The function of this unit just follows from the definition “mixing and dosing”.
The selection of a suitable dosing and mixing valve depends on assessment criteria of the same importance which are mentioned without rating in the following:
- easy, but operationally reliable handling
- easy metering, 0 – 100% if possible
- wear resistance constructive formation of the abrasive channel so that there will be a possibly constant stream of the abrasive into the air stream
- the opportunity of an immediate interruption of the abrasive intake must be reasonably taken into account by using a watchdog circuit required by the professional association.
The individual behaviour of the blasting unit has been an obstacle to an optimal compromise regarding these items; this has been mainly shown by analyses of the used abrasive.
DV80 dosing valve
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ADV80 dosing valve
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Many blasting units do not dose correctly. They open the dosing valve too widely so that a too high loading factor, i.e. amount of abrasive to amount of compressed air, is operated, but this does not lead to a respectively higher abrasive blasting performance. As regards normal pressure values of 7 – 8 b analyses of used abrasive show that only 30 – 35% burst upon impact so that they cannot be used once again. Hardness and shape of the used abrasive play a decisive role in this regard.
60% of the abrasive can be used at blasting pressures of 10 – 12 b. The non-optimal position of the abrasive dosing valve below the blasting pot is a second important factor for its wrong assessment because the pot is often installed too far from the nozzle and this causes inevitably a separation in the blasting hose. This one becomes bigger if the hose cross-section, the coupling systems and the placing of the hose are incorrect (e.g. many turns, jams, kinking).
That’s why utmost care has to be taken over dosing the abrasive. The always increasing costs for disposal of the used abrasives partially mixed with toxic paints have to be mentioned as well.