Vibrating sifter machines are used to separate a desired product from the contaminants it may contain (pieces of wood, metal, plastics, wire threads...). Those machines are typically circular or rectangular and have wire mesh tasked to stop foreign bodies from entering the process. Vibration is applied to help the good product go through the sieve and also direct the contaminants towards a rejection. This Engineering guide explains the design of vibrating sifters, how to integrate them in a production line and how to troubleshoot potential operating issues.
The simplest way to check a material to verify it does not contain big foreign particles or pieces is to have it go through a static grid. This method is simple but does not prove to be always efficient. The solids (powder or larger particles like pellets) has to be very free flowing to be able to pass through the grid without further action.
Thus, these grids are usually quite coarse, from 10 to 40 mm. Such grids are particularly interesting when tipping bags of 25 kg. They allow the operator to support the bag and avoid that big parts of the bag fall in the product stream. They can also be located at the discharge of Big Bags where they will prevent the liner (if any) to be sucked to the product stream. For these 2 applications however, it is important to make sure that the materials don't lump in the bags, otherwise, it will be particularly difficult to the make them pass through the grid, which may endanger the performance of the process.
In contrary to the static grid, the vibrating sieves are equipped with a motor allowing to vibrate the sieve. The vibration action allows the material to go through the sieve. As a consequence, vibrating sieves can achieve higher throughput, at smaller mesh size. They allow to optimize process safety and process performance. The design of vibrating shifters is however not trivial and many examples have shown that vibrating sieves not carefully designed, specified or maintained can actually bring or release foreign body to the good product. See below the parts on vibrating sieve mechanical design to know how to properly specify such equipment.
Ultrasonic sieving machines : in specific cases, for small to very small mesh sizes (which can be the case when the vibrating screen is used as a classifier), the material being sieve can get stuck in the apertures which leads the sieve to be clogged. For such application, the addition of an ultrasonic system is helping to unblock the material and keep the sieve running for longer. Many suppliers are proposing ultrasonic sieving as an option.
Rectangular vibrating sieves have similar capabilities as the round ones. Vibrations allow the product to go through a sieve in order to check it regarding the presence of foreign bodies. However, it brings an additional function : the possibility to move the powder laterally from 1 point to another. The vibrations are indeed designed to apply an horizontal movement to the particles. Such rectangular vibrating sieve can therefore be used, in some situation, as a feeder. The design of the machine is very important, especially its supply, since a proper control of the flow is also necessary to avoid a "flooding" of the sieve.
Note : classifying
Classifying is another form of product sieving that is not detailed here. Multi deck round vibrating sifters can be used for this purpose. This can be used for classifying pellets, beans or smaller powders.
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