The Dry-Blending Operation

The dry-blending operation is a critical process in case of low-dosage forms. In order to obtain a high degree of mixing, cohesive powder components have to be disagglomerated. For this purpose, it is often advantageous to proceed as follows  

sieving of the blend through a sieve with an appropriate mesh for disagglomeration, 

The shear forces at work during the sieving step are important for dis-agglomeration of the finer cohesive material and/or favoring contacts between finer and coarser particles. 

In the case of an active substance at very low dose, i.e., requiring a high dilution with the auxiliary substances (e.g., 1 100), the blending operation may be divided into two steps primary 1 10 dilution and then a second 1 10 blending step in order to obtain the final dilution. 

The content uniformity which can be obtained depends, according to established theoretical considerations, on the particle size of the active substance (19,20). As a rough estimate for the obtainable relative standard deviation 5 rei of the content of the active substance, the following rule can be applied, based on Poisson statistics  

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