How can metallic impurities be eliminated?
mo explains the use of metal separators
Even the smallest of metal particles in the melt can cause costly damage. There is an extreme danger for nozzles and hot runner systems as well as for the tools, which means waste as well as loss of production. In addition, dosing systems are also endangered, the failure of which also leads to loss of production. High costs may be involved if large metallic impurities damage the screw feeder or the cylinder and, in the worst case, destroythem.
There are various sources of contamination: damage to the processing or production systems or negligence of personnel, for example, who perhaps inadvertently drop a screw into the open day tank. As a result, not only regrind or recycled material is affected. Metal particles may also occasionally be found in virgin material, in some cases even contained in individual granules of granulate. These are both magnetic and non-magnetic substances, which is important for detection and removal and therefore for the selection of a suitable metal separator.
In practise, this means that a hopper magnet is often not sufficient, because it can only remove ferromagnetic particles from the product flow.
As an alternative, melt filters have the advantage that they also collect foreign matter such as glass or small stones, but then the impurities are already in the melt. For this reason, melt filters are frequently used in extrusion systems.
In order to identify even the smallest of metallic, magnetic and non-magnetic impurities upstream of the extractor and to remove them from the material flow, metal separators with a detection coil can be used. If a metal particle falls through the opening of the search coil, this disturbs the magnetic field and a signal is triggered. This in turn activates a separating unit – for example a flap or slide gate – which very quickly ejects the foreign body together with a small amount of good material. When constructing this, it is important to select a design which is suitable for the pipe cross-section and throughput. Otherwise, the impurity may not be detected.
The design of metal separators depends on individual circumstances and subsequently the installation situation. For example, it must be decided whether the separator is to remove the impurities from stationary or free-falling granulated bulk material or from pneumatically conveyed bulk material. The devices can be installed directly above the material inlet of the processing machine or in the pipeline of the machine, to the dryer or to the day tank. After the optimum scanning sensitivity and separation time have been set, the separator operates fully automatically without interrupting the material flow.
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