80 TABLETING
Solving the challenges of tableting
Gerd Hüffmann discusses effervescent tablet compression using a rotary tablet press.
T
here are a number of problems that often occur when compressing
effervescent tablets. Te effervescent powder is highly hygroscopic (it attracts water). Tis causes the following problems: the tablet powder becomes sticky (sticks to punch faces and die walls); or the tablet expands after compression, making it difficult to eject the tablet from the dies (high ejection force required, which leads to increased mechanical wear on cams).
Tere are some precautions/ countermeasures that can be taken. With regard to the environment, controlling the humidity in the tableting room (the entire room, not just the inside of the tablet press!) is paramount. Te ideal conditions are <20°C and <20% humidity.
With regard to the tablet press itself, to counter the sticking problem of effervescent materials to the punch tips, there are two main options. Te first is the ‘traditional’ method, where upper and lower punches are fitted with rubber inserts, so the powder will not stick to the punch tips easily. When the tablet is compressed the rubber insert is compressed as well. When the tablet is released (from the compression force) the rubber (previously compresses) expands and this ‘expansion movement’ separates the powder/ tablet form the punch tip.
Killian’s stearate spray system.
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Tere are three disadvantages of this approach. Firstly, the rubber wears out over time and needs replacing. Next, the hardness of the rubber must match the operating condition of the press (higher compression forces
requires a harder rubber insert, lower compression forces require a softer rubber) otherwise the rubber does not ‘move’ enough between contraction under pressure and expansion after the pressure is released. Tirdly, as the rubber is being used and wears out, its surface becomes rough and hence the surface of the tablet may show some roughness too.
Te second option is the use of an external stearate spraying system, such as the Killian PKB 2300, is used to spray/coat the punch tips with magnesium stearate. Te layer/coating of magnesium stearate prevents the powder/
tablet from sticking to the punch faces. Te disadvantage is the investment cost for the stearate system. However, the advantages are multiple. No rubber inserts are needed (only chrome-plated punch faces). Te tablet surface is always smooth and shiny. Also, hardly any stearate needs to be mixed into the granule/powder, which means less cost for stearate/ improved tablet hardness /shorter dissolution time of the tablet.
To counter the high ejection forces inside the die there are also two choices. Here the traditional method involves fitting the lower punches with either a felt or a woven material to absorb
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