Mann+Hummel ProTec: Precisely dosed for high quality fleece fabric

Process-stabilised dosing of all components directly during the manufacturing process – that is one of MANN+HUMMEL ProTec’s core competencies. The fleece manufacturer RKW Gronau relies on exactly this competence in its new nonwoven fabric plant. The SOMOS® Gramix S9 gravimetric dosing and mixing system, designed for up to nine formula components, is integrated in the plant.

For reasons of cost effectiveness and to maintain flexibility during product changes, individually prepared manufacturing compounds are generally never or hardly ever used during nonwoven fabric manufacture. RKW Gronau always mixes the corresponding additive into the base material – matched to the use in the product – inline during the process. The SOMOS® Gramix S9 series gravimetric dosing and mixing system put into operation there at the end of 2008 in the new nonwoven fabric plant is designed for a total of six formula components and a total material throughput rate of 1,000 kg/h. A wide range of high quality nonwoven fabrics, matched to the most varied applications, are being produced on the plant.

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The five dosing stations for the minor constituents consist of one hopper scale each plus a proportioning screw adapted to the mass flow rate – the centrally conducted main component freely flows, after the hopper scale, to the material feed in the extruder. Any possible non-compliance from the target specifications in the material flow is corrected without any delay. The weighing cells work with a precision of ± 0.015 %, ensuring high dosing and repetition accuracy.

As RKW Gronau also apportions functional additives such as UV stabilisers and flame retardants as a master batch in small concentrations, the decision fell in favour of gravimetric dosing. Purely in terms of costs, volumetric dosing, although cheaper, would have increased the risk that possible fluctuations in the bulk weight due to irregularities in the grain size distribution in the additive would also be reflected in temporal-local attribute fluctuations in the manufactured fleece.

Specialised in manufacturing synthetic nonwoven fabrics, the Gronau site, part of RKW SE (www.rkw-group.com), Frankenthal, is manufacturing mostly nonwovens based on polypropylene (PP). At present there are three lines producing with a total capacity of 12,500 tons per year. With the latest plant addition in 2008, RKW Gronau can now choose between two technologies for compacting the produced random orientation fleece after the spinneret: thermobonding (spunbonding), just as in the present nonwoven fabric plants, and now also the newer water jet compacting (spunlace method). During this, the filament layers are pressurised in several stages with wafer thin water jets with a pressure of 250 bar. Instead of welding the filament layers together point by point as is conventional, they entangle under the water jet treatment intensively with each other in all directions.

Water jet compacted fleece is very much softer and more supple to handle than spunbond fleece. In addition to improved haptics, improved mechanical characteristic values result at the same time: especially the maximum lateral force increases by entangling the filaments. One of the decisive prerequisites to do that is created by the Gramix S9 dosing station; it supplies the extruder with a continuous, true-to formula and longitudinal homogeneous mass flow.

Configured from modular building blocks, capable of solving the task
The dosing station weighing funnels can be individually adapted from one graduated standard-size modular system to each processing machine. In the system installed for the nonwoven fabric plant, the weighing container for the major constituent has a volume of 100 l, suitable for flow rates of 100 to 1,200 kg/h. For the minor constituents there are two smaller dosing units with 6 l tank volumes (for throughputs of 1 to 60 kg/h) and three 25 litre weighing units for throughputs of 12 to 300 kg/h. The infinitely variable servomotors driving the proportioning screw placed under the hopper scales continuously convey the minor components in the dead-space-free mixing zone where it flows together with the incoming major constituent. For safety reasons, in the nonwoven fabric plant a metal separator follows to eject any possibly dragged-along, even smallest metal particles from being fed into the material in the extruder. The extruders are always run in the overfed state in order not to risk any break-off of individual filaments on the spinning beam due to insufficient material supply.

If the material volumes in the weighing funnels sink to a preset minimum, material is refilled from the storage containers located above automatically and in the shortest time. These containers are equipped with suction conveyor stations to immediately convey new material from the silo or the storage containers. A central vacuum generator (rotary piston compressor station) supplies the negative pressure required for vacuum conveying at the conveyor stations combined with a central filter to collect and eject dust particles carried along in the conveyor air.

All functions and components for material conveyance and material dosing are continuously monitored with the powerful, user-friendly Simatic S7 microprocessor controller. Communication with the nonwoven fabric plant control takes place via a Profibus interface. However, the S7 controller can even additionally control the extruder and the subsequent trigger unit.

Mann+Hummel ProTec GmbH, Bensheim

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