In addition to easy operation, GRAVIMAX blenders offer extreme accuracy, made possible by RTLS (Real Time Live Scale) real-time weighing. With the help of this method, the gravimetric blenders from WITTMANN reach a metering accuracy of 0.006%, keeping the desired mixing ratio of virgin material and additives on a constant level.
Deviations can be reduced to an absolute minimum, which enables processors to limit the material content to the minimum quantity in each case. The new functions of the GRAVIMAX blenders – available in different sizes for different throughput rates – contribute further to noticeable cost cuts in complex metering processes.
In practice, plastics processors often tend to exceed the sufficient dosage of additives by up to 30%, that is, to administer a massive overdose. A practice based on the fear that, due to fluctuations in metering, the dosage could fall below the required minimum quantity; and of course, the scrap rate in production must be kept low. In other words: production often takes place with a colour additive content of 6 or even 6.5% instead of the required 5%. With a price of EUR 2.80 per kilogram of additive and material throughput of 50 kg per hour, this excessive consumption adds up to about EUR 11,300.– per annum. If an additive costs EUR 10.– or even EUR 25.– per kilogram, the cost difference quickly reaches a six-digit figure.
The touch display terminal of the GRAVIMAX control panel offers a simple procedure for selecting different types of material, setting the correct metering quantities and saving them in the control system as part of the preparations for production. Here, the control system distinguishes between the material types: virgin material, regrind and additive.
Where it is necessary to blend in additives in highly accurate quantities – to prevent both colour fluctuations and excessive material costs – the metering mode of Reference Additive can be set. Strong vibrations of the machine or sudden shocks can lead to blending in deviating quantities of additives, which inevitably leads to the production of parts of different quality. In the Reference Additive blending mode, the GRAVIMAX blender instantaneously adjusts the quantity of virgin material to be blended into the actual quantity of additive, so that the percentage ratio between the two types of material corresponds again to the ratio defined in the original parameter settings for the production lot. So if an overdose of additive is blended in, this is compensated by adding more virgin material.
Accurate metering is generally very difficult to achieve where long glass-fibre (LGF) reinforced plastics are processed. Here, the top priority must always be given to the correct quantity of material with LGF content on the one hand, but on the other hand, there is an added risk of “bridging” on the part of the material. To counteract this effect, GRAVIMAX blenders are equipped with pulsating metering valves. In this way, LGF materials are kept moving during metering. And again, there is a way to prevent the effect of an overdose – in this case, that of the virgin material – using a specially selected metering mode: Reference Virgin. This metering mode ensures that – depending on the exact quantity of virgin LGF material metered in – the quantities of the other components to be added are adjusted accordingly.