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A ZSK polypropylene (PP) compounder has been equipped with the UG 925 underwater pelletiser from Coperion and is achieving an output of 72 tonnes per hour.

The PP compounder went into service in Saudi Arabia at the beginning of 2009.

When it comes to increasing the output capacity of a ZSK compounder for PP, the die plate plays an important role.

Since PP melt is more tacky than PE melt and its viscosity range is wider, the hole spacing must be larger and the passage of the melt longer.

This, in turn, increases the pressure requirement for the pelletising operation.

Therefore, a twin-screw compounding extruder with a 380mm screw diameter (ZSK 380) is used and is capable of building up the necessary pressure without any support from a melt pump.

The ZSK 380 is currently the largest model of the ZSK series.

The screw speed and the 18.5MW drive output of the compounder in Saudi Arabia are high.

The Suprex planetary gearbox for the screw drive permits the variation of the screw speed and hence the optimisation of the compounding process.

The output capacity of each of two ZSK NT compounders supplied to Saudi Arabia is 55 tonnes per hour for the particularly demanding compounding of bi-modal and multi-modal high-density polyethylene (HDPE).

The two-stage ZSK NT was conceived by Coperion for the compounding of such PE grades, which combine a high strength and a high stress-cracking resistance.

In the first stage, a ZSK 320, the material is melted.

In the second stage, a ZSK 380, the relatively low screw speed ensures a long residence time and hence the high degree of homogenisation necessary for this particular material.

Two smaller ZSK NT compounders, equipped with 250/320mm- and 250/350mm-diameter screws respectively, have already been delivering a successful service since the end of 2008.

Both the concept of the high-capacity underwater pelletiser and that of the ZSK NT compounder have met demanding requirements.

With further developments in the pipeline, the output target of 100 tonnes per hour for the compounding of polyolefins is now said to be within reach.

Among the engineering challenges posed by the development of these high-capacity compounding systems were the design of components using FEM methods and the handling and precision machining of large and heavy metal parts.

Coperion Waeschle

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