Nuga: An innovative technique for granulating PET bottles

The use of PET bottles in bottling plants is spreading relentlessly throughout the world. After they have been used, they must be shredded for recycling purposes. For this demanding task, NUGA AG, Balgach (Switzerland) has developed a new series of CentriCut 44 granulators (Fig.), which are specially designed to grind PET bottles.

In the process, the bottles are directed into the CentriCut granulator by way of a conveyor belt, as whole bottles in bulk, released from bales in perforated, compressed form or as individual briquettes. By means of a horizontally arranged screw conveyor, the material is forced axially on to the rotor from inside. The rotor is open, with three or five rows of rotor blades around the circumference.

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The centrifugal force generated by the turning rotor throws the material through the open rotor onto the granulator compartment wall (consisting of a screen and the stator blades) which surrounds the fully enclosed granulating compartment. Here, it is ground between the rotor and stator blades into cleanly cut, almost dust-free granulate. In this way, the tendency of the lightweight PET bottles to 'dance' on the rotor or the ejection of partially shredded material, which occurs with conventional granulators, is completely eliminated.

The rotor and stator blades consist of identical, precisely fitting carbide cutting tool blades which can be rapidly replaced. No adjustment of the gap between cutters is required, since the blade clearance is constant over the entire length, excluding adjustment errors.

The flakes, to which the ground bottles have been reduced, are now rapidly extracted through the large, non-rust screen (electropolished and thus wear-resistant) surrounding the cutting chamber, by means of a powerful, integrated vacuum extraction system. The brief dwell-time thus achieved means that the flakes are not subject to any thermal stress. At the same time, specific energy consumption is low, the cutter blades sustain very little wear and, finally, throughput is exceptionally high.

An electronic, load-dependent control system between the rotor and the screw conveyor shaft prevents the granulating compartment from becoming overfilled and material from building up against the screen, as well as permitting startup from a standstill with residues of material still in place.

Depending on the condition of the returned PET bottles, the size of the rotor and the screens, the throughput of the CentriCut 44 is 1000-2000 kg per hour. The drive power employed to achieve this is only 30-55 kW. A rotor diameter of 440 mm and depth of 270 or 480 mm provide for an exceptionally compact design. Weighing around 2.5 t, the machine features a housing mounted on castors, making it mobile and capable of flexible use.

The integral extraction system permits the complete, rapid cleaning of the machine. The shredding compartment can be opened without the use of tools by means of sideways swivelling screen baskets, which also make it possible to replace the screens in the minimum of time. The machines are fitted with a fully integrated, safety-type, noise insulated housing.

K’2004, Düsseldorf, 20.-27.10.2004, Hall 9, Stand B66

NUGA AG Kunststoffschneidmühlen, Balgach/Schweiz

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