Cutter Mill

The cutter mill (Fig. 1) is very similar to the hammer mill in operation, except that sharp blades are used in place of blunt surfaces. Hammer and cutter mills are especially suited for brittle or fibrous materials and can be considered the most commonly encountered mills for roller compaction (in dry granulation).

Cutter mill
Fig. 1: Cutter mill.

Construction and Working of Cutter Mill

The rotary knife cutter features a horizontal rotor equipped with two to twelve uniformly spaced knives along its periphery, rotating at speeds between 200 and 900 rpm. It is housed within a cylindrical casing that contains several stationary knives. A screen at the bottom of the casing regulates the size of the material discharged from the milling zone. The feed material should be less than 1 inch thick and should not exceed the length of the cutting knife. For sizes smaller than 20-mesh, a pneumatic product collection system is necessary. Under optimal operating conditions, the maximum size limit for a rotary cutter is 80-mesh.

A disc mill comprises two vertical discs, which can either rotate in opposite directions (double-runner disc mill) or have only one disc rotating (single-runner disc mill) with an adjustable clearance between them. The discs may feature cutting faces, teeth, or convolutions. The material, typically premilled to around 40-mesh size, is usually suspended in a stream of air or liquid before being fed into the mill.

The shear rates present in cutter mills are useful in producing a coarse degree of size reduction of dried granulations prior to tableting.

Advantages and Disadvantages of Cutter Mill

Cutter mills offer versatility, capable of handling a variety of materials, including fibrous, tough, and hard substances. They produce consistent and uniform particle sizes, and the output can be adjusted by varying the screen size and rotor speed. These mills are highly efficient, capable of high-speed operation leading to high throughput, and their simple design makes cleaning and maintenance straightforward. Additionally, they are scalable, suitable for both small-scale laboratory use and large-scale industrial applications.

However, cutter mills have some disadvantages. Their high-speed operation can lead to significant energy consumption and heat generation, which may affect heat-sensitive materials. They can also produce considerable noise and vibration, necessitating soundproofing and sturdy installation. The blades and screens can wear out over time, requiring regular replacement. Furthermore, cutter mills require the feed material to be less than 1 inch thick, which may necessitate pre-processing. Lastly, they are not suitable for achieving extremely fine particle sizes, with a practical limit around 80-mesh.

Reference:

  • Aulton, M. (2018). Aulton’s pharmaceutics, the design and manufacture of medicines. Edinburgh. : Elsevier.
  • Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd.