Advancements in mixing technologies--Part 2.

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.

Advances in the design of the batch mixer

The hopper assembly

Both Intermix and Banbury mixers can be fitted with either a pneumatic or hydraulic hopper assembly. In the past, both machines were offered with a pneumatic hopper assembly. Variability of the plant air supply to the pneumatic hopper assembly will create variability in the mixing process. The hydraulic hopper was developed primarily to eliminate the variability in the mixing process caused by the varying of the plant air supply (figure 2). A continual monitoring of the pneumatic system is required to assure efficient mixing and batch-to-batch repeatability are achieved. The Banbury has the hydraulic hopper assembly available as an option, while the Intermix is sold with the hydraulic hopper as standard. The identified market for the Intermix machine dictates a philosophy of a quality first approach to mixing. Therefore, a hydraulically actuated ram is supplied as standard. The hydraulic hopper assembly has the advantage of isolating the mixer from the problems associated with a varying plant air supply that will affect batch to batch consistency, as well as productivity. In some cases, it can eliminate the need and cost for compressed air in the mixing facility.

[FIGURE 2 OMITTED]

The ram position indicator shows the position of the ram or plunger in the hopper of the mixer, and is considered part of the hopper assembly, it is an extremely valuable tool for optimizing the batch weight for both Banbury and Intermix mixers. The device is positioned within the air cylinder in those mixers equipped with a pneumatically actuated ram, and is positioned under the shrouding of the hopper assembly for mixers equipped with a hydrualic ram actuator. In either installation, the device is located in a protected area. It monitors the position of the weight or plunger during the mixing process. Plotting of the ram position, mixer power and indicated mix temperature can footprint and allow documentation of an optimized mixing cycle.

The mixer sides

The mixer sides for the Banbury and Intermix are similar in design and construction. The sides have been optimized for close temperature control of the working surface and efficient heat transfer (figure 3). Both mixers have the sides coated with a similar proprietary hard surfacing for resistance to wear, and are designed to accept multiple oil injectors. Recent developments in the materials of construction have led to the application of new hard surfacing alloys. The new hard coatings have a more uniform micro structure that has the potential to extend the usable life of the mixing chamber. The new materials of construction were developed in anticipation of the more widely accepted compounding of highly abrasive fillers, such as silica, and the desire to extend mixer life for those applications where non-abrasive materials are being compounded.

[FIGURE 3 OMITTED]

The mixer rotor end plates

The Intermix rotor end plates are steel plates located at the mixing chamber ends (figure 4a). The mixing action of the Intermix forces compound against the end plates. Therefore, the chamber end plates are drilled for water cooling as standard. The working surfaces of the end plates are hard surfaced for wear resistance. The rotor end plates also have precision-machined bottom sealing surfaces to mate with the contour of the drop door (figure 4b). This eliminates powder leakage between the drop door ends and the rotor end plates.

[FIGURE 4 OMITTED]

The Banbury standard rotor end plates are of a single piece construction (figure 5). They are normally chrome plated to eliminate the possibility of stock contamination. Water-cooled design is available as an option, improving dust stop life and overall mixer performance. As with the Intermix, various hard surface coating options are now available for improved abrasive and chemical wear applications.

[FIGURE 5 OMITTED]

The rotors

The primary difference between the Banbury and the Intermix is the design of the rotors. The Intermix rotors are of the intermeshing/interlocking type. The rotors must be operated at the same speed/rpm. A differential rotor surface speed between the rotors is created due to the interaction of the minor diameter of one rotor working the product mix against the major diameter of the opposing rotor. This is where high shear dispersive mixing occurs. This action is quite similar to the action that occurs in the nip of a two-roll mill. It is the squeezing action that optimizes the contact of the rubber against the rotor surfaces. As with a mill, this action allows for an extremely efficient heat transfer mechanism between the rubber and the metal surface of the rotor. Critical placement of the rotor hogs allows for movement of material in the mixing chamber. Each rotor directs materials being mixed to one end of the mixing chamber bore (figure 6). The cross-sectional geometry of the high technology NR-5 rotor nog has been changed from previous designs. The new design allows for improved shear flow of the mix over the nog tip and body. This change not only increases the net chamber volume of the mixer, but also allows for additional dispersive mixing between the nog and the body bore of the sides of the mixer (similar to that action of the Banbury mixer) (figure 7). The minor diameter and major diameter of the NR-5 rotor have been optimized for heat transfer and metal temperature control. A heat channel labrynth arrangement within the rotor body maximizes the heat transfer capability of this unique rotor design.

[FIGURES 6-7 OMITTED]

The Intermix can produce a quality product under a broad range of operating parameters, making it an easy machine to use to produce a quality mix. However, due to its ability to remove heat from the mix, it is not uncommon for over-mixing or excessive energy to be consumed during the mixing process. In order to be efficient and not use excessive energy during the mixing process, attention must be paid to the selection of the mixer metal temperature profile.

This article will be continued in the March 2007 issue.