MIXERS | MACHINERY
Schematic representations from Mixaco show the vertical, radial, and horizontal vectors that combine to create the vortex in a container mixer Source: Mixaco
cally for the mixing needs of plastics compound- ing: the Container Mixer CMQ (EP 3 342 480 B1); and the High Intensity Mixer FM-T. It says the CMQ Container Mixer incorporates an
innovative mixing principle that can cut up to 80% from normal cleaning time. This is achieved through the use of a flat easy-to-wipe mixing head and mixing tool with a high bottom and wall clearance that prevents material build-up. The FM-T High Intensity Mixer is described as a quick-to- clean flexible stationary. Its design incorporates a two-piece completely jacketed divided mixing bowl configured with a hydraulic unit for lifting the upper bowl part and for lifting and turning the lid. Good access to the inside of the mixer is said to allow easy cleaning and maintenance, without the need for confined space permits (with the US OSHA framework).
In the vortex Mixaco of Germany makes a range of container mixers – following the principle that the sooner a mix is removed from the mixer, the quicker the machine is back in operation. The company says that 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 deter- mined 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 – which
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improve friction and blending performance. Mixaco says the vortex is affected by many factors during mixing. 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. This causes the quality of the blend to suffer significantly. The filling level of the heating mixer is also an
important factor 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 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 reduced. 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.
CLICK ON THE LINKS FOR MORE INFORMATION: �
www.mti-mixer.de �
www.zeppelin-systems.com �
www.mixaco.com
April 2021 | PIPE & PROFILE EXTRUSION 37
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