Causes for Weight Variations during Tableting

Weight variations of the tablets produced is a relatively common problem of the tableting process. But each tableting process aims at producing tablets with a constant weight. The actual cause for this problem is the lack of weighing systems that are sufficiently fast to weigh or dose the required weight for each single tablet. Since this is not possible, each tablet press doses a certain amount of powder into the die and this powder is then pressed into tablets by the upper and lower punch. This means that a volume is dosed, but the quality requirement is the weight. Hence, weight variations in a limited extent are quite normal due to variations in the density of the powder material and to a partially incomplete filling of the dies.

The pharmacopoeias specify the acceptable level of weight variations. If the weight variations are too high the level of active ingredient in each tablet might be too high or too low and then the tablets don't comply with the specifications any more. It must therefore be aimed at tableting a powder that can be dosed quickly and consistently in the dies. For this reason very good flow characteristics are extremely important. That is why tableting often is preceded by a granulating process in order to give the feed material better flow characteristics than the physical mixture has. Is the particle size distribution of the powder rather broad a separation of the components of the powder must be prevented that might occur for example because of vibrations in the feed container. Hence it has to be prevented that the bigger particles are dosed first because they slipped downwards and the smaller particles afterwards. In this case a mechanical decoupling of the press from the feed container might be helpful. A further rule of thumb is: the biggest particles should not exceed 20 % of the diameter of the die. The upstream granulation should produce no bigger particles; if necessary a further grinding/sieving process must be carried out.

But sometimes the situation can also be improved by means of a forced filling. Usually the lower punch already is pulled down before the die reaches the area of the filling unit. This means that the material falls into the die only due to gravitation. In the case of a forced filling however, the lower punch is flush with the die table. The lower punch is pulled down into its target position only below the filling unit. The material is sucked into the die. This often makes it possible to work with a higher pressing speed even if the material doesn't flow optimally.

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