machinery | Multi-screw extruders
Right: Keimei’s triple-screw arrangement
provides three intermeshing points between the screws,
rather than one in twin-screw arrangements
Screw model is a double-decker eight-screw version. The company says both intermeshing co-rotating screw designs evolved out of process data it had accumulated from its twin-screw extruder models. “While a twin-screw extruder has considerable
performance compared with a single-screw extruder, certain limits are still associated with the twin-screw extruder when additional performance is needed,” Technovel says. It says the Quad Screw and Octa Screw units make possible what is not possible with a twin-screw design. However, no further details on the machines were available at the time of writing.
Stepping up to twelve The RingExtruder from Extricom of Germany has no fewer than 12 intermeshing screws. Rather than arranging them in straight lines – as in the Technovel units - Extricom places them, as the name suggests, in a ring around a fixed core. The company says the increased number of screw spindles provides a range of processing benefits. Prime among them is the number of intermeshing sections – because each screw in a RingExtruder intermeshes with two others the number of intermeshing sections is twelve times higher than for a twin-screw model. The complex flow processes in the intermeshing sections show a higher extensional flow rate, which can be used very efficiently for dispersive mixing processes while also saving energy, Extricom says. In contrast to this, the screw channels outside the intermeshing sections mostly show straightforward shear flows. “RingExtruders make separation and mixing
Below: Keimei uses “three start” screws on its triple- screw extrud- ers to improve mixing and keep LD ratios to the minimum
processes more efficient. Moreover, the special RingExtruder geometry provides a larger heat transfer area across the entire output area, which means better temperature control, i.e. cooling or heating,” the company claims.Extricom recently reported on a series of tests carried out with Bayer MaterialScience (now Covestro) comparing twin-screw extruders with its RingExtruder for the compounding of polycarbonate and polycarbonate blends. According to the machinery maker, some new product developments at Bayer had pushed the twin-screw extruders it normally uses to
their limits and it was looking at ways to increase output rates with consistent or even higher quality. The BMS tests were conducted on an Extricom RE 3
XP RingExtruder with 28D screws of 30mm diameter. The output rate for this configuration is comparable with a co-rotating twin-screw extruder with a screw diameter of between 55 and 58 mm. Extricom says screw changeover times on its machine were much shorter than on a twin-screw, and it adds that the RingExtruder also allowed significantly shorter product changeover times due to its narrower residence time range. According to Extricom, standard setup and metering of all additives/fillers into the main metering section allowed high loads of pigments and organic fillers as well as low-viscosity additives to be processed without torque fluctuations, which it says are a frequent challenge for twin-screw extruders processing such a blend of materials. PC/ABS blends were successfully compounded in a RingExtruder at melt temperatures slightly below those required by a twin-screw. As to whether BMS invested in a RingExtruder,
Extricom remains tight lipped. The company says it is is bound by a secrecy agreement and Covestro declines to say. Extricom Sales Director Thomas Bauer will only say that the results from the BMS trial were in line with those from trials with other customers—again, covered by NDAs. One independent compounder known to be using the RingExtruder, however, is Lehmann & Voss. It uses the technology for production of masterbatches and confirms that the unit gives very good pigment dispersion and also avoids the temperature peaks that can cause degradation. Bauer says the Extricom extruder has proven to be particularly good at producing thermally conductive compounds, where filler loadings can be quite high. “There is more to come in this application,” he says, noting the growth in LED lights and the growing preference for heat sinks in conductive plastics rather than metals. All these compounds require good dispersive mixing, which the RingExtruder is able to provide without overheating and the associated risk of
16 COMPOUNDING WORLD | June 2016
www.compoundingworld.com
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