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|2. Dry filling
Filling a material in a packaging is most often the last step in a manufacturing process. The material that has been produced, or just handled, in previous process steps, is filled in a packaging that will ensure its protection and its safe transport and handling. However, problems at this stage can have very serious consequences for the whole factory as a drop of performance at the last stage, means that the whole process will decrease in throughput or quality.
Managing the filling process is thus a key focus for every factory operators handling bulks solids. This page is focusing on filling machines designed with an Auger filling system or a cup dosing system and discusses key problems that can happen on the line and how to solve them.
Most of the issues that are listed below can actually be tracked down to a common underlying root cause : the product flowability. Powder fillers, especially for the smallest packaging are very sensitive to a powder that is cohesive. It is all the more true in cup dosing system which rely only on gravity to fill and empty the cups.
The flow is the hopper right above the filling head must be good, otherwise the bulk solid will reach the filling head on an irregular pattern causing strong deviation in the actual weight packaged.
How to solve
In order to help the flow, make sure that the hopper cone (or slope if it is a wedge hopper) is steep enough. If necessary, consider the use of agitators that will help break product bridging or ratholes. Using small vibrators can also be a possibility.
The filling process is (most of the time) volumetric. It means that the volume of product actually moved by the dosing system to a pack must be very well controlled as it has to translate in a constant weight, which is what customers are paying for.
However, many factory operators are observing a variability in the actual weight filled (net weight) : this can lead to complaints, even non compliance, and can lead to losses for the manufacturer has he has to set the filling weight actually higher than target to make sure the filled weight is above declaration.
One possible root cause is a bad setting of the volume of powder to be dosed, that can vary from one stroke to another ; it can especially be the case for cup dosers.
However bad weight control are typically related to the product flow : if the volumetric dosing tool is not filled all the time the same way, the weight will change. Irregular filling can be due to cohesive powders or bad flow in the buffer hopper. Another possible issue is to have the level of product fluctuating above the dosing system, the flow of the product is then changing depending on the height of product in the hopper.
How to solve
The flowability issue can be adressed by playing on the formulation of the product to be filled (use a flowing agent for example) or by reviewing the design of the machine to add discharging aids such as agitators in hoppers, or small vibrating systems. If it is observed that the level in the filling hopper is changing a lot over time, it can be improved by changing the control of the refilling of the hopper.
The sealing of the pack must be done without powder sticking in between the 2 sides of the packaging material.
If some product is trapped, the sealing will not be done correctly which will lead to some leakage more or less high. It can simply cause spillages if the system is really not closed (quite rare, this kind of occurrence will more happen if the sealing jaws have an issue), or can cause micro leakages which will be more or less hazardous for the product in the pack. Typically, for perishable goods, such as foods, this kind of leakages must be avoided.
A typical root cause for a back sealing is that the product when falling in the pack is generating dust, if the dust has not settle at the time the jaws are closing for sealing, then leakages can happen.
How to solve
To avoid this issue, the material filled must be less dusty
(lower quantity of fines), and / or a dust aspiration system must
be implemented by the filling machine manufacturer.