When using planetary ball mill, how to select ball size and how much material should be taken in mill pot?
The ball size chosen depend on the impact energy requirement. Bigger balls help in size reduction if the feed particles are big like chunks. If the milling materials are powder, then smaller ball size is preferred.
There should be at least 25% empty place in the mill pot after feeding balls and materials.
There is no such thumb rule for powder to ball ratio. It is generally decided by nature of materials to be milled and ball size, ball to material weight ratio, milling time, RPM,etc