Conventional dry size reduction in the pharmaceutical industry is accomplished by impact. This impact size reduction generally falls into two categories: mechanical impact and impact via fluid energy. Examples of mechanical impact mills are hammer mills and pin mills; while spiral jet mills, loop jet mills, and fluidized bed jet mills are examples of micronizers or fluid energy mills.
The types of feed devices used in the industry vary from rotary valves to vibratory tray devices and screw feeders (single and twin screw types). However, as with any process, the product flow characteristics will often determine the best feed method. Since most pharmaceutical powders and formulations can have difficult flow characteristics, the twin screw feeder is often the method of choice.
Volumetric vs. Gravimetric?
Depending upon the accuracy of feed required, the screw feeder can be supplied in either a volumetric or a gravimetric configuration.