Hammer Mill
Milling is commonly employed to reduce particle size of granulations, bulk drug substances, and excipients to facilitate uniformity of powder mixes.
This process is also used to manipulate the dissolution profile of the dosage form. Following wet granulation, wet milling is often employed to improve the granule surface area for more efficient drying. Sizing of granulation is typically accomplished using either low energy mills (oscillating granulators) or high energy mills (hammer, conical and centrifugal impact mills). The hammer mill is the most common and versatile machine used in the manufacture of solid dosage forms. it consists of either a swinging or fixed rotor, which forces the material against a fixed screen. Various factors such as rotor shaft configuration (vertical/horizontal, fixed/swinging), granulation feed rate, blade type (hammer or knives), rotor speed, and screen size and type (mounting, screen thickness, rasping, or regular screens) are to be considered while scaling up a hammer mill process. The particle size of the milled material is smaller than the corresponding screen size since particles enter the screen holes tangentially. This effect is more pronounced at higher rotor speeds. Narrow particle size distributions are obtained at medium and high speeds compared to low speed. Several scale-up factors for Fitzpatrick hammer mills are summarized in TABLE 7.
a conical mill ( Comill )consists of a conical screen inside a milling chamber along with a rotating impeller. comills use less energy than hammer mills and are well suited for milling heat sensitive and difficult to mill materials. The impeller configuration(knife, round, or sawtooth edges), impeller speed, and screen size affect the properties of the milled product in a comill.