Process machinery: advancements in mixing technologies--part 1.

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. Over the years, specific applications have emerged that identify the areas of accepted performance for each mixer. As the initial strengths and weaknesses of each design became obvious, on-going development work has resulted in changes and improvements to both technologies. These efforts have resulted in the broadening of the mixing capabilities of both mixers. Both the Intermix and Banbury are used for medium viscosity rubber formulations, general rubber goods and general thermoplastic applications. In addition to this, Intermix mixers are used for specialty (low temperature) rubber compounding, extreme quality applications and single step mixing applications, and the Banbury is used for high volume applications, multiple step mixing applications (especially final mix of high viscosity rubber compounds) and plastic compounding applications.

The most current Intermix (intermeshing rotor mixer) is the Mark 5 Series Intermix. To meet the ever increasing demands of the rubber industry, the mixer is sold as a package that includes a redesigned hydraulic hopper, high technology NR-5 rotors, a ram position indicator and a three-zone temperature control unit. This machine is designed to maximize heat transfer, to impart controlled squeeze and shear flow mixing, and to eliminate variability that can occur due to changes in the plant air supply, typically creating variability in the mixing process where pneumatic hoppers were used. The machine is targeted for single pass rubber mixing processes and other specialty mixing applications where there is a quality first mentality.

The most current Banbury mixer (tangential rotor mixer) is equipped with either an ST or NST rotor design. It is packaged with a high efficiency tip cooled synchronous technology rotor, a three-zone high efficiency temperature control system, a ram position indicator, a pneumatically actuated ram or an optional hydraulically driven hopper assembly. The new tangential rotor mixers offer significantly improved performance as compared to older tangential mixers with conventional two or four wing rotor designs. The newly introduced NST rotor design bridges the gap between the intermeshing and tangential technology. Although it is still considered a tangential mixer rotor design, it exhibits many of the characteristics of the intermeshing style mixer. The Banbury mixer design is targeted for efficient compounding where the emphasis is on productivity for multiple and single pass mixing processes.

The mixing system and the mixing quality objective

The batch mixer was designed to combine a number of raw materials to produce an intermediate composite. By the nature of the batch mixer, the product is in the form of a large mass or lump at discharge that must be put into strip, pellet, granule or sheet form to make it suitable for fabrication into the final product part. The mixer is put in tandem with another machine such as a mill, granulator or extruder (a post-mixer forming device) to accomplish this task. The mixer and the post-mixer forming device can be referred to as the mixing system.

Generally, the post-mixer forming machinery is not considered as a machine that significantly contributes to the quality level of the intermediate composite. There are, however, specific applications where the post-mixer forming machinery and the primary mixer work together to achieve a specific level of quality of the completed intermediate composite. The use of a post-mixer mill to incorporate the cure package into a rubber masterbatch produced in the batch mixer is just one example. In most applications, the post-mixer forming device is expected only to change the form of the mixed product and to remove heat from it.

The intermediate composite must have certain measurable characteristics which will identify it as being at a specific level of quality required by the fabrication machinery and that of the final part to be produced. The quality concerns can be identified in two specific areas that must be considered when the formulation or the composite product is being developed. The intermediate composite must be easily formed and/or cured (if required) into the final product geometry by conventional fabrication machinery having a specific level of processability. It also must exhibit product potential or, in other words, have physical properties such that the fabricated part will meet the requirements of the final product in application.

An understanding of both the materials making up the product and the machinery mixing and forming the intermediate composite is required to produce a quality intermediate composite. New raw materials are continually being developed that place greater demands on the mixing machinery to produce a quality mix. More abrasive fillers such as silica are being used. New polymers and polymer blends with unique processing requirements are now expected to be mixed efficiently in the batch mixer. Reactive compounding applications such as silanization are now commonplace in the tire industry and require better temperature control of the reacting materials during the mixing process. How the product is mixed, as well as the condition and the design of the mixer, can have a dramatic effect on the quality and uniformity of the intermediate composite. Advancements in batch mixing technology have and are continually being made to meet the higher demands of these new materials and processes. Typical advancements include:

* High efficiency rotor designs for improved heat transfer and control over the dispersive and distributive factors of the mixing process for both tangential and intermeshing mixing technologies;

* hydraulically driven mixer rams or plungers providing uniform and more efficient application of batch pressure during the mixing process (the control of the effective batch pressure can directly relate to the intensity of shear mixing during the batch mixing process);

* RPI--ram/plunger position indication and documentation for process monitoring and improvements in the optimization process (the data acquired from the RPI in conjunction with mixer power demand, mix temperature and mixer speed data can allow for a more rapid optimization process, leading to a higher quality product being produced in a shorter mixing cycle);

* improved heat transfer of all internal contacting surfaces, allowing better temperature control of the contacting metal surfaces, as well as improved heat removal from the mix during compounding;

* improved materials of construction to extend the life of the internal mixer;

* accurate and efficient temperature control systems to permit optimum heat transfer and mixer metal temperature control; and

* variable speed mixer drives for improved control of the mixing process and increased productivity.

