The accuracy of dosing systems when packaging bulk and bulk cargo goods in bags
To achieve consistent fill weights.
Frank Goris
Technical and Commercial Director
Do you remember our last technical blog in which we discussed the interaction between the dosing and weighing system? This time we will take a closer look at the accuracy of net and gross dosing systems
Accuracy of a weigher
The accuracy of weighing is determined by:
The weighing appliance
- With net filling systems the dose is collected in a weighing hopper. The weighing appliance consists of an electronic weighing module that registers the weight of the product in the weighing hopper.
- With a gross filling system the dose is immediately collected in the bag. The weighing appliance then consists of the electronic weighing module that registers the weight of the product in the bag suspended at the filler opening.
The stable weighing appliances built by Arodo in combination with suitable weighing electronics result in high weighing accuracy.
Consistent product flow from the dosing system
It is up to the manufacturer to choose the right dosing system and the right product hopper for the dosing system, where it is important that the dosing system guarantees a continuous and consistent product flow. The more constant the product flow, the more accurate the final weighing.
- There are products that can become so aerated that they can start to behave like water. Then the dosing screw no longer has the product under control. The silo pressure then determines the speed at which the product flows from the dosing screw. Arodo uses a variety of techniques to avoid this undesired situation.
- There are also products that are difficult to dose with a tilting disc check valve. If the product has an erratic form, product outflow may sometimes not be nice and uniform.
The adhesion of product to weighing or non-weighing parts.
This obviously greatly depends on the type of product and often also the conditions in which the product is packaged (ambient factors such as relative humidity).
- With gross dosing the product can adhere to the following surfaces:
- Dosing head: product accumulating here has no effect on weighing. This is unless the accumulated product falls into the bag/filler opening at a random time.
- Filler opening: the product accumulating here is immediately weighed. It is logical that everything accumulating here is weighed, but it is not in the bag.
- With net dosing there are more contact surfaces so the system is less suitable for sticky products:
- Dosing head: product accumulating has no effect on weighing. This is unless the accumulated product falls into the net weigher at a random time.
- Net weigher: product accumulating here can be compensated by electronic zero-setting. However, if the product sticks the one time, and falls from the wall another, the weights will be inaccurate.
- Hopper and filler opening: the product accumulating here and that again detaches like with the net weigher has a direct influence on weighing.
- A net weigher will be less suitable for sticky products because a net weigher has more contact surfaces with the product.