Planetary ball mills share the same design as other basic ball mills – a grinding jar filled with media and rotated on its own axis. But their unique design takes advantage of centrifugal force and the Coriolis effect to grind materials to a very fine or even micron size.
These forces come into play as the grinding jar in a planetary ball mill rotates on its own axis in the opposite direction of the disc (commonly termed a sun wheel) it’s affixed to. These opposing movements, along with the difference in rotating speeds, result in the powerful combination of friction and impact forces required for the fine level of grinding afforded by a planetary ball mill.
Appropriate applications include a wide range of materials, such as ceramics, polymers, limestone, clay minerals, limestone, pigments paint and metal oxides. Like other ball mills, planetary ball mills provide a slight advantage in metal-free grinding, because an operator only needs to line the tank and use ceramic media to achieve a metal-free environment (as opposed to internally agitated ball mills, which require the arms and shaft to be sheathed, which is easily done).
Planetary ball mills are well suited to grind small samples in laboratory settings. But the great expense of scaling them up to full production size has so far limited the widespread use of planetary milling in actual manufacturing. In comparison, internally agitated high energy ball mills, such as a batch process Attritor, scale up easily and efficiently to large production machines capable of processing hundreds of gallons per batch. Attritors also can run in continuous mode, which is another advantage over planetary mills, which cannot run in continuous mode.
Because they employ stationary tanks that don’t require shrouding, attrition mills provide another significant advantage over planetary ball mills. The lack of shrouding allows the use of a water jacket on the tank in an internally agitated ball mill, providing cooling or heating to aid in the grinding process.
Unlike planetary ball mills, Attritors offer the advantage of being able to sample the material during milling without having to stop the process. Formulation adjustments and grind aid additions can be performed “on the fly” without having to stop the milling process.