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How to shred offcut waste of special plastics

the cutting unit

More and more industries are relying on technical plastics – high-performance materials typically used in an engineering context – and the shredding of these materials is becoming an increasing challenge.

Take polyamides (PA) as an example. These engineering ther­moplastics can usually withstand permanent temperatures of up to 130°C. They are characterised by their high strength, toughness and damping properties.

In the automotive industry, these materials are increasingly replacing metal. PA, like polyoxymethylene (POM), polyethylene terephthalate (PET) and Cast Pol­yamide 6, with their excellent mechanical properties, are also used to build machines. PET is also very popular in the beverage and food industry.

But increasing demand is lead­ing to a tense supply situation. It can now take up to four months for a materials order to be deliv­ered. Prices are also rising.

One solution is to return material back into the production cycle in the form of purgings or rejects. This ‘in-house recycling’ means companies spend less on new materials, and there is no need to dispose of them in exter­nal recycling facilities.

With the right technology, qual­ity can be produced that is compa­rable to, or even better than, the original material. This is where shredding comes into its own.

Take GF 30 glass fibre-rein­forced plastic as an example. The fibres inserted in this material must have a certain length to ensure the required rigidity. If they are wrong, the quality suffers.

Viscosity also plays a role when other materials, such as PET, are to be compounded in an extruder. Material flow behaviour is improved if shredding takes place in a single-stage rather than a multi-stage process. The output quality and the throughput per­formance must both be efficient enough to enable companies to work economically.

Plant and machinery business Vecoplan’s developers work closely with customers to find the right solution for the various engineer­ing plastics.

Plants can be adapted to input and output requirements by changing the rotors and cutters and selecting the right screen. For example, a larger rotor diameter might be selected, which can be equipped with hardened tools that can be used several times and changed quickly.

For a more stable and tougher tool design, the core can be increased and the mounting can be adapted to the task at hand. Vecoplan technicians can also use bolted and hardened counter-knives to make the counter knife bar more solid.

For the rotors, Vecoplan uses the HiTorc drive, which is available in various performance classes. It works dynamically, with a strong start-up phase and a high torque. No mechanical elements such as gearboxes, belts, clutches or hydraulic units are required.

In contrast to units with gear drives, the strong vibrations and shocks that occur during the often-difficult shredding process pose no great challenge for the HiTorc direct drive. Compared with mechanical and hydraulic drives, wear is significantly lower and maintenance work is less.

And thanks to the direct mounting of the drive on the rotor shaft, there is no loss of power in the power train and that means the HiTorc can achieve a higher level of efficiency.

For safe operation, the braking resistor enables a fast rotor stop if there is a risk of an accident occur­ring and if impurities are detected. Positive temperature coefficient thermistor technology protects the main motor from fire.

Machines can be strengthened with a reinforced design to pre­vent premature wear during the shredding of certain materials. The housing can be constructed with thick-walled ribs, for exam­ple, with more solid sidewalls and a more robust base.

In one of its plants, Vecoplan increased the distance between rotor end face and side wall, in both the rotor area and screening chamber. This prevents tapes and wires from winding on to the shaft. The free passage of material downwards also reduces wear between rotor and sidewall.

Depending on the application, the company can develop new components or optimise existing ones. In this way, it can respond effectively to the different materi­als and throughput requirements of customers.

Martin Klotz is area sales man­ager at Vecoplan’s factory in Bad Marienberg, Germany

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