The batch mixer

The internal batch mixer is the primary mixing device in the mixing system (figure 1). It is common knowledge that the proper design, condition and operation of the mixer are imperative to produce a quality product. There are currently two technologies of batch mixers that are available today for compounding rubber and rubber related formulations. They are referred to as the intermeshing type mixer (the Intermix) and the tangential type mixer (the Banbury). Both batch mixers are composed of a hopper assembly for the loading of the raw materials of a rubber compound into the mixing chamber.

[FIGURE 1 OMITTED]

The hopper assembly contains a chute and hopper door for loading materials into the mixer. In the hopper assembly, there is a ram (Banbury terminology) or plunger (Intermix terminology) that can be raised or lowered, allowing raw materials to be added into the mixer and then forced down into the mixing chamber. It is the pressure applied by the bottom of the ram or plunger during ram down mixing that can affect both mixing time (affecting productivity) and the type and amount of shear affecting the distributive and dispersive mixing process (affecting product quality).

The mixing chamber of both the Intermix and the Banbury mixer contain two counter rotating rotors for the intensive and extensive mixing of the product. Based on the mixer design, all or only selected critical components of the internal working surfaces of the mixer are temperature controlled. Once the materials to be mixed are within the mixing chamber, it is the action of the rotors inside the mixing chamber that creates movement of the mix, initiating and completing the mixing process. In order to prevent material leakage between the rotating rotors and the stationary mixing chamber, dust stop seals are required. The dust stop can be of a lubricated or non-lubricated design. Within the mixing chamber there is at least one thermocouple that allows the change of temperature of the product mix during the mixing process to be tracked or monitored. Energy input and mixing time can and are also used to monitor the progress of the mixing process. Oil injectors can be located through the walls of the mixing chamber or through the rotor end plates for the direct injection of liquids into the mix. Once the mixing process is complete, the batch must be discharged. All batch mixers have a means of discharging the batch by either a drop door assembly located at the bottom of the mixing chamber (the most common method) or by raising the hopper assembly and tilting the body (mixing chamber) of the mixer.

The rotors are most commonly driven by an electric motor connected to them by means of a power transmission (speed reduction) unit. The reduction unit used for both mixers is commonly referred to as a unidrive. It takes the high speed output from the electric motor and reduces it to a speed appropriate for the mixing process. The unidrive reducer has two output shafts that turn both rotors in a counterclockwise rotation. There are a few batch mixers today that are driven by hydraulic motors directly, but these are few and far between, as the technology is in its infancy and there are some technical concerns that are currently being investigated.

Intensive (dispersive) and extensive (distributive) mixing occurs within the mixing chamber as the rotors turn. The mix temperature increases as mechanical deformation of the polymer based mix occurs. During the mixing process, the mix is usually under pressure by the ram or plunger, confining the materials within the mixing chamber and creating an effective mixing mechanism in the critical shearing zones of the mixer. Kinetic energy from the motor is imparted into the mixing material and is converted mostly to thermal energy, raising the temperature of the mix within the mixing chamber. Some of this thermal energy is removed from the mix by the contacting internal metal surfaces of the mixing device, while the remainder of the energy remains in the mix, building the temperature of the composite. This build up of the mix temperature in many cases is a limiting factor of the mixing operation due to thermal degradation or the potential pre-cure of the rubber mixture. It is within the mixing process that differences between rotor designs can be seen. In order to produce acceptable product, operating parameters and sometimes mixing procedures will differ based on rotor design.

The strength of the Intermix is in its ability to move materials aggressively within the mixing chamber, effectively shearing the mix, with a remarkable ability to remove heat generated during the mixing process. The strength of the Banbury is in its ability to move materials aggressively within the mixing chamber, effectively shearing the mix, with the ability to accept most available feed forms of raw materials and previously mixed rubber. It is able to aggressively mix a broad range of products in a short mixing cycle.

This article will be continued in the February 2007 issue.

by Frank Borzenski, Farrel