66 - What is the purpose of a level control in the collection bin for loss-in-weight systems?
There are two customary methods of adjusting the throughput of the dosing system to the throughput of the extruder. Usually, a signal of the extruder that in linear to the extruder rotational speed is used. For example, 1-10 volt or 0-24 mA would then correspond to 0-max. of the dosing system’s throughput range. If the rotational speed of the extruder changes, the dosing system is updated correspondingly.
The second method is a level control in the collection bin. Here a signal transmitter in the collection bin (usually installed directly at the extruder intake) monitors the fill level. The dosing controls then regulate the throughput of the dosing system so that a specified fill level is guaranteed. With this method a signal of the extruder is not necessary.
Level regulation in the collection bin can only be used with overfed extruders. In this context, overfed means that a surplus of material always needs to be present at the intake of the extruder in order to ensure undisturbed operation. The throughput of the extruder is determined only by the rotational speed of the extruder, the dosing system’s control adjusts the throughput of the dosing systems according to the throughput of the extruder automatically.
The advantage of level control in the collection bin is that the material supply above the intake can be specified exactly. This is often indispensable for demanding processes where the material can separate or rehumidify (e.g. when manufacturing spinning fibres). Residual moisture, recipe accuracy and high mix consistency are extremely important in these cases. That is why the material level above the extruder needs to be a slow as possible. However, at the same time, extruder throughput changes, when turning spin pumps on or off, for example when cleaning the spin nozzles, must not affect the material level.
Signal transmitters can be testers, infrared sensors or load cells (pre-weighed collection bins). The advantage of a pre-weighed collection bin that dust in the material, various dielectric properties or static charge of the material don’t influence the pre-weighed material. The continuous weighing of the content means that the control can react directly to throughput changes and adjust the throughput of the dosing device.
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