machinery feature | Twin-screw extruders
Steer focuses on optimising screw elements, including its DSE, FME and FKB designs (left to right)
non-uniformity of energy exposure. Though process pressure will force material forward through the element, some material is forced back upstream. Says Roden: “Several families of mixing elements
are now available that focus energy input during mixing on a specifi c task. These tasks can be identifi ed as stirring or elongation, kneading, and shearing”. He goes on to explain: “If viscoelastic materials such
as molten plastic are stirred in a partially-fi lled extruder, a high degree of elongation of the materials occurs. One can visualize this action by imagining a twin-armed taffy mixer in operation. Taffy is stretched uniaxially then recombined when the two arms cross paths. Such mixing action is prevalent in co-kneaders where materials are moved around static pins by oscillating kneading fl ights”. Dynamic Stirring Elements (DSE) are designed to
reproduce this action in a twin-screw extruder. They are basically forward conveying elements that possess corresponding pins and grooves in the root of the element. As material is transported forward it is repeatedly stretched and recombined producing highly effective and effi cient elongational mixing. The forward lead of the screw fl ight ensures that the elements run partially fi lled, thus limiting the amount of shear introduced to the materials. To provide kneading action, Steer has developed the
Fractional Mixing Element (FME). This is available with three, four or fi ve lobes, which correspondingly provide low, moderate or high kneading energy input. Biaxial elongation and planar shear combine to provide an excellent environment for the controlled kneading of materials. For high-shear mixing, Steer offers the Fractional Kneading Block (FKB). It says that the fractional design of the element reduces ineffi cient and damaging shear peaks associated with typical two-lobe kneading blocks. The neutral arrangement of the lobes ensures that the element runs full and that materials are exposed to a very uniform mixing environment. Elements are available in three- or four-lobe arrangements depend-
40 COMPOUNDING WORLD | March 2014
ing on the level of shear desired. Zeppelin is another company with an alternative take on mixing elements. Its Henschel RHC compounders have special conical-shaped, very low-shear mixing elements that are said to provide a gentle incorporation of fi llers. In particular, they are said to be highly effective for shear-sensitive materials. Machines have an OD/ID ratio of 1.66 and a torque density of 15 Nm/cm3
. Markus
Brunkau , general manager for compounders in the company’s Henschel Mixing Technology business unit, says that use of these elements both in the process direction and the other way around creates a very high effective volume inside the process unit. Although there is a big gap between the ends of the mixing elements, they have a self-cleaning effect than enables them to be used for PVC where dead spots would lead to unaccepta- ble overheating of polymer. Brunkau says that the RHC compounders can
incorporate conventional kneading blocks, “but we are trying to reduce them inside the screw confi guration or even eliminate them completely for some applications like PVC or WPC. For other applications like black or colour masterbatch with a load of 40% pigments, kneading blocks are ideal.” Zeppelin Systems has recently applied for a patent
covering the continuous and inline production of plasticized PVC formulations, in which the mixing process is combined with the extrusion process. Brunkau says the new process uses much less energy than the more conventional two-step process, since there is no cooling step between mixing and compounding. He says that a key part of this development is the
application and combination of the conical shaped mixing elements in the fi rst part of the twin-screw extruder. In this part of the machine, the absorption of the plasticizer within the PVC resin as well as the homogenization takes place in a temperature range of below 120°C. In the subsequent process step, the intermediate product – the equivalent of the dry-blend in a conventional PVC mixing process – is melted without passing through any additional interfaces, and
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