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TABLETING


MILLING TRIALS


SMALL-SCALE


Hartmut vom Bey & Barbara Fretter provide a comparison between a small-scale mill and roller compaction production unit


material. Sometimes the densification step for the small amount of material can be simulated by a compaction simulator as well. Using conventional small lab scale mills for milling the compacts results in different particle size distributions as the functional principle is not comparable to later production scale roller compactor.[1] To allow more reliable predictions during early development, the company has developed a small-scale mill (manual mill) that eliminates the described


W


FIG. 1. Ranking of process parameters influencing the particle size distribution of granulates


hen developing a dry granulate with Gerteis roller compactors, the first trials are performed with very small amounts of


difficulties. Te Small Scale Mill inherits the same milling principle as the company’s Pactor production machines. Te particle size distribution (PSD) from the Small Scale Mill provides reliable indications about the granule characteristics of the future production. Terefore, the mill is an excellent scale-up tool.


INFLUENCING FACTORS OF THE MILLING STEP ON PARTICLE SIZE DISTRIBUTION One important influencing factor on the PSD of granulates is, of course, the material itself. API and mixtures have a huge impact on the PSD.


Beside the material/formulation, roller compaction process parameters have also affected the PSD of the granulate. A ranking of the influencing process parameter for roller compaction is given in Fig. 1.[1]


Te most important


parameter is the ribbon density, which is a result of the specific roll force and gap used in the roller compactor. Tis applies for production roller compactors from Gerteis in the same way as for lab roller compactors. A higher ribbon density, which is achieved by increasing the specific roll force, results in stronger ribbons and therefore larger particles. Of course, the mesh size used has also an important influence on the final particle size distribution, because it limits the upper particle size. During development the influence of the mesh size is often examined.


INFLUENCE OF SPECIFIC ROLL FORCE ON PARTICLE SIZE DISTRIBUTION


Coming back to the influence of the specific roll force on the PSD, which has the major impact on the roller compaction granulate qualities and should therefore be the focus of the development work for the dry granulation process. In Fig. 2 Avicel 101 is taken as an example for this effect. Increasing the densification by using higher specific roll forces results in coarser particles, although the width of their distribution remains constant.


44 www.scientistlive.com


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