Advancements in mixing technologies--part 3.

Since the introduction of the Banbury mixer in 1916 and the introduction of the Intermix in 1934, both machines are said to have acquired an excellent reputation as being rugged and effective compounding devices. As the initial strengths and weaknesses of each design became obvious, on-going development work has resulted in changes and improvements to both technologies.

The rotors (continued)

The Banbury mixer has a tangential rotor design. The rotors do not intermesh/interlock and can turn independently of one another. Within the Banbury mixing chamber, dispersive mixing occurs, primarily between the rotor tip and the body bore. To assure proper shear flow, it is important that the materials being mixed flow over the tip of the rotor wing within the mixing chamber bore. The helix of the rotor wings creates axial flow of the mix within the mixing chamber. The interaction of the two rotors creates a degree of squeeze flow between the rotors when the rotor wings align tip to tip. When one rotor wing enters the window of interaction (the area identified by a vertical plane between the two rotors between the bottom of the weight and the top of the door top), there is an exchange of the mix from one body bore to the other, creating a distributive mixing action (figure 8). This can only occur through proper selection of the metal temperatures of both the body bore and rotor. It is imperative that the material being mixed releases from the rotor and slightly adheres to the body bore for this action to occur. The proper selection of metal temperature not only assures efficient flow over the rotor tip, but also insures the mixing material is moved axially in the bore of each rotor from one side of the mixer to the other (distributive mixing). Older Banbury mixers operate with friction ratio gearing. A typical friction ratio is 1.125/1. As the rotors turn, they interact in a cyclic pattern in the window of interaction. This interaction creates a squeeze flow (dispersive mixing) and transfer flow (distributive mixing) of material between the two rotors, as previously discussed. New tangential mixers are commonly equipped with even speed gearing in the unidrive. When even speed gearing is used, critical alignment of the rotor wings is required to optimize the mixing action. Numerous studies have shown that even speed operation of the tangential mixer can improve product quality and mixer productivity. When properly aligned, a specific mix flow pattern develops that insures efficient mixing.

[FIGURE 8 OMITTED]

The rotor-to-rotor interaction, along with the characteristic geometry of the tangential rotor design, aggressively accepts and pulls the fed rubber into the mixing chamber. This is the strength of this rotor technology. High ram pressure is normally not required for the rapid ingestion of feed materials at the beginning of the cycle.

The new high technology ST and NST tangential rotors are designed to turn at the same speed and are designed for high efficiency wing tip temperature control (figure 9). Exact positioning of the rotor wings for both the ST and the NST rotor is required. The NST (wing function technology rotor) moves material axially within the mixing chamber very aggressively. One rotor wing on each rotor is dedicated to the movement of material axially within the mixing chamber. One wing on each rotor is designed to efficiently shear the mixing materials. The orientation of the rotor wings on each rotor is such that there is a continual exchange of mixing rubber from one bore of the mixing chamber to another as the rotors turn. The NST rotor design is one that can be considered as distributive mixing intensive. The flow pattern was modeled after that of the Intermix.

[FIGURE 9 OMITTED]

The ST rotor is an evolved rotor design from conventional tangential two- and four-wing rotor technology (figure 10). The primary rotor wings on each rotor share almost equally in the distributive and dispersive mixing process. The ST rotor has optimized wing placement, optimized wing tip temperature control and optimized rotor to rotor interaction with even speed gearing, along with optimized wing tip geometry.

[FIGURE 10 OMITTED]

The NR-5 Intermix rotor and the ST and NST Banbury rotor have taken both intermeshing and tangential rotor technology to an advanced level to meet the current and near future demands being placed on the batch mixing process.

The mixer drive

A variable speed drive is recommended for efficient compounding in both Banbury and Intermix mixers. As a rule of thumb, the mixer rpm should be set to the maximum speed possible that will allow the highest quality of product to be produced in the shortest period of time. Normally, a compromise has to be made between productivity and quality. There also are a number of compounding applications where a changing of the speed during the mixing cycle is required for efficient compounding. In many cases, the speed required to break down or blend polymers of different viscosities is significantly different than the speed required to incorporate fillers, oils or plasticizers into the product mix. The use of variable speed motors is growing rapidly in the rubber industry and is becoming the standard.

The drive type and selection thereof is considered to be a separate topic and not within the scope of this article. However, deserving specific mention is the gradual increase of the use of variable frequency AC rather than SCR/DC drives for driving batch mixers. There can be a significant energy cost savings by the use of variable frequency AC drives. The power factor (the ratio of the average [or active] power to the apparent power of an alternating-current circuit) can directly relate to the cost of electric energy to turn the rotors to mix rubber. The power factor for variable frequency AC drives is in most applications higher than that for SCR/DC drives.

Mixer metal temperatures

Both the Banbury and the Intermix are equipped with a three-zone heating and cooling system, referred to as a temperature control unit, or TCU. The control of the metal temperature is critical for efficient mixing. The Banbury mixer has one zone dedicated to the rotors, a second to the sides and a third to the door top.

For the Banbury mixer, recent studies have shown that the material being mixed should release from the rotor, ram and drop door, and slightly adhere to the body bore for optimum dispersive and distributive mixing. This action is similar to the action required by an extruder to move rubber down the length of the barrel. If the rubber sticks to the screw and slides on the barrel, the rubber will turn with the screw and no movement will occur. If, however, the rubber slides on the screw and slightly sticks to the barrel, the rubber will be moved efficiently. This movement of material in an extruder is similar to the movement of material in the mixing chamber, and is required to create efficient mixing.

One example of the dramatic effect of using differential temperatures between the rotors and sides of the Banbury was demonstrated when final mixing of a synthetic rubber tread stock was being undertaken. All operating parameters for mixing, except for rotor set temperatures, were unchanged. Productivity was unchanged, but product uniformity was significantly improved.

The Intermix is also suppled with a three-zone heating or cooling TCU. The rotors and rotor end plates are fed by one zone, the mixing chamber by the second, and the drop door and plunger by the third. Briefly, when compounding the first batch, the zones of the mixer should be set at temperatures that allow for the product mix to be released from the mixing chamber at discharge. Because of the large working metal surface area to net chamber volume ratio of the Intermix, and the intermeshing rotor design with optimized heat transfer capability, a large amount of heat can be removed from the mix during the mixing process. It is not uncommon for compounders to take the conservative approach by running the mixer cool when using the Intermix. Operating the Intermix in this way can result in excessive energy consumption and higher mixer torque (power) than is required to make the product efficiently. With experience, one will find that the mixer should be operated at the highest temperature setting permissible that will allow a rapid discharge of product from the mixing chamber, while still producing a quality product. The proper selection of TCU/mixer temperature settings for the Intermix will allow acceptable product quality, while minimizing the power and energy requirements to mix.

Conclusion

The design of the Banbury and the Intermix is continually being evaluated and improved. Changing technology is in response to the increased demand placed on the machinery by evolving mixing requirements. With the introduction of new polymers, fillers, rubber compounds, the need to accomplish reactive mixing processes and the demand for increased productivity, the design of the batch mixer has changed. New rotor technology, hydraulically driven hopper assemblies, improved metal temperature control and heat transfer, improved materials of construction, the use of variable speed drives for the mixing process and the ability to improve mixing efficiency by using data from an RPI system are advancements in batch mixing technology.