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Material dosing units are used for adding a set amount of a single additive (a fixed percentage of colour masterbatch, for example) to a base raw material. Units are typically located between the main material in-feed hopper of a machine and the throat of the machine. Cheaper volumetric units use a small auger type screw that runs for a set amount of time at a constant speed. More accurate gravimetric units work by weighing the additive before depositing it into the machine throat.
Gravimetric blenders can be specified to weigh and also mix several additives into the primary raw material. These units are typically machine mounted and have a number of hoppers, one for each additive and one for the primary material. Once each component has been weighed and collected in a chamber, it is mixed by a rotating paddle.
Gravimetric batch blenders can be specified to weigh and also mix a number of additives into the primary raw material. These units are typically machine mounted and have several hoppers, one for each additive and one for the primary material. Once each component has been individually (sequentially) dosed and weighed within a collection chamber, it is mixed by a rotating paddle before being released into the processing machines feed throat.
This is a cyclical process, so is best suited to equipment that also has a production cycle rather than a continuous process. Injection moulding would be a typical application.
Loss-in-weight blenders are a type of gravimetric blender that is a better solution for continuous processes such as plastic extrusion.
Each material hopper is mounted on its own load cell and has a dosing auger. A computer monitors the reduction in weight of each hopper and adjusts the speed of each auger’s rotation to maintain the correct flow rate. A static mixer combines the different materials as they flow through the mixing chamber and into the feed throat of the processing machine.
Extrusion control blenders combine both gain-in-weight and loss-in-weight gravimetric blending technology. Individual materials are sequentially deposited into a weighing/mixing chamber, providing the highest levels of dosing accuracy. The chamber has a loss in weight load cell that then monitors the rate that material is taken up by the production line. In effect, the output of the production equipment (typically an extrusion line) is used to adjust the output of the blender.
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