Gerd Hüffmann discusses effervescent tablet compression using a rotary tablet press
There are a number of problems that often occur when compressing effervescent tablets. The effervescent powder is highly hygroscopic (it attracts water). This 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).
There 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. The 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. The 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.
There 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. Thirdly, 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.
The 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. The layer/coating of magnesium stearate prevents the powder/tablet from sticking to the punch faces. The disadvantage is the investment cost for the stearate system. However, the advantages are multiple. No rubber inserts are needed (only chrome-plated punch faces). The 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 lubricating oil, eg vegetable oil or almond oil, which is sprayed onto the lower punches with a spray gun. This oil lubrication helps to reduce the ejection forces in the dies, preventing cams, punches and tablets from being damaged. The disadvantages are that excess oil drops down to the bottom, and the machine needs to be placed onto a drip tray. Operating this way can be quite messy due to the oil in the drip tray. Also the felt/material will get worn and need replacing.
The alternative method involves an external stearate spraying system being used, to spray/coat the die walls (in addition to the punch tip) with magnesium stearate. The layer/coating of magnesium stearate prevents the powder/tablet from sticking to the die walls, thus reducing the ejection force. Again, the only disadvantage with this approach is the investment cost for the stearate system.
The advantages are: no special lower punches (e.g. with felt inserts) required; no drip tray required; and no contamination due to dripping oil.
There are a number of other general measures to be considered relating to the machine configuration.Firstly, pre-compression force measurement is a ‘must-have’ function to prevent tablets from capping. Ejection force monitoring meanwhile, is a ‘should-have’ option. Effervescent tablets are usually rather large (20mm to 25mm in diameter) and difficult to eject. The measured ejection force is a very good indication if there is sufficient lubrication between the tablet and the die wall during ejection.
Another ‘should-have’ option is a punch pressure- or dosage relief system, to allow the press to start up easily without stopping right away due to too high compression force, caused by dies becoming overfilled during a machine stoppage. With a dosage relief function, the dosing (powder fill) is reduced until the machine has reached operating speed, when the dosing increases back to the original values/settings.
Punches and dies
Punches should have EU1”-441 punch heads, offering a larger contact area on the punch head in order to provide increased compression time (dwell time). Meanwhile, dies should have an increased taper to reduce ejection forces.
When it comes to measures in respect to the tablet handling after the tablet press, there are also a number of considerations. As effervescent tablets are rather soft and brittle they cannot be de-burred/de-dusted in a mechanical de-duster. In some cases however, a vibrating de-duster can be used, after a thorough evaluation of the mechanical durability of the tablets.
After the tablets exit from the tablet press, they cannot be allowed to just drop down from the exit chute into a receiving drum or bin. The tablets will need to be supported right into a receptacle. Ideally a conveyor belt is placed under the tablet exit chute to transport the tablets directly to the packaging machine (in-line packaging).
Finally, the tablets need to remain in a humidity-controlled environment until they are packed into a tube or a strip pack.
Gerd Hüffmann is with Killian Tableting in Germany.
