What Is a Bead Mill?
Bead Mills are machines for agitating and pulverizing wet powders and other media using particles called beads, which enable very fine grinding down to the nano-level. They are a technology generally manufactured within Japan.
The history of bead mills dates back to the postwar period in Japan, when Igarashi Machine Manufacturing (now AIMEX) completed Japan’s first bead mill based on technology owned by DuPont de Nemours, Inc. and mixed with its own original technology. After that, it went through a period of expansion due to Japan’s rapid economic growth, and a period of maturity after the Heisei era, leading up to the current state-of-the-art bead mill.
Uses of Bead Mills
The main example of actual use of bead mills in the market is the grinding of various objects. The objects are food, metal oxides for glass layers on the surface of ceramics, iron oxide for magnetic tape, barium titanate for ceracon in laminates, and various other objects, which are used in many grinding applications.
Recently, nano-dispersion needs have been increasing. For example, the bead mill is being utilized to disperse particles of 100 mm or smaller in the dispersion of organic pigments used in LCD color filters, zirconia for hard coatings, and titanium dioxide for cosmetic applications.
Principle of Bead Mills
In a wet bead mill, a vessel is filled with a slurry in which beads and particles are dispersed, and the slurry is agitated at high speed by a stirring rotor, causing the beads and particles in the slurry to collide with each other, resulting in particle size reduction.
Bead Mills make it possible to adjust the amount of energy imparted to the particles by changing the size of the beads used.
For example, grinding hard particles or reducing them to a smaller particle size requires strong energy, which requires relatively large beads to be brought into contact with each other at high speed.
On the other hand, with smaller beads, the collision energy is lower, but the processing speed can be increased because the number of collisions with particles is increased by increasing the bead volume.
Smaller beads are also more suitable for dispersion, since strong milling with larger beads results in excessive particle milling and re-agglomeration of particles. Thus, the energy of grinding and dispersing affects the size and hardness of particles that can be subdivided as well as the processing speed, and the frequency with which beads and particles collide with each other also affects the speed at which they are processed.
The size of the space in which the beads move is also an important factor that also has a significant impact.
How to Select a Bead Mill
In the principles, we mentioned the influence of bead size and speed on milling performance. On the other hand, it is necessary to separate the beads from the milled particles, and the performance of the bead mill is also important for this method. There are three main separation methods: slitting, screen, and centrifugal separation.
The slit method separates slurry by passing it through a narrow gap. Basically, beads of 0.3 mm or larger are used and can be used stably even with viscous slurries. The screen method is similar to the slit method in that it uses a gap to separate slurry, and beads of 0.1 mm or larger are used.
The centrifugal separation method uses centrifugal force to separate slurry and beads, and can be applied to beads smaller than 0.1 mm.
As explained above, the size of the beads greatly affects the grinding capacity. Slit and screen methods are often employed for submicron-level milling, while centrifugal separation methods are often employed for nano-sized milling and dispersion.