EuroWire March 2021

Technology News

Dosing for extrusion of cross- linked and foamed insulations

Some cable manufacturers still prepare material mixtures by hand: the risk of wrong mixtures or separation of material is high, and reproducibility cannot be guaranteed. Among the earliest methods of automation for the dosing of materials was the implementation of disc dosing units, mounted on the main material hoppers. Disadvantages included high maintenance, comparatively difficult handling and the risk of material separation. These systems were gradually replaced by screw feeding dosing units, installed between the main material hopper and the material inlet of the extruder. The separation of material was eliminated, and changeover periods were reduced. Handling was improved and the spreading of material around the equipment was stopped. In comparatively basic production methods, when the speed of the extruder is rarely changed (ramping) and the requirements of the quality of the mixture are minimal, this way of having a dosing unit synchronised to the speed of the extruder is still common. However, as property requirements increase, new products need highly precise dosing solutions. Cross-linked products require a uniform cross-linking over the entire production run, and foamed (chemical foaming) products need a uniform cell structure and a regular foaming result.

Mixing station systems are required when precise dosing is necessary over the complete speed range of the extruder, if very small amounts/percentages of additives are to be dosed or if non-visible additives are to be handled. In this type of system, all materials are dosed and controlled. Mixing stations work independently of the extruder speed and material consumption. Since the 1970s, Woywod Kunststoffmaschinen has developed this technology to supply Plasticolor equipment to the market. Two main versions are available: Plasticolor volumetric mixing stations and Plasticolor gravimetric mixing stations (loss-in- weight principle). Advantages of the volumetric and gravimetric mixing stations include good handling of plastic materials, ranging from free- flowing powder and microgranules to coarse regrind; precise dosing of each individual component at any stage of production; quick cleaning and short changeover times; a low volume of pre- mixed material; and adherence to the preset recipe over the full rpm range of the extrusion line. For special applications and for the extrusion of special materials, synchronised Plasticolor mixing stations that offer a constant material level in the neckpiece are available. Woywod Kunststoffmaschinen GmbH & Co Vertriebs-KG https://woywod.de

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