MACHINERY | MIXERS
Coperion’s Mix-A-Lot bulk mixer offers high- speed and cost-efficient homogenisation
Inside the vortex The company says producing a quality PVC dry blend at speed requires the creation of an optimal vortex in the mixer. In hot mixers the vortex describes the flow of the material as it moves inside the mixing bowl, the company says, and is determined by the different movements (combined vectors) achieved by the design of mixing tool and vessel. The bottom blade creates a mainly vertical upwards movement and pushes the material to the sides while the middle tool creates a radial force. The velocity of the blades creates a centrifugal force (radial vector) and the material gets pressed to the side walls of the bowl, forming a vacuum in the centre. As a result, material is dragged to the shaft (horizontal vector) and pulled down (vertical vector). These three vectors create the vortex, which can be further optimised by the bowl design and turbulence created by the mixing tools. Mixaco says the vortex is affected by many factors during the mixing process. One is the tip speed of the mixing tool. The ideal tip speed depends on the raw materials, blade and vessel design. If it is too low (less than 20 m/s) no vacuum is created in the centre of the bowl resulting in no horizontal movement of material. The mixer may also start vibrating badly. If the tip speed is too high (more than 35-40 m/s) the material is pushed up and to the side walls, creating a huge vacuum in the centre. The result is no top-down movement of material and a high radial motion —referred to as doughnut-mixing. The quality of the blend suffers significantly. The filling level of the heating mixer is also an
important parameter in creating the optimal vortex. In general, a heating mixer is designed to work with a certain amount of material inside (Mixaco’s recommended volume is 85% and vessels, tools and motors are engineered based on this). With too low a filling level the mixer cannot create much downforce and the blend is pushed upwards and
54 COMPOUNDING WORLD | November 2020
remains at the top of the vessel. The result is that the tools are moving but not in contact with material, so the mixing effect is much less. However, the vortex needs some space to expand (free volume) as the PVC particles heat up and increase in volume. If the mixer is overfilled, the vortex will not move efficiently and there is a risk of blowing the filter or lid connections. Finally, the company says the vortex is affected by changing blade design or enhancing blade configuration. If the blend used is light or has a wide spread of bulk densities, it may be helpful to change blade configuration. Light materials — or highly fluidised blends — tend to stay at the top of the mixer and do not homogenise well. In this case, lowering the blades can provide more space for the materials to collapse in to the centre of the bowl to create a better vortex. High speed, gentle and cost efficient homogenisation were prime design requirements for the Mix-A-Lot bulk material mixer from Coperion. “As an optional addition to compounding plants made by Coperion, the unit ensures particularly efficient, high-speed and gentle homogenisation of the fed material,” says Jürgen Rumschick, Project and Sales Engineer at Coperion. The Mix-A-Lot design is available in four sizes for
throughput rates of up to 5 tonnes/h (an ATEX version is available). Rumschik says the unit provides very good mixing performance and short mixing cycles, which he attributes to its optimised mixing rotor design. “Its low circumferential velocities and optimum design of the gap to the housing permit gentle handling, without particle destruction or heating of the product. The large inspection door on the front of the Mix-A-Lot provides easy access to the entire process chamber. An additional opening at the discharge flap also facilitates inspection and cleaning of this section,” he says.
Simplified feeding Coperion says using the Mix-A-Lot to premix ingredients prior to introduction to the compounding extruder can provide the opportunity to reduce capital investment and plant maintenance costs. “Up to now it has generally been standard practice to feed each individual component to the extruder through a separate differential loss-in-weight feeder with a buffer hopper. However, using this technique, the Mix-A-Lot first produces the specific pellet mixture, for which one single loss-in-weight feeder is then sufficient,” he says.
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