78 TABLETING
two-tip tool configuration results in a 19% increase in tablet strength, and another 19% increase in strength from a two-tip to a
Fig. 2. Tablet breaking force versus compression force
the amount of powder in the die cavity and the distance between the upper and lower punch tips. Assuming the concentric pressure rollers and punches are within working length tolerance, the resulting compression force will be similar for each turret station if the powder is uniform in fill. It is of special importance that the multi-tip punch working lengths are within tolerance since these force systems have no way of determining which tip is causing the issue.
Real-world case study At the Natoli Institute for Industrial Pharmacy Research & Development located in the Arnold and Marie College of Pharmacy at Long Island University in Brooklyn, New York, a series of experiments were conducted to study the compression force impact from a single-tip, two-tip and four-tip 0.25” (6.35mm) round tool. Te tools are illustrated in Fig.1. A direct compression blend consisting of 25% w/w of APAP and 74.5% SMCC 50 with 0.5% magnesium stearate was used for the study.
A compaction profile study
was performed on a benchtop rotary tablet press at 25rpm turret speed. Tablets were collected at varying force levels for each tool configuration. Resulting tablets were measured for thickness, weight and breaking force. Fig. 2 depicts the compaction
profile for the three tools. It is clear that as the number of tips are increased, the compression force of multi-tip tooling must also increase to achieve the same tablet breaking force. One might think that since the area of the tool has doubled, it would require twice the force to achieve the same tablet breaking force as illustrated in Fig.3, but this may not be the case. Fig. 4 is the compaction
profile normalised for the tablet geometry and punch tip area. Tis gives a clearer picture of the resulting tablet strengths at different applied compaction pressures. If in fact the force is doubled when the tool area is doubled then the plots would superimpose. At the same compaction pressure, the
four-tip configuration. In this case study, the applied pressure or compression force would be less than double to achieve the same tablet strength.
Ejection
After the tablet is compressed, the ejection cam will push the lower punch upward to eject the tablet. Te ejection force is a combination of the residual radial die wall and coefficient of friction between the tablet belly band surface and the die wall surface. Excessive ejection force levels will cause premature wear to the lower punches, cams and may cause visual striations on the tablet’s belly band. Multi-tip punches increase the belly band surface due to the increased number of tablets. Tis inherently increases ejection force levels and the likelihood of having the aforementioned issues. Common ways to reduce ejection force include: increased levels of powdered lubricant in the formulation (i.e. magnesium stearate); tapered dies; and lined dies of different materials. Although this was a relatively small case study, it provides enough intriguing information about the necessary compression forces for multi-tip tooling to continue further testing on a larger scale, which Natoli is now doing. As manufacturers look to reduce costs and increase productivity, wear on tablet presses and tooling is always a concern. With the potential ability to reduce compression forces necessary to produce quality tablets, multi-tip punch opponents may have one less reason for concern.
Fig. 4. Tablet strength versus compaction pressure
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For more information ✔ at
www.scientistlive.com/eurolab Fig. 3. Pressure calculation
Robert Sedlock is director of technical training and development at Natoli Engineering.
www.natoli.com
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