In a planetary mixer, the blade tears the mass apart, and shear is applied between a moving blade and a stationary wall. The mixing arm moves in two ways, around its own axis, and around the central axis, so that it reaches every spot of the vessel. The plates in the blade are sloped so that the powder makes an upward movement. Therefore, tumbling (convective) motion is also obtained.
Construction of Planetary Mixer
The construction of a planetary mixer is shown in Figure 1. It consists of a vertical cylindrical shell, which can be removed either by lowering it beneath the blade or raising the blade above the bowl. The mixing blade is mounted from the top of the bowl. The mixing shaft is driven by a planetary gear train, as indicated in Figure 1. It rotates around the ring gear, which further rotates around the mixer blade. It is normally built with a variable speed drive.
In the planetary mixer, the agitator has a planetary motion. It rotates on its own and around the central axis so that it reaches all parts of the vessel. The beater is shaped to pass with close clearance over the side and bottom of the mixing bowl. Therefore, literally, there are no dead spaces in the mixing bowl. The blade tears the mass apart and shear is applied between the moving blade and the stationary wall. The plates in the blade are sloped so that the powder makes an upward movement. Therefore, tumbling (convective) motion is also obtained. Since it is a variable speed drive, initially the blade moves slowly for premixing and finally at increased speed for active mixing. Thus high shear can be applied for mixing.
Emptying the bowl may be done by hand (scooping) or by dumping mechanism.
Uses: Planetary mixer produces precise blends, in addition, to breaking down agglomerates rapidly. Low speeds are used for dry blending and faster speeds for the kneading action required in wet granulation. Steam jacketed bowls are used in the manufacture of sustained-release products and ointments.
Advantages: Speed of the rotation can be varied at will, so it is advantageous over sigma blade or ribbon type blenders. This is more useful for the wet granulation process. There are no packing glands in contact with the product.
- Mechanical heat is built up within the powder mix.
- It requires high power.
- It has a limited size and is useful for batch work only.
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