The double planetary mixer is gaining popularity for mixing materials of high viscosity. In terms of its capability of handling high viscosities, it is next only to the double arm kneader mixer (e.g. sigma mixer). The introduction of new blade designs has extended the double planetary mixer’s working capacity from about 1 million centipoise to about 8 million centipoises, thereby resulting in a fitting alternative to double arm kneaders. The simple construction, operation, and relatively lower cost make it a preferred choice over the double arm kneader mixer.
Construction and Operation
The Double Planetary Mixer includes two blades that rotate on their own axes, while they orbit the mix vessel (also known as bowl) on a common axis. The material to be mixed is loaded into a cylindrical vessel with a nearly flat bottom. The blades continuously advance along the periphery removing material from the vessel wall and transporting it to the interior. After one revolution the blades have passed through the entire vessel, after three revolutions most materials have been mixed, and after only 36 revolutions, the blades have contacted virtually the entire batch (see figure 1).
The drive system of a double planetary mixer consists of a motor and a gearbox that drives the planetary head. Each planetary blade is generally driven by gears that rotate due to the motion of the planetary head. Variable-speed drives can be provided to operate the mixer over a wide range of speeds, ensuring high torques even at lower speeds. Located below the drive system is a cylindrical cover for the bowl. This cover may be provided with nozzles, liquid spray arrangement, or viewing ports. A major advantage of this vertical type mixer is the shaft seals do not directly come in contact with the product material.
Material may be charged into the mixer either through the nozzles on the top cover or directly loaded into the mixer bowl. Depending on the size of the mixer, either the bowl or the drive system of the mixer is lifted or lowered using hydraulic arrangement. The mixer bowl may be jacketed for circulation of heating or cooling media. The mixer can be designed for operation under pressure or vacuum.
After mixing, the material is discharged through a bottom valve, or by manual scooping of the material from the bowl. For extremely viscous materials, hydraulically operated automatic discharge systems are available that push the material out through the discharge valve. Figure 2 is the photograph of a 750 litre working capacity, double planetary mixer with a hydraulic arrangement for lifting and lowering of the bowl and a pneumatically actuated flush bottom discharge valve.
Depending on the properties of the material to be mixed and the blade type, the specific power for the double planetary mixer ranges from 30 to 50 kW/m3. Because of the vertical configuration of the mixer, the material fill levels in the double planetary mixer can be as low as 20% to a maximum of 85% of the total bowl volume.
The blade profiles used for the double planetary mixer are the rectangular type, the finger blades and the helical blades. Figure 3 shows the different blade profiles.
Rectangular blades are the most common and preferred choice for majority of the applications. The blade profile comprised of the vertical flights and the horizontal lower crossbar is shaped to produce both radial and axial flow of material. Teflon scrapers may be provided to remove the materials that may stick to the vessel walls, thereby ensuring thorough mixing and efficient heat transfer.
The rectangular blades are used for viscous materials like dental composites and other material with viscosities nearly up to 1 million centipoises. However, at higher viscosities, and with extremely sticky materials like sealants, materials loaded with fibrous fillers, the rectangular blades are known to have limitations. In such cases, the material tends to climb up the blades thereby resulting in lower mixing efficiency.
Finger blades are best suited when faster top-to-bottom mixing is required or when faster wetting of powders is needed. The horizontal paddles, which are referred to as fingers, are directed downwards creating a vigorous axial flow of material. The disadvantage of the finger blade design is that the mixer cannot be provided with scrapers for clearing the material deposited on the vessel walls. The finger blades are preferred for mixing of delicate solids and when the mixture is loaded with fibers.
The helical blade consists of a pair of helical flights. As the blade rotates, each flight passes very close to the vessel wall and pushes the material in the forward direction, directed downwards, constantly moving material from the wall to the vessel interior. Unlike the rectangular blade, the helical blade does not have the horizontal rectangular bar that offers resistance to material movement. As a result, the energy required for mixing dense materials with a helical blade is lower. In other words, the helical blade can handle material of much higher viscosities as compared to the rectangular blade. The design of the slope and spiral of the helical blade are critical. It is because of this unique design that the helical blades are capable of excellent top-to-bottom mixing. The helical blade design has enhanced the capability of the double planetary mixer to handle viscosity in excess of 1 million centipoise.
The double planetary mixer is efficient at mixing heavy materials like dense powders, filled slurries and composites. Some of the common applications of the double planetary mixer are listed below.
- Dental composites
- Metal Powders
- Thick film inks
- Solder masks
- Automotive catalysts
- Sheet molding compounds
- Pharmaceutical granulations
- Face masks
- Urethane sealants
- Hot melts
- Conductive inks
- Specialty coatings
- Transfer inks
- Grinding wheels
- Metallic slurries
- Syntactic foams
- Waste solidification
- Caulking compounds
- Fuel cells
- Metal cleaning compounds
- Engineered plastic resins
- Epoxy resins
Photographs – Courtesy – Unique Mixers & Furnaces Pvt. Ltd., India (www.uniquemixer.com)