Magnets for ferrous-magnetic contaminations in plastics
Bars and grilles
rollers and drums
Neodymium, the permanent magnet
Neodymium Nd is a rare metal and was discovered in 1885. It has multiple application uses and is identified with NdFeB (N35, N45, N52) .
It generates a magnetic field with strong attraction to magnetic metals and ferrous, such as splinters, screws or bolts, even in dusty form such as filings.
separation of ferrous-magnetic contaminations
In the processing of plastics, whether in recycling or in the production of granules, it often happens that the product is contaminated with ferrous and/or magnetic metal shavings.
The presence of these metal contaminations reduces the quality of the product and causes additional problems, such as clogging of filters or injector nozzles, breakage of extrusion screws, etc.
The use of magnetic separators ensures that the plastic is thoroughly cleaned of all ferrous impurities.
magnets against ferrous-magnetic contaminations
different applications, specific solutions
Bars, grilles and magnetic filters
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Magnetic rods vary in length and are usually 25 mm in diameter. They can have different applications during the various stages of the plastics processing:
- the simplest is for manual use, as a check and control. The bar can for example be immersed in a big bag or big-bag containing the plastic granule, any presence of magnetic metal components will be captured and will be visible on the surface of the bar;
- The second use is to use different bars to construct a customised magnetic grid as desired. In fact, the magnetic grids used for plastic inspection consist of magnetic bars mounted in parallel on a frame.
Magnetic gratings consist of individual magnetic bars mounted in parallel on a frame. The diameter of the magnetic bars is usually 25 mm. The grids come in different sizes and shapes, and can consist of 2 or more bars, and be round or square in shape, to fit the various parts of the system.
The grid is the most widely used type of magnet in the plastics industry because it guarantees the effective separation of ferrous-magnetic metal from plastic. The close spacing of the bars generates a constant magnetic field, a real barrier-filter. The plastic product, whether granules, powder or flakes, is dropped by gravity and passed through the bars, all ferrous particles are attracted and deposited on the bars.
Application points for magnetic grids are usually at the outlet of pipes or granulators, at the inlet of collection hoppers or at the outlet of screw conveyors.
Magnetic tube filters are an evolution of magnetic grids. These accessories are generally cylindrical or square in shape and, as they have an inlet and outlet hole, can be installed directly on the pipes through which the plastic passes.
The installation is purely vertical, so that the plastic product passes through the system by gravity. Inside the magnetic tube filters there can be one or more grids, mounted in succession, in parallel and with the bars staggered. This increases the size and intensity of the magnetic field, and therefore the effectiveness of retaining ferro-magnetic impurities.
In addition to grids, magnetic filters can also consist of a single central magnet, or two side magnetic plates. These solutions are suggested in cases where the plastic product is dusty or otherwise not very flowing and the use of magnetic bars would hinder the constant flow of the plastic.
The application points can be different: loading and unloading of silos, product outlet from granulators or on the feed lines of extruders or moulds. In some cases the use of a gravity metal separator with a magnetic tube filter is suggested. The combination of the two systems offers different benefits, especially in the presence of a heavily polluted plastic product.
It is recommended to first install the magnetic filter and then the gravity metal separator. The product is first cleaned of all magnetic metal contaminations, including ferrous dust and filings, when it passes through the magnetic filter. Then the metal separator controls and rejects all other non-magnetic metals that have not been retained by the magnetic filter. In this way two separate controls are obtained and the gravity metal separator will reject less polluted plastic product.
EASY CLEAN SYSTEMS
The easy clean construction method basically consists of sliding tubes that cover the magnetic bars. The magnetic bar or grid in the easy clean design therefore consists of 2 parts: the magnetic bar or grid and a second frame consisting of tubes.
During use, the magnetic bars are inserted inside the tubes and the assembly is placed inside the magnetic filters or directly in contact with the plastic granule to be controlled. The tubes, also called casings, are made of AISI304 or a non-magnetic material, are in direct contact with the plastic and during use the ferrous magnetic impurities obviously stick to them.
At the end of the processing cycle, the removal of the ferrous contaminants is done by removing the magnetic rods from the casings rather than by hand, which is always a long and uncomfortable procedure. As soon as the two parts are separated, the ferrous impurities fall downwards and become detached from the tubes because the magnetic field is lost.
Plates, drums and magnetic systems
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Magnetic plates have two distinct uses. They can be placed suspended over a conveyor belt on which there is plastic. As they pass through, all the magnetic contaminations are attracted and stick to the plate.
Another use of the plates is as slide plates. The plastic is slid over the plate, and while the magnetic contaminations remain attached to the magnetic plate, the cleaned product then proceeds to fall.
The use of these systems is recommended if the use of magnetic grids is not possible, for example with large plastic products that would not pass through the bars.
ROLLERS AND DRUMS
Magnetic rollers and drums are generally used in conjunction with conveyor belts. It is common to use a magnetic roller at the end of a conveyor belt on which plastic containing ferrous contaminations is transported.
At the end of the belt, the plastic will proceed on its downward trajectory to the front of the roller. All ferrous and magnetic impurities, on the other hand, will remain attached to the belt under the attraction of the magnetic field. I
The belt, after rotating around the roller, as soon as it starts its return path under the roller, will push away the ferrous contaminations by letting them fall downwards, in a different direction to where the plastic fell. In the lower part of the roller, underneath, at the end of the conveyor belt, a bulkhead or plate is usually installed, which will facilitate the separation and falling down of the plastic from its magnetic contaminations.
The self-cleaning magnetic systems consist of magnetic grids, with two planes, both made with the easy clean system. The two magnetic grids and their casings are connected to hydraulic pistons that control their movement and the separation and extraction of the bars from the casings.
The hydraulic pistons are finally driven in their movement by a pump, which is controlled by an electronic control unit. The control unit, with a timer and set work cycles, independently manages the sliding and cleaning of one and the other grating.
The casing of the system in question has an upper plastic inlet hole and two lower outlet holes, one for the ferrous contaminations and one for the plastic product purified from magnetic contaminations.