The Selection of Milling Machine

Milling selection: Hammer and screen mill

Check to see if a low-cost technology can meet the requirements, One of the least costly types of milling machinery for food products is also the most frequently used. The hammer and screen mill style uses the impact of swinging hammers that rotate at high speeds inside a round housing to reduce the particle size. When the particles meet the targeted size, they fall through a screen and into collection. Screens are available in a variety of sizes and configurations to suit dry, sticky, fibrous or abrasive ingredients. Today's most effective hammer mills grind down to D90 <75 µm. If a finer end product is required, a more advanced impact systems may be considered.

Advanced systems achieve fine grinding down to D97 <35 µm. They replace the hammer rotor with a pin disc style rotor and force the material to collide with a series of pins set in concentric circles protruding from high-speed rotating discs. This style excels in milling crystalline, fatty and oily products such as spices that may buildup and blind screens. The latest models allow these fatty, oily and other heat-sensitive products to be processed without separate cooling equipment.

Other systems replace the hammers with knife rotors that granulate the material. These perform well for coarse granulation to medium fine size reduction and excel with softer products.

For finer milling of soft and/or heat-sensitive products, attrition mills offer an alternative. These mills grind the product between two, tapered plates that rotate at high speeds, reducing the particle size as the product moves from the infeed at the center to the periphery for discharge and collection.