EuroWire March 2023

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Fundamental considerations in dosing

the material in the weighing hopper. For the dosing of larger amounts, a quantity test is not necessary. At the beginning of the dosage, however, in each case only the correct dosage is given, even if a roughly good material factor is stored. With a wrong material factor, dosing will not be correct at the beginning and for a certain amount of time (balancing). The duration of the incorrect dosage in respect of balancing by the gravimetric algorithms depends on the set dosing rate. The smaller the dosing rate, the longer the balancing process takes. In some cases, it can take a long time.

The experts at Woywod Kunststoff- maschinen have provided a useful guide to the concepts behind the company’s dosing technology. It is the basic task of dosing systems to provide specified amounts of material at a defined time in the defined ratio. For recipe adherence, it is not the volume but the mass of the dosing product that is important. The dosing result, however, depends on the “bulk material specification”, the ambient conditions and the dosing process. In terms of process technology, a distinction must be made between the volumetric and the gravimetric dosing principle. The dosing capacity depends on the volume, the speed and the bulk density of the dosing material, and properties such as the coefficient of friction of the dosing material, shape and size. The expected performance is determined by quantity tests, usually made during the commissioning, and then stored as note, recipe, material database or similar. A material factor has to be stored. With volumetric dosing an adjustment is not made – it is assumed that the performance-determining factors do not change significantly. With gravimetric dosing, an automatic adjustment takes place via gravimetric control. If performance-determining factors change, the speed of the motor will be adjusted. It is therefore necessary that gravimetry recognises the change. This is done via a load cell, which determines the weight loss of

Gravimetric mixing station

cell measures (weighs) the material to be dosed. The only measure is therefore the weight. The target/actual comparison regulates the dosage, which is why gravimetric systems automatically compensate for any deviations in the bulk density. Another advantage is the weight-based recording of the dosed quantities, which can be used for business purposes. With volumetric units this is only possible indirectly – it is not a measured value. It is not usually one single factor that determines which dosing method is used. The following parameters can be decisive in combination: investment costs; property requirements of the final product; properties of the material to be dosed; production duration; plant structure (height and complexity); operational safety; maintenance intervals; necessary cleaning effort; and staff requirements. With volumetric mixing, the investment costs, the space required and the cleaning times are all lower, the handling is simpler and the operational reliability is higher. In the case of gravimetric mixing, investment costs are higher, more space is required, cleaning times are longer, and handling is more complex. Due to the complexity and additional electronics, operational safety is more dependent on the operating personnel. Woywod Kunststoffmaschinen GmbH & Co Vertriebs-KG

In the case of volumetric dosing, the material discharge is based exclusively on the volume. Because volumetric dosing devices do not measure the mass, the dosing device must be calibrated to the respective material before use, and it must be determined how much mass the dosing unit doses in a defined period of time. The same applies to material and batch changes. In addition, volumetric dosing systems cannot automatically compensate for material property changes such as bulk density variations. With the gravimetric or weight-controlled dosing principle, an integrated load

Volumetric mixing station


March 2023

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