the die table. It is only lowered to its final position beneath the filling unit. As a result, the material is actively “carried along,” which is especially advantageous for poorly flowing powders – and allows for higher tablet press speeds.