MACHINERY | ALTERNATIVE EXTRUDERS
Figure 1: Schematic showing the arrangement of 12 intermeshing and co-rotating screws in the CPM RingExtruder RE with a computer simulation (right) confirming it achieves elongational flow Source: CPM Extrusion Group
screws (Figure 1), delivers maximum degassing capacity for decontamination of rPET in bottle-to- bottle and bottle-to-melt food-grade applications. “Compared to twin-screw extruders, the RE system has the increased surface/volume relationship necessary for efficient devolatilisation, as well as the lower specific energy needed to preserve the intrinsic viscosity [IV] for the resulting PET melt,” says Dreiblatt. “At high production capacities, large diameter
twin-screw extruders become less efficient for degassing and temperature control and reach a limitation above 4 tonnes/h. At these production rates, the degassing efficiency and temperature control of the RingExtruder RE is maintained by virtue of the increased surface area of the twelve screws,” he says. “The twelve intermesh regions existing between
the co-rotating screws produce a high degree of elongational flow, a unique feature of the RingEx-
truder RE that cannot be duplicated in twin-screw extruders, which are dominated by shear flow. This translates to significantly lower energy input for melting and dispersive mixing, a major advantage when compounding materials with high viscosity and/or temperature-sensitivity. When processing such materials, twin-screw extruders must operate at low screw speed, and therefore low production rates, to minimise frictional heat and this is a key point of differentiation for the RingExtruder RE,” according to Dreiblatt.
Proven technology More than 500,000 tonnes/yr of food-grade rPET resin is produced globally using RingExtruder RE technology, according to CPM. The technology is also widely used for continuous mixing of rubbers. However, new applications for the RingExtruder RE are currently under current development that expand on the benefits of elongational mixing in reducing shear-induced degradation. For example, the company says compounding of ABS and PC resin on large-scale twin-screw machines is currently limited by high melt temperature and the accompanying shift in colour/Yellow Index. It says these compounds can be produced with improved properties and lower specific energy using RingEx- truder RE technology. “We do not see RingExtruder RE technology competing in today’s traditional markets and applications, since existing twin-screw technology is both familiar and widely accepted,” says Dreib- latt. “However, increased demand for recycling of mixed plastics and chemical recycling both present significant challenges for twin-screw
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