MACHINERY | ALTERNATIVE COMPOUNDERS
tion rate using twin-screw extruders is limited by screw speed (because of shear heating) for temperature-sensitive applications, he adds, citing compounding of TPV, POM, PMMA and TPU as particular examples.
Above: The schematic of an MRS shows the large single screw section in which slots house eight satellite screws. These rotate around and in the opposite direction to the main screw, breaking up and renewing the surface area of the polymer at an extremely high rate. When combined with a large vacuum window, this can provides very good devolatilisation performance
capacity. CPM engineers are evaluating a larger screw diameter, as well as increasing the torque density for the RE7 to reach these new targets.” Dreiblatt says the low specific energy and low
processing temperature characteristic of the RingExtruder RE also offers tremendous benefits for compounding of the newest generation of biopolymers. “Many of these materials are currently compounded using twin-screw extruders at lower screw speeds to minimise shear heating, but the low screw speed translates to low production rate for a given size machine. Since the RingExtruder RE uses elongational flow for melting and mixing, lower processing temperatures are achieved at commercial production rates.” These same characteristics are being applied to conventional materials where the current produc-
Degassing systems The MRS Multi Rotation System Extruder was designed by Gneuss from the outset for high perfor- mance devolatilisation of polymer melts. The first application sectors included use with hygroscopic polymers but, in the meantime, the decontamination performance has been shown to be advantageous in other applications. These include the extraction of harmful volatiles to enable post-consumer recycled material to be used for food contact applications and also for extraction of odours. The operating principle of the MRS extruder
provides low shear and distributive mixing, making it very effective at blending polymers. The com- pany says the machine is not intended to compete with twin-screw systems in mainstream compound- ing applications where high kneading performance is needed. However, while twin screw extruders are generally equipped with vents, there are applica- tions that demand devolatilising performance beyond the scope that these can support. Gneuss has developed a modular system in which the first stage, plasticising, is carried out using a conventional twin screw extruder before the melt passes into a special version of the MRS extruder, where intense devolatilisation of the polymer takes place. The polymer is then further transported via a short single-screw extrusion section, which develops the pressure needed for
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IMAGE: GNEUSS
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