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Gravimetric dosing is the principle whereby the components are dosed in a balanced way instead of moving a certain volume. There are two variants of gravimetric dosing, one weighing the decrease in weight (loss in weight principle) and the other weighing the increase in weight (gain in weight principle).
The loss in weight systems are usually volumetric dosing machines with a weighing cell under the hopper containing the material to be added. In practice, these weighing cells have a very large range, sometimes even up to 20kg. As a result, the machine is often unable to detect a single dose. Only after a number of doses, the weighing cell shows a decrease in weight and can adjust the next doses to these values. This can make start-up time long, which often results in rejection.
In addition, it is still not possible to dose at half speed. The advantage of such a system is that it is self-calibrating and, therefore, in practice, much more accurate than volumetric dosing, but just as easy to use.
With the gain in weight principle, all the components are weighed out one by one and combined in one batch, which is why these machines are also called batch blenders. To dose the same example product as above, a batch will be made of 500 grams. The weight to be added is now 7.5 grams, which is approximately equal to 300 grains. If it is not 300 but 330, there is a deviation of 10%. However, this can now be compensated in the second component, which is also weighed. Of this, 19,700 grains were requested but are now adjusted to 21,670 (+10%). Also with the last component, a deviation can occur, suppose 20 grains are dosed too much then the deviation is only 0,09%. This makes a batch blender much more accurate than a volumetric dosing machine or a loss-in-weight dosing machine. Finally, everything is weighed, resulting in almost perfect traceability and reproducibility.
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Within the plastic processing industry, granulate is available in all kinds of packaging forms. Natural is usually available in big bags or in loads that are subsequently blown into silos. Additives and masterbatch often arrive in bags or big bags. A material transport system is needed to get the material from the bags, big bags or silos to the injection moulding machine, extruder, or blow moulding machine. There are several options in the market. There are screw conveyor systems, blow conveyor systems and vacuum conveyor systems.
Screw conveyors are often used for powders and flakes. Blow transport systems are often used to fill a silo. Vacuum conveying systems are often used to transport the material to an intermediate buffer or the processing machine. Vacuum conveying systems can be divided into two systems, decentralised systems and centralised systems.
In a central system, the hopper loaders do not have their own filter or pump, in contrast to the single-stage hopper loaders. The filter and pump are shared with other hopper loaders. This makes maintenance of the filter and pumps easier. In addition, these filters and pumps are much more robust than the filters and pumps in a stand-alone hopper loader, resulting in far fewer failures and downtimes. Finally, it removes the dust from the process so that not everything is smeared. A central system consists of 4 different parts:
A decentralised system is also called a standalone or single-stage hopper loader system. The name hopper loader literally means hopper filler. These standalone hopper loaders each have their own pump and filter. The hopper loader brings itself under vacuum, which sucks up the material. The advantage of this type of hopper loader is that it is easy to use without the need for a vacuum line. The disadvantage of these systems is that there will always be dust left behind in the hopper loader, which can lead to smearing. In addition, these hopper loaders often have a compressed air self-cleaning system which causes overpressure in the hopper loader. This pressure has to go somewhere and often causes a lot of dust around the hopper loader. Finally, these hopper loaders are often very maintenance sensitive, so the chance that the machines come to a standstill is high.
Dryers are used for drying hygroscopic materials such as PA (Nylon), PC and PET. The drying of these materials is necessary to preserve certain properties. Moisture causes air bubbles in the product. This is because water boils at 100 degrees, and therefore vapour is created, which is trapped in the product. This does not benefit the quality of the end product. A dryer works with a dew point. A dew point is expressed in degrees—the lower the dew point, the lower the humidity. Low humidity is necessary so that the hygroscopic material can release the moisture back into the air. This requires a dew point of at least -18°C. A frequently asked question is whether a dew point of -40°C is twice as good as a dew point of -20°C. The answer is short and simple, no it is not.
This is because the relationship between the dew point and humidity is not linear. Richard Mollier, a German professor of applied physics and mechanics, was a pioneer in the study of thermodynamics, particularly water, steam and humid air. He also studied the relationship between dew point and humidity.
What has become clear is that a dew point of -40°C is not twice as good as a dew point of -20°C. The absolute zero point of the moisture content starts at -273°C. Up to -20°C, the course is almost linear. From -20°C, the curve starts. At -20°C, the moisture content is approximately 0.75 g/kg, which means that each degree of temperature difference corresponds to a difference in moisture content of approximately 0.003 g/kg. The moisture content at -40°C is approximately 0.69 g/kg. The difference is only 0.06 g/kg. It is not necessary for many materials to dry at a dew point lower than -20°C. It costs a lot of energy to get a lower dew point, but ultimately it has a minimal influence on the end product.
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