The size of the output granulate is controlled by the diameter of the holes in the screen. Screen hole size is heavily dependent on material composition and machine size. When running at slower rotor speeds, smaller screen holes and a thinner screen will produce more uniform particle sizes and highest throughput. At higher speeds, larger screen holes are needed to achieve throughput and reduce fines, although they may increase the range of particle sizes, particularly with brittle materials.
Standard screens are typically made from hot rolled steel which is suitable for most general purpose applications. For more abrasive materials, heat-treated steel is used to increase wear resistance.
Thinner “perf” or milled back screens are used with materials that tend to bind in or clog the holes. The thinner material in the working area of the screen allows the rotor to push material through the holes.
For some beside the press granulators, it’s not uncommon to generate longer thin pieces of material called “longs” that can also clog screen holes. This can be addressed by screenless granulators. However, screenless units have their own limitations in granulate size control. Another way to address the issue is with angled holes in the screen that minimize the problem with longs, though this will somewhat reduce the throughput rate of a granulator